Industrial controllers , basic theory


Derivative Action

The third response found on controllers is the derivative mode. Whereas proportional response responds to the size of the error and reset responds to the size and time duration of the error, the derivative mode responds to how quickly the error is changing. In figure 12, two derivative responses are shown.

The first is a response to a stop change of the measurement away from the set point. For a step, the measurement is changing infinitely fast, and the derivative mode in the controller causes a very large change in the output, or spike, which dies immediately because the measurement has stopped changing after the step. The second response shows the response of the derivative mode to a measurement which is changing at a constant rate. The derivative output is proportional to the rate of change of this error. The greater the rate of change, the greater the output due to derivative. The derivative holds this output so long as the measurement is changing. