MODBUS Communication

1.1Introducing Modbus Protocol
Modicon programmable controllers can communicate with each other and with other devices over a variety of networks. Supported networks include the Modicon Modbus and Modbus Plus industrial networks, and standard networks such as MAP and Ethernet. Networks are accessed by built-in ports in the controllers or by network adapters, option modules, and gateways that are available from Modicon. For original equipment manufacturers, Modicon ModConnect partner programs are available for closely integrating networks like Modbus Plus into proprietary product designs.
The common language used by all Modicon controllers is the Modbus protocol. This protocol defines a message structure that controllers will recognize and use, regardless of the type of networks over which they communicate. It describes the process a controller uses to request access to another device, how it will respond to requests from the other devices, and how errors will be detected and reported. It establishes a common format for the layout and contents of message fields.
The Modbus protocol provides the internal standard that the Modicon controllers use for parsing messages. During communications on a Modbus network, the protocol determines how each controller will know its device address, recognize a message addressed to it, determine the kind of action to be taken, and extract any data or other information contained in the message. If a reply is required, the controller will construct the reply message and send it using Modbus protocol.
On other networks, messages containing Modbus protocol are imbedded into the frame or packet structure that is used on the network. For example, Modicon network controllers for Modbus Plus or MAP, with associated application software libraries and drivers, provide conversion between the imbedded Modbus message protocol and the specific framing protocols those networks use to communicate between their node devices.
This conversion also extends to resolving node addresses, routing paths, and error-checking methods specific to each kind of network. For example, Modbus device addresses contained in the Modbus protocol will be converted into node addresses prior to transmission of the messages. Error-checking fields will also be applied to message packets, consistent with each network's protocol. At the final point of delivery, however-for example, a controller-the contents of the imbedded message, written using Modbus protocol, define the action to be taken.
Figure 1 shows how devices might be interconnected in a hierarchy of networks that employ widely differing communication techniques. In message transactions, the Modbus protocol imbedded into each network's packet structure provides the common language by which the devices can exchange data.
Figure 1 Overview of Modbus Protocol Application
1.1.1 Transactions on Modbus Networks
Standard Modbus ports on Modicon controllers use an RS-232C compatible serial interface that defines connector pinouts, cabling, signal levels, transmission baud rates, and parity checking. Controllers can be networked directly or via modems.
Controllers communicate using a master-slave technique, in which only one device (the master) can initiate transactions (queries). The other devices (the slaves) respond by supplying the requested data to the master, or by taking the action requested in the query. Typical master devices include host processors and programming panels. Typical slaves include programmable controllers.
The master can address individual slaves, or can initiate a broadcast message to all slaves. Slaves return a message (response) to queries that are addressed to them individually. Responses are not returned to broadcast queries from the master.
The Modbus protocol establishes the format for the master's query by placing into it the device (or broadcast) address, a function code defining the requested action, any data to be sent, and an error-checking field. The slave's response message is also constructed using Modbus protocol. It contains fields confirming the action taken, any data to be returned, and an error-checking field. If an error occurred in receipt of the message, or if the slave is unable to perform the requested action, the slave will construct an error message and send it as its response.
1.1.2 Transactions on Other Kinds of Networks
In addition to their standard Modbus capabilities, some Modicon controller models can communicate over Modbus Plus using built-in ports or network adapters, and over MAP, using network adapters.
On these networks, the controllers communicate using a peer-to-peer technique, in which any controller can initiate transactions with the other controllers. Thus a controller may operate either as a slave or as a master in separate transactions. Multiple internal paths are frequently provided to allow concurrent processing of master and slave transactions.
At the message level, the Modbus protocol still applies the master-slave principle even though the network communication method is peer-to-peer. If a controller originates a message, it does so as a master device, and expects a response from a slave device. Similarly, when a controller receives a message it constructs a slave response and returns it to the originating controller.
1.1.3 The Query-Response Cycle
Figure 2 Master-Slave Query-Response Cycle
The Query
The function code in the query tells the addressed slave device what kind of action to perform. The data bytes contain any additional information that the slave will need to perform the function. For example, function code 03 will query the slave to read holding registers and respond with their contents. The data field must contain the information telling the slave which register to start at and how many registers to read. The error check field provides a method for the slave to validate the integrity of the message contents.
The Response
If the slave makes a normal response, the function code in the response is an echo of the function code in the query. The data bytes contain the data collected by the slave, such as register values or status. If an error occurs, the function code is modified to indicate that the response is an error response, and the data bytes contain a code that describes the error. The error check field allows the master to confirm that the message contents are valid.
1.2 Two Serial Transmission Modes
Controllers can be setup to communicate on standard Modbus networks using either of two transmission modes: ASCII or RTU. Users select the desired mode, along with the serial port communication parameters (baud rate, parity mode, etc), during configuration of each φ-controller. The mode and serial parameters must be the same for all devices on a Modbus network.
The selection of ASCII or RTU mode pertains only to standard Modbus networks. It defines the bit contents of message fields transmitted serially on those networks. It determines how information will be packed into the message fields and decoded.
On other networks like MAP and Modbus Plus, Modbus messages are placed into frames that are not related to serial tranasmission. For example, a request to read holding registers can be handled between two controllers on Modbus Plus without regard to the current setup of either controller's serial Modbus port.
1.2.1 ASCII Mode
When controllers are setup to communicate on a Modbus network using ASCII (American Standard Code for Information Interchange) mode, each eight-bit byte in a message is sent as two ASCII characters. The main advantage of this mode is that it allows time intervals of up to one second to occur between characters without causing an error.
Coding System
V Hexadecimal, ASCII characters 0 ... 9, A ... F
V One hexadecimal character contained in each ASCII character of the message
Bits per Byte
V 1 start bit
V 7 data bits, least significant bit sent first
V 1 bit for even / odd parity-no bit for no parity
V 1 stop bit if parity is used-2 bits if no parity
Error Check Field
V Longitudinal Redundancy Check (LRC)
1.2.2 RTU Mode
When controllers are setup to communicate on a Modbus network using RTU (Remote Terminal Unit) mode, each eight-bit byte in a message contains two four-bit hexadecimal characters. The main advantage of this mode is that its greater character density allows better data throughput than ASCII for the same baud rate. Each message must be transmitted in a continuous stream.
Coding System
V Eight-bit binary, hexadecimal 0 ... 9, A ... F
V Two hexadecimal characters contained in each eight-bit field of the message
Bits per Byte
V 1 start bit
V 8 data bits, least significant bit sent first
V 1 bit for even / odd parity-no bit for no parity
V 1 stop bit if parity is used-2 bits if no parity
Error Check Field
V Cyclical Redundancy Check (CRC)
1.3 Modbus Message Framing
In either of the two serial transmission modes (ASCII or RTU), a Modbus message is placed by the transmitting device into a frame that has a known beginning and ending point. This allows receiving devices to begin at the start of the message, read the address portion and determine which device is addressed (or all devices, if the message is broadcast), and to know when the message is completed. Partial messages can be detected and errors can be set as a result.
On networks like MAP or Modbus Plus, the network protocol handles the framing of messages with beginning and end delimiters that are specific to the network. Those protocols also handle delivery to the destination device, making the Modbus address field imbedded in the message unnecessary for the actual transmission. (The Modbus address is converted to a network node address and routing path by the originating controller or its network adapter.)
1.3.1 ASCII Framing
In ASCII mode, messages start with a colon ( : ) character (ASCII 3A hex), and end with a carriage return-line feed (CRLF) pair (ASCII 0D and 0A hex).
The allowable characters transmitted for all other fields are hexadecimal 0 ... 9, A ... F. Networked devices monitor the network bus continuously for the colon character. When one is received, each device decodes the next field (the address field) to find out if it is the addressed device.
Intervals of up to one second can elapse between characters within the message. If a greater interval occurs, the receiving device assumes an error has occurred. A typical message frame is shown below.
Figure 3 ASCII Message Frame
Exception
With the 584 and 984A/B/X controllers, an ASCII message can normally terminate after the LRC field without the CRLF characters being sent. An interval of at least one second must then occur. If this happens, the controller will assume that the message terminated normally.
1.3.2 RTU Framing
In RTU mode, messages start with a silent interval of at least 3.5 character times. This is most easily implemented as a multiple of character times at the baud rate that is being used on the network (shown as T1-T2-T3-T4 in the figure below). The first field then transmitted is the device address.
The allowable characters transmitted for all fields are hexadecimal 0 ... 9, A ... F. Networked devices monitor the network bus continuously, including during the silent intervals. When the first field (the address field) is received, each device decodes it to find out if it is the addressed device.
Following the last transmitted character, a similar interval of at least 3.5 character times marks the end of the message. A new message can begin after this interval.
The entire message frame must be transmitted as a continuous stream. If a silent interval of more than 1.5 character times occurs before completion of the frame, the receiving device flushes the incomplete message and assumes that the next byte will be the address field of a new message.
Similarly, if a new message begins earlier than 3.5 character times following a previous message, the receiving device will consider it a continuation of the previous message. This will set an error, as the value in the final CRC field will not be valid for the combined messages. A typical message frame is shown below.
Figure 4 RTU Message Frame
1.3.3 How the Address Field is Handled
The address field of a message frame contains two characters (ASCII) or eight bits (RTU). Valid slave device addresses are in the range of 0 ... 247 decimal. The individual slave devices are assigned addresses in the range of 1 ... 247. A master addresses a slave by placing the slave address in the address field of the message. When the slave sends its response, it places its own address in this address field of the response to let the master know which slave is responding.
Address 0 is used for the broadcast address, which all slave devices recognize. When Modbus protocol is used on higher level networks, broadcasts may not be allowed or may be replaced by other methods. For example, Modbus Plus uses a shared global database that can be updated with each token rotation.
1.3.4 How the Function Field is Handled
The function code field of a message frame contains two characters (ASCII) or eight bits (RTU). Valid codes are in the range of 1 ... 255 decimal. Of these, some codes are applicable to all Modicon controllers, while some codes apply only to certain models, and others are reserved for future use.
When a message is sent from a master to a slave device the function code field tells the slave what kind of action to perform. Examples are to read the ON / OFF states of a group of discrete coils or inputs; to read the data contents of a group of registers; to read the diagnostic status of the slave; to write to designated coils or registers; or to allow loading, recording, or verifying the program within the slave.
When the slave responds to the master, it uses the function code field to indicate either a normal (error-free) response or that some kind of error occurred (called an exception response). For a normal response, the slave simply echoes the original function code. For an exception response, the slave returns a code that is equivalent to the original function code with its most significant bit set to a logic 1.
For example, a message from master to slave to read a group of holding registers would have the following function code: 0000 0011 (Hexadecimal 03)
If the slave device takes the requested action without error, it returns the same code in its response. If an exception occurs, it returns: 1000 0011 (Hexadecimal 83)
In addition to its modification of the function code for an exception response, the slave places a unique code into the data field of the response message. This tells the master what kind of error occurred, or the reason for the exception.
The master device's application program has the responsibility of handling exception responses. Typical processes are to post subsequent retries of the message, to try diagnostic messages to the slave, and to notify operators.
1.3.5 Contents of the Data Field
The data field is constructed using sets of two hexadecimal digits, in the range of 00 to FF hexadecimal. These can be made from a pair of ASCII characters, or from one RTU character, according to the network's serial transmission mode.
The data field of messages sent from a master to slave devices contains additional information which the slave must use to take the action defined by the function code. This can include items like discrete and register addresses, the quantity of items to be handled, and the count of actual data bytes in the field.
For example, if the master requests a slave to read a group of holding registers (function code 03), the data field specifies the starting register and how many registers are to be read. If the master writes to a group of registers in the slave (function code 10 hexadecimal), the data field specifies the starting register, how many registers to write, the count of data bytes to follow in the data field, and the data to be written into the registers.
If no error occurs, the data field of a response from a slave to a master contains the data requested. If an error occurs, the field contains an exception code that the master application can use to determine the next action to be taken.
The data field can be nonexistent (of zero length) in certain kinds of messages. For example, in a request from a master device for a slave to respond with its communications event log (function code 0B hexadecimal), the slave does not require any additional information. The function code alone specifies the action.
1.3.6 Contents of the Error Checking Field
Two kinds of error-checking methods are used for standard Modbus networks. The error checking field contents depend upon the method that is being used.
ASCII
When ASCII mode is used for character framing, the error checking field contains two ASCII characters. The error check characters are the result of a Longitudinal Redundancy Check (LRC) calculation that is performed on the message contents, exclusive of the beginning colon and terminating CRLF characters.
The LRC characters are appended to the message as the last field preceding the CRLF characters.
RTU
When RTU mode is used for character framing, the error checking field contains a 16-bit value implemented as two eight-bit bytes. The error check value is the result of a Cyclical Redundancy Check calculation performed on the message contents.
The CRC field is appended to the message as the last field in the message. When this is done, the low-order byte of the field is appended first, followed by the high-order byte. The CRC high-order byte is the last byte to be sent in the message.
Additional information about error checking is contained later in this chapter. Detailed steps for generating LRC and CRC fields can be found in Chapter .
1.3.7 How Characters are Transmitted Serially
When messages are transmitted on standard Modbus serial networks, each character or byte is sent in this order (left to right): Least Significant Bit (LSB) ... Most Significant Bit (MSB)
With ASCII character framing, the bit sequence is:
Figure 5 Bit Order (ASCII)
With RTU character framing, the bit sequence is:
Figure 6 Bit Order (RTU)
1.4 Error Checking Methods
Standard Modbus serial networks use two kinds of error checking. Parity checking (even or odd) can be optionally applied to each character. Frame checking (LRC or CRC) is applied to the entire message. Both the character check and message frame check are generated in the master device and applied to the message contents before transmission. The slave device checks each character and the entire message frame during receipt.
The master is configured by the user to wait for a predetermined timeout interval before aborting the transaction. This interval is set to be long enough for any slave to respond normally. If the slave detects a transmission error, the message will not be acted upon. The slave will not construct a response to the master. Thus the timeout will expire and allow the master's program to handle the error.
Note: A message addressed to a nonexistent slave device will also cause a timeout.
Other networks such as MAP or Modbus Plus use frame checking at a level above the Modbus contents of the message. On those networks, the Modbus message LRC or CRC check field does not apply. In the case of a transmission error, the communication protocols specific to those networks notify the originating device that an error has occurred, and allow it to retry or abort according to how it has been setup. If the message is delivered, but the slave device cannot respond, a timeout error can occur which can be detected by the master's program.
1.4.1 Parity Checking
Users can configure controllers for Even or Odd Parity checking, or for No Parity checking. This will determine how the parity bit will be set in each character.
If either Even or Odd Parity is specified, the quantity of 1 bits will be counted in the data portion of each character (seven data bits for ASCII mode, or eight for RTU). The parity bit will then be set to a 0 or 1 to result in an Even or Odd total of 1 bits. For example, these eight data bits are contained in an RTU character frame: 1100 0101
The total quantity of 1 bits in the frame is four. If Even Parity is used, the frame's parity bit will be a 0, making the total quantity of 1 bits still an even number (four). If Odd Parity is used, the parity bit will be a 1, making an odd quantity (five).
When the message is transmitted, the parity bit is calculated and applied to the frame of each character. The receiving device counts the quantity of 1 bits and sets an error if they are not the same as configured for that device (all devices on the Modbus network must be configured to use the same parity check method).
Note that parity checking can only detect an error if an odd number of bits are picked up or dropped in a character frame during transmission. For example, if Odd Parity checking is employed, and two 1 bits are dropped from a character containing three 1 bits, the result is still an odd count of 1 bits.
If No Parity checking is specified, no parity bit is transmitted and no parity check can be made. An additional stop bit is transmitted to fill out the character frame.
1.4.2 LRC Checking
In ASCII mode, messages include an error-checking field that is based on a LRC method. The LRC field checks the contents of the message, exclusive of the beginning colon and ending CRLF pair. It is applied regardless of any parity check method used for the individual characters of the message.
The LRC field is one byte, containing an eight-bit binary value. The LRC value is calculated by the transmitting device, which appends the LRC to the message. The receiving device calculates an LRC during receipt of the message, and compares the calculated value to the actual value it received in the LRC field. If the two values are not equal, an error results.
The LRC is calculated by adding together successive eight-bit bytes of the message, discarding any carries, and then two's complementing the result. It is performed on the ASCII message field contents excluding the colon character that begins the message, and excluding the CRLF pair at the end of the message.
In ladder logic, the CKSM function calculates a LRC from the message contents. For applications using host computers, a detailed example of LRC generation is contained in Appendix C.
1.4.3 CRC Checking
In RTU mode, messages include an error-checking field that is based on a CRC method. The CRC field checks the contents of the entire message. It is applied regardless of any parity check method used for the individual characters of the message.
The CRC field is two bytes, containing a 16-bit binary value. The CRC value is calculated by the transmitting device, which appends the CRC to the message. The receiving device recalculates a CRC during receipt of the message, and compares the calculated value to the actual value it received in the CRC field. If the two values are not equal, an error results.
The CRC is started by first preloading a 16-bit register to all 1's. Then a process begins of applying successive eight-bit bytes of the message to the current contents of the register. Only the eight bits of data in each character are used for generating the CRC. Start and stop bits, and the parity bit, do not apply to the CRC.
During generation of the CRC, each eight-bit character is exclusive ORed with the register contents. Then the result is shifted in the direction of the least significant bit (LSB), with a zero filled into the most significant bit (MSB) position. The LSB is extracted and examined. If the LSB was a 1, the register is then exclusive ORed with a preset, fixed value. If the LSB was a 0, no exclusive OR takes place.
This process is repeated until eight shifts have been performed. After the last (eighth) shift, the next eight-bit byte is exclusive ORed with the register's current value, and the process repeats for eight more shifts as described above. The final contents of the register, after all the bytes of the message have been applied, is the CRC value.
When the CRC is appended to the message, the low-order byte is appended first, followed by the high-order byte.
In ladder logic, the CKSM function calculates a CRC from the message contents. For applications using host computers, a detailed example of CRC generation is given on page .

PNEUMATIC

Basic Advantages
Why are pneumatic power systems so popular in such a wide range of work functions? Electronic systems certainly have a much faster response to control signals. Mechanical systems can be more economical. Hydraulic systems can be more powerful.
The answer lies in the unusual combination of advantages pneumatic systems offer. A basic advantage is their high efficiency. For example, a relatively small compressor can fill a large storage tank to meet intermittent high demands for compressed air. Unlike hydraulic systems, no return lines are required.
Other advantages include: high reliability, mainly because of fewer moving parts; compactness; forces, torques and speeds readily variable over a widely useful range; easy control and coordination with other machine/system functions; low cost; easy installation and maintenance; and the availability of a wide range of standard sizes and capacities.
Another, often decisive, advantage in some applications is that air devices create no sparks in explosive atmospheres. They can also be used under wet conditions with no electrical shock hazard.
It is often advantageous to add pneumatic power to machines that have electricity as their primary power source. This may be done to economically provide supplementary functions such as automatic clamping, locking, closing, opening, etc., of various components or devices. The design problems involved are usually not difficult to solve, and equipment selection procedures are simple and straightforward. Installation is simple, too.
When the air compressor or vacuum pump is driven by a power takeoff from the machine it is being added to, the mounting location may be critical (although accessory air components may be placed almost anywhere). But if the unit is provided with its own drive motor, then virtually the entire system can be installed at any point—even away from the machine—as long as that point can be reached by a hose, pipe, or tube to transmit the pneumatic power.
What are some of the drawbacks of pneumatic power systems?
One is the need for a compressor and for distribution lines, compared to the convenience of plugging an electric motor into an existing electric system.
Another is the inevitable energy loss in converting electrical or chemical energy into pneumatic energy, which is then used to do work that the prime mover could have done directly. In addition, pneumatic work devices are often not very energy efficient (20 percent efficiency is typical of air motors, for example). But in variable-load applications, this is offset by pneumatic devices drawing only the power actually needed. Most electric motors, by contrast, draw almost the same power regardless of load.
And, of course, reasonably sized pneumatic devices cannot exert the forces and torques that hydraulic devices can. In most applications, a horsepower rating somewhere in the tens represents the crossover point at which the increasing size and cost of pneumatic devices begin to exceed the basic cost of a hydraulic power generation and transmission system.
In summary, then, electric, hydraulic, and pneumatic systems each have their place. But the advantages of pneumatic power make it the system of choice in many applications

HYDRAULİC SYSTEMS

Hydraulic machinery are machines and tools which use fluid power to do work. Heavy equipment is a common example.
In this type of machine, high pressure hydraulic fluid is transmitted throughout the machine to various hydraulic motors and hydraulic cylinders. The fluid is controlled directly or automatically by control valves and distributed through hoses and tubes.
The popularity of hydraulic machinery is due to the very large amount of power that can be transferred through small tubes and flexible hoses, and the high power density and wide array of actuators that can make use of this power.

Hydraulics used to increase force and torque.
Contents

1 Hydraulic power
2 Force Multiplication
3 Hydraulic circuits
4 Constant pressure system versus load-sensing system
5 Hydraulic pump
6 Control valves
7 Actuators
8 Reservoir
9 Accumulators
10 Hydraulic fluid
11 Filters
12 Tubes Pipes and Hoses
13 Seals, fittings and connections
14 Oil cooler
15 Calculation of the required diesel engine power, mobile machinery
16 See also
17 External links

Hydraulic power
The science of Fluid pressure provides some of the theory of hydraulics.
A force acting on a small area can create a much larger force by acting on a larger area by virtue of hydrostatic pressure.
A large amount of energy can be carried by a small flow of highly pressurized fluid.
Hydraulic machinery offers a very large amount of power and force with relatively small components. A typical hydraulic cylinder with a 75 mm (3 inch) bore, for example, can supply 89 000 N (20,000 lbf). The power transmission in a hydraulic system is easily controlled with valves.
Some parts of a hydraulic system will operate at about 2000 kPa (300 psi) (pilot controls, vehicle brakes). The main hydraulic actuators (for example, cylinders or fluid motors) will typically operate in the range of 7000 - 42000 kPa (1000 - 6000 psi). With advances in materials and design, there is a trend toward higher pressure, with some systems operating to 100 000 kPa (15,000 psi) and some exotic systems with titanium hardware operating at over 350 000 kPa (50,000 psi).

Force Multiplication
An interesting thing about hydraulic systems is the ability to apply force multiplication. Imagine if cylinder one (C1) is one inch in diameter, and cylinder two (C2) is ten inches in diameter. If the force exerted on C1 is 10 lbs, the force exerted by C2 is 1000 lbs because C2 is a hundred times larger in area (S=pi*radius*radius) as C1. The downside to this is that you have to move C1 a hundred inches to move C2 one inch.

Hydraulic circuits

A simple open center hydraulic circuit.
The equivalent circuit schematic.
For the hydraulic fluid to do work, it must flow to the actuator and or motors, then return to a reservoir. The fluid is then filtered and re-pumped.
The path taken by hydraulic fluid is called a hydraulic circuit of which there are several types.
Open center circuits use pumps which supply a continuous flow. The flow is returned to tank through the control valve's open center; that is, when the control valve is centered, it provides an open return path to tank and the fluid is not pumped to a high pressure. Otherwise, if the control valve is actuated it routes fluid to and from an actuator and tank. The fluid's pressure will rise to meet any resistance, since the pump has a constant output. If the pressure rises too high, fluid returns to tank through a pressure relief valve. Multiple control valves may be stacked in series[1]. This type of circuit can use inexpensive, constant displacement pumps.
Closed center circuits supply full pressure to the control valves, whether any valves are actuated or not. The pumps vary their flow rate, pumping very little hydraulic fluid until the operator actuates a valve. The valve's spool therefore doesn't need an open center return path to tank. Multiple valves can be connected in a parallel arrangement and system pressure is equal for all valves.



Constant pressure system versus load-sensing system
Open loop and closed loop circuit.
The closed center circuits exist in two basic configurations, normally related to the regulator for the variable pump that supplies the oil:
Constant pressure systems (CP-system), standard. Pumppressure always equals the pressure setting for the pumpregulator. This setting must cover the maximum required load pressure. Pump delivers flow according to required sum of flow to the consumers. The CP-system generates large power losses if the machine works with large variations in load pressure and the average system pressure is much lower than the pressure setting for the pump regulator. CP is simple in design. Works like a pneumatic system. New hydraulic functions can easily be added and the system is quick in response.
Constant pressure systems (CP-system), unloaded. Same basic configuration as 'standard' CP-system but the pump is unloaded to a low stand-by pressure when all valves are in neutral position. Not so fast response as standard CP but pump life time is prolonged.
Load-sensing systems (LS-system) generates less power losses as the pump can reduce both flow and pressure to match the load requirements, but requires more tuning than the CP-system with respect to system stability. The LS-system also requires additional logical valves and compensator valves in the directional valves, thus it is technically more complex and more expensive than the CP-system. The LS-system system generates a constant power loss related to the regulating pressure drop for the pump regulator:
Power loss = ΔpLS · Qtot; The avarage ΔpLS is around 20 bar (290 psi). If the pump flow is high the extra loss can be considerable. The power loss also increase if the load pressures varies a lot. The cylinder areas, motor displacements and mechanical torque arms must be designed to match in load pressure in order to bring down the power losses. Pump pressure always equals the maximum load pressure when several functions are run simultaneously and the power input to the pump equals the (max. load pressure + ΔpLS) x sum of flow.
There exists basically 4 type of load-sensing system:
(1) Load sensing without compensators in the directional valves. Hydraulically controlled LS-pump.
(2) Load sensing with up-stream compensator for each connected directional valve. Hydraulically controlled LS-pump.
(3) Load sensing with down-stream compensator for each connected directional valve. Hydraulically controlled LS-pump.
(4) Load sensing with a combination of up-stream and down-stream compensators. Hydraulically controlled LS-pump.
System type (3) gives the advantage that activated functions are synchronized, flow relation remains independent of load pressures even if pumps reach the maximum swivel angle. This feature is important for machines that often run with the pump at maximum swivel angel and with several activated functions, such as with excavators. With type (4) system, the functions with up-stream compensators have priority. Example: Steering-function for a wheel loader.
Other basic system configurations:
Open-loop: Pump-inlet and motor-return (via the directional valve) are connected to the hydraulic tank.
Closed-loop: Motor-return is connected directly to the pump-inlet. To keep up pressure on the low pressure side, the circuits have a charge pump (a small gearpump) that supplies cooled and filtered oil to the low pressure side. Closed-loop circuits are generally used for hydrostatic transmissions in mobile applications. Advantages: No directional valve and better response, the circuit can work with higher pressure. The pump swivel angle covers both positive and negative flow direction. Disadvantages: The pump cannot be utilized for any other hydraulic function and cooling can be a problem due to the limited exchange of oil flow. Closed loop systems generally have a 'flush-valve' assembled in the hydraulic motor in order to exchange more flow than the basic leakage flow from the pump and the motor, for increased the cooling and filtering effects. The leakage flow as well as the extra flush flow must be supplied by the charge pump. Closed loop systems in mobile equipment are generally used for the transmission as an alternative to mechanical and hydrodynamic (converter) transmissions. The advantage is a stepless gear ratio (hydrostatic gear ratio) and a more flexible control of the gear ratio depending on load and operating conditions.


Hydraulic pump
A magnified view of an external gear pump.
Hydraulic pumps supply fluid to the components in the system. Pressure in the system develops in reaction to the load. Hence, a pump rated for 5,000 psi is capable of maintaining flow against a load of 5,000 psi.
Pumps have a power density about ten times greater than an electric motor (by volume). They are powered by an electric motor or an engine, connected through gears, belts, or a flexible elastomeric coupling to reduce vibration.
Common types of hydraulic pumps to hydraulic machinery applications are;
Gear pump: cheap, durable, simple. Less efficient, because they are constant displacement, and mainly suitable for pressures below 200 bar (3000 psi).
Vane pump: cheap and simple, reliable (especially in g-rotor form). Good for higher-flow low-pressure output.
Axial piston pump: many designed with a variable displacement mechanism, to vary output flow for automatic control of pressure. There are various axial piston pump designs, including swashplate (sometimes referred to as a valveplate pump) and checkball (sometimes referred to as a wobble plate pump). The most common is the swashplate pump. A variable-angle swash plate causes the pistons to reciprocate.
Radial piston pump A pump that is normally used for very high pressure at small flows.
Piston pumps are more expensive than gear or vane pumps, but provide longer life operating at higher pressure, with difficult fluids and longer continuous duty cycles. Piston pumps make up one half of a hydrostatic transmission.


Control valves
Directional control valves route the fluid to the desired actuator. They usually consist of a spool inside a cast iron or steel housing. The spool slides to different positions in the housing, intersecting grooves and channels route the fluid based on the spool's position.
The spool has a central (neutral) position maintained with springs; in this position the supply fluid is blocked, or returned to tank. Sliding the spool to one side routes the hydraulic fluid to an actuator and provides a return path from the actuator to tank. When the spool is moved to the opposite direction the supply and return paths are switched. When the spool is allowed to return to neutral (center) position the actuator fluid paths are blocked, locking it in position.
Directional control valves are usually designed to be stackable, with one valve for each hydraulic cylinder, and one fluid input supplying all the valves in the stack.
Tolerances are very tight in order to handle the high pressure and avoid leaking, spools typically have a clearance with the housing of less than a thousandth of an inch. The valve block will be mounted to the machine's frame with a three point pattern to avoid distorting the valve block and jamming the valve's sensitive components.
The spool position may be actuated by mechanical levers, hydraulic pilot pressure, or solenoids which push the spool left or right. A seal allows part of the spool to protrude outside the housing, where it is accessible to the actuator.
The main valve block is usually a stack of off the shelf directional control valves chosen by flow capacity and performance. Some valves are designed to be proportional (flow rate proportional to valve position), while others may be simply on-off. The control valve is one of the most expensive and sensitive parts of a hydraulic circuit.
Pressure relief valves are used in several places in hydraulic machinery; on the return circuit to maintain a small amount of pressure for brakes, pilot lines, etc... On hydraulic cylinders, to prevent overloading and hydraulic line/seal rupture. On the hydraulic reservoir, to maintain a small positive pressure which excludes moisture and contamination.
Pressure reducing valves reduce the supply pressure as needed for various circuits.
Sequence valves control the sequence of hydraulic circuits; to insure that one hydraulic cylinder is fully extended before another starts its stroke, for example.
Shuttle valves provide a logical or function.
Check valves are one way valves, allowing an accumulator to charge and maintain its pressure after the machine is turned off, for example.
Pilot controled Check valves One way valve that can be opened (for both directions) by a foreign pressure signal. For instance if the load should not be hold by the chack valve anymore. Often the foreign pressure comes from the other pipe that is connected to the motor or cylinder.
Counterbalance valves A counterbalance valve is in fact a special type of pilot controlled checkvalve. Whereas the checkvalve is open or closed, the counterbalance valve acts a bit like a pilot controlled flow control.
Cartridge valves is in fact the inner part of a check valve; they are off the shelf components with a standardized envelope, making them easy to populate a proprietary valve block. They are available in many configurations; on/off, proportional, pressure relief, etc. They generally screw into a valve block and are electrically controlled to provide logic and automated functions.
Hydraulic fuses are in-line safety devices designed to automatically seal off a hydraulic line if pressure becomes too low, or safely vent fluid if pressure becomes too high.
Auxiliary valves. Complex hydraulic systems will usually have auxiliary valve blocks to handle various duties unseen to the operator, such as accumulator charging, cooling fan operation, air conditioning power, etc... They are usually custom valves designed for the particular machine, and may consist of a metal block with ports and channels drilled. Cartridge valves are threaded into the ports and may be electrically controlled by switches or a microprocessor to route fluid power as needed.

Actuators
Hydraulic cylinder
Rotary actuator (hydraulic)
Motor (a pump plumbed in reverse)
hydrostatic transmission
Brakes


Reservoir
The hydraulic fluid reservoir holds excess hydraulic fluid to accommodate volume changes from: cylinder extension and contraction, temperature driven expansion and contraction, and leaks. The reservoir is also designed to aid in separation of air from the fluid and also work as a heat accumulator to cover losses in the system when peak power is used. Design engineers are always pressured to reduce the size of hydraulic reservoirs, while equipment operators always appreciate larger reservoirs.
Some designs include dynamic flow channels on the fluid's return path that allow for a smaller reservoir.

Accumulators
Accumulators are a common part of hydraulic machinery. Their function is to store energy by using pressurized gas. One type is a tube with a floating piston. On one side of the piston is a charge of pressurized gas, and on the other side is the fluid. Bladders are used in other designs.
Examples of accumulator uses are backup power for steering or brakes, or to act as a shock absorber for the hydraulic circuit.


Hydraulic fluid
Also known as tractor fluid, hydraulic fluid is the life of the hydraulic circuit. It is usually petroleum oil with various additives. Some hydraulic machines require fire resistant fluids, depending on their applications.
In addition to transferring energy, hydraulic fluid needs to lubricate components, suspend contaminants and metal filings for transport to the filter, and to function well to several hundred degrees Fahrenheit or Celsius.


Filters
Filters are a very important part of hydraulic machinery. Metal filings are continually produced by mechanical components and need to be removed, along with other contamination.
Filters may be positioned in a variety of locations. The filter may be located between the reservoir and the pump intake. Blockage of the filter will cause cavitation and possibly failure of the pump. Sometimes the filter is located after the pump, and before the control valves. This arrangement is more expensive, since the filter housing is pressurized, but eliminates cavitation problems and protects the control valve from pump failures. The third common filter location is just before the return line enters the reservoir. This location is relatively insensitive to blockage and does not require a pressurized housing, but any contaminants that may enter the reservoir (from external sources) are not filtered until they pass through the system at least once.


Tubes Pipes and Hoses
Hydraulic tubes are seamless steel precision pipes, specially manufactured for hydraulics. The tubes have standard sizes for different pressure ranges and the standard diameters go up to some 100 mm. The tubes are supplied in length of 6 m, cleaned, oiled and plugged. The tubes are interconnected by different types of flanges (especially for the larger sizes and pressures), welding cones/nipples (with o-ring seal), several types of flare connection and by cut-rings. In case the sizes are larger, Hydraulic pipes are used.Direct welding of 2 tubes together is not acceptable because one cannot check the inside surface.
Hydraulic pipe is used in case standard hydraulic tubes are not available. In general these pipes are used for low pressure. They can be connected by threat connections, but mostly by welding. Because of the larger diameters, in general the pipe can be inspected internally after welding. Steel suppliers carry black pipe, which is non-galvanized and suitable for welding.
Hydraulic hose is graded by pressure, temperature, and fluid compatibility. Hoses are used when pipes or tubes can not be used. Usually to provide flexibility for machine operation or maintenance. The hose is built up with rubber and steel layers. A rubber interior is surrounded by multiple layers of woven wire and rubber. The exterior is designed for abrasion resistance. The bend radius of hydraulic hose is carefully designed into the machine, since hose failures can be deadly, and violating the hose's minimum bend radius will cause failure. Hydraulic hoses generally have steel fittings swaged on the ends. The weakest part of the high pressure hose is the connection of the hose to the fitting. Another disadvantage of hoses is the shorter life of rubber which requires periodic replacement on the order of every 5- 7 years.
Tubes and pipes for hydraulic applications are internally oiled before the system is commissioned. In general the steel piping is painted outside. In case flare- and other couplings are used, under the nut, there are spots where the paint is removed; here the rust process will start. For this reason, for Marine and Offshore use, more and more piping (especially outside and especially small size)is made from stainless steel.
Seals, fittings and connections
In general, valves, cylinders and pumps have female threaded bosses for the fluid connection, and hoses have female ends with captive nuts. A male-male fitting is chosen to connect the two. Many standardized systems are in use.
Fittings serve several purposes;
To bridge different standards; O-ring boss to JIC (hydraulic), or pipe threads to face seal, for example.
To allow proper orientation of components, a 90°, 45°, straight, or swivel fitting is chosen as needed. They are designed to be positioned in the correct orientation and then tightened.
To incorporate bulkhead hardware.
A quick disconnect fitting may be added to a machine without modification of hoses or valves
A typical piece of heavy equipment may have thousands of sealed connection points and several different types of seals, below are some of the most common types;
Pipe fittings, the fitting is screwed in until tight, difficult to orient an angled fitting correctly without over or under tightening.
O-ring boss, the fitting is screwed into a boss and orientated as needed, an additional nut tightens the fitting, washer and o-ring in place.
Flare seal, a metal to metal compression seal with a cone and flare mating.
Face seal, metal flanges with a groove and o-ring are fastened together.
Beam seal, an expensive metal to metal seal used mostly for aircraft.
Swaged seals, tubes are connected with fittings that are swaged in place (non-serviceable). Primarily used in aircraft.
Elastomeric seals (O-ring boss and face seal) are the most common types of seals in heavy equipment and are capable of reliably sealing 6000+ psi (41368+ kPa) of fluid pressure.

Oil cooler
See section below.
Calculation of the required diesel engine power, mobile machinery
Calculation of the required max. power output for the diesel engine, rough estimation:
(1) Check the max. powerpoint, i.e. the point where Pressure x Flow reach the max. value.
(2) Ediesel = (Pmax · Qtot) ÷ η.
Qtot = calculate with the theoretical pumpflow for the consumers not including leakages @ max. power point.
Pmax = actual pumppressure @ max. power point.
Note: η is the total efficiency = (output mechanical power ÷ input mechanical power). For rough estimations, η = 0.75. Add 10-20% (depends on the application) to this power value.
(3) Calculate the required pumpdisplacement from required max. sum of flow for the consumers in worst case and the dieselengine rpm in this point. The max. flow can differ from the flow used for calculation of the diesel engine power. Pump volumetric efficiency avarage, piston pumps: ηvol= 0.93.
Pumpdisplacement Vpump= Qtot ÷ ndiesel ÷ 0.93.
(4) Calculation of prel. cooler capacity: Heat dissipation from hydraulic oiltanks, valves, pipes and hydraulic components is less than a few percent in standard mobile equipment and the cooler capacity must include some margins. Minimum cooler capacity, Ecooler = Ediesel· 0.25
At least 25% of the input power must be dissipated by the cooler when peak power is utilized for long periods. In normal case however, the peak power is used for only short periods, thus the acual cooler capacity required might be considerably less. The oilvolume in the hydraulic tank is also used as a heat accumulator when peak power is used. The system efficiency is very much dependant on the type of hydraulic pumps and motors used and powerinput to the hydraulics may vary a lot. Each circuit must be evaluated and the load cycle estimated. New system designs and system modifications must always be tested in practice and the tests must cover all type of load cycles.

Hydraulics
From Wikipedia, the free encyclopedia
Table of Hydraulics and Hydrostatics, from the 1728 Cyclopaedia.
Hydraulics is a topic of science and engineering dealing with the mechanical properties of liquids. Hydraulics is part of the more general discipline of fluid power. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. Hydraulic topics range through most science and engineering disciplines, and cover concepts such as pipe flow, dam design, fluid control circuitry, pumps, turbines, hydropower, computational fluid dynamics, flow measurement, river channel behavior and erosion.
The word "hydraulics" originates from the Greek word ὑδραυλικός (hydraulikos) which in turn originates from ὕδραυλος meaning water organ which in turn comes from ὕδωρ (water) and αὐλός (pipe).
Contents
[hide]
1 History
2 Hydrostatic power transmission
3 See also
4 References
5 External links

History
The earliest masters of this art were Ctesibius (flourished c. 270 BC) and Hero of Alexandria (c. 10–70 AD) in the Greek-Hellenized West. In ancient China there was Sunshu Ao (6th century BC), Ximen Bao (5th century BC), Du Shi (circa 31 AD), Zhang Heng (78 - 139 AD), and Ma Jun (200 - 265 AD), while medieval China Su Song (1020 - 1101 AD) and Shen Kuo (1031 - 1095). The ancient engineers focused on sacral and novelty uses of hydraulics, rather than practical applications. In ancient Sri Lanka, the Sinhalese used hydraulics in many applications, in the ancient kingdoms of Anuradhapura and Polonnaruwa. The discovery of the principle of the valve tower, or valve pit, for regulating the escape of water is credited to Sinhalese ingenuity more than 2,000 years ago. By the first century A.D, several large-scale irrigation works had been completed. Macro- and micro-hydraulics to provide for domestic horticultural and agricultural needs, surface drainage and erosion control, ornamental and recreational water courses and retaining structures and also cooling systems were in place in Sigiriya, Sri Lanka.
In 1690 Benedetto Castelli (1578–1643), a student of Galileo Galilei, published the book Della Misura dell'Acque Correnti or "On the Measurement of Running Waters", one of the foundations of modern hydrodynamics. He served as a chief consultant to the Pope on hydraulic projects, i.e., management of rivers in the Papal States, beginning in 1626.[1]
Blaise Pascal (1623–1662) study of fluid hydrodynamics and hydrostatics centered on the principles of hydraulic fluids. His inventions include the hydraulic press, which multiplied a smaller force acting on a smaller area into the application of a larger force totaled over a larger area, transmitted through the same pressure (or same change of pressure) at both locations. Pascal's law or principle states that for an incompressible fluid at rest, the difference in pressure is proportional to the difference in height and this difference remains the same whether or not the overall pressure of the fluid is changed by applying an external force. This implies that by increasing the pressure at any point in a confined fluid, there is an equal increase at every other point in the container, i.e., any change in pressure applied at any point of the fluid is transmitted undiminished throughout the fluids.

Hydrostatic power transmission
A hydrostatic power transmission system makes use of fluid under pressure to drive a mechanical load. In this sense, hydrostatic means that energy transfer is brought about by fluid flow and pressure, but not from the kinetic energy of the flow (the latter would be characteristic of a hydrodynamic drive, such as a fluid coupling or torque converter).
A basic hydrostatic power transmission system consists of a positive displacement pump driven by the prime mover, a positive displacement hydraulic motor, interconnecting piping (which may be a combination of steel tubing, actual pipe and hoses), and a reservoir. Additional components, such as valves and filters, are often part of such a system, the former to provide control of the transmission system and the latter to protect precision machined parts from damage due to oil-borne contaminants.
Motion is transmitted by the pump drawing oil from the reservoir, pumping it into the motor, with the discharge returning to the reservoir. The flow of oil causes the motor to rotate at a speed that is proportional to the pump speed. Any resistance to motor rotation will cause system pressure to rise due to the use of the positive displacement pump, which will translate as torque at the motor.
The maximum torque that can be exerted by the motor is determined by the maximum pressure in the system, as well as the ratio between the displacement of the pump and the displacement of the motor, displacement being expressed in cubic inches or cubic centimeters per revolution. For example, a pump specified as displacing 10 cubic inches per revolution will (in theory) pump exactly 10 cubic inches of oil for each revolution (the actual output will be lower due to internal leakage in the pump). If said pump is mated with a motor that displaces 20 cubic inches per revolution, the drive ratio will be 2:1 and the motor will run at one half the speed of the pump, but develop approximately twice the torque applied to the pump. Hence hydrostatic power transmission behaves in a fashion similar to that of a purely mechanical equivalent of gears and shafts.
Hydrostatic power transmission is widely used in industrial machinery and earthmoving equipment, and has found some application in transportation. A principal advantage of hydrostatic power transmission systems is the flexibility of pump and motor positioning within the equipment. Since the only connection between the pump and motor is through the piping, which can be routed in whatever fashion is convenient for the machine designer, hydrostatic motors can often be used to drive machinery placed in difficult to access areas.
The main disadvantage of hydrostatic drive is its inefficiency relative to other power transmission systems. Most of the inefficiency is brought about by resistance to fluid flow through the piping and fittings. The resulting turbulence wastes some of the energy imparted to the fluid as heat.

Hydraulic press
Hydraulic force increase.
A hydraulic press is a hydraulic mechanism for applying a large lifting or compressive force. It is the hydraulic equivalent of a mechanical lever, and is also known as a Bramah press after the inventor, Joseph Bramah. Hydraulic presses are the most commonly-used and efficient form of modern press.

How it works
The hydraulic press depends on Pascal's principle: the pressure throughout a closed system is constant. At one end of the system is a piston with a small cross-sectional area driven by a lever to increase the force. Small-diameter tubing leads to the other end of the system. A fluid, such as oil, is displaced when either piston is pushed inward. The small piston, for a given distance of movement, displaces a smaller amount of volume than the large piston, which is proportional to the ratio of areas of the heads of the pistons. Therefore, the small piston must be moved a large distance to get the large piston to move significantly. The distance the large piston will move is the distance that the small piston is moved divided by the ratio of the areas of the heads of the pistons.
For example, if the ratio of the areas is 5, a force of 100 newtons on the small piston will produce a force of 500 newtons on the large piston, and the small piston must be pushed 50 cm to get the large piston to rise 10 cm. This is how energy, in the form of work in this case, is conserved. Work is force times distance, and since the force is increased on the larger piston, the distance the force is applied over must be decreased. The work of the small piston, 100 newtons multiplied by 0.5 meter (50 cm) is 50 joules (J}, which is the same as the work of the large piston, 500 newtons multiplied by 0.1 meter (10 cm).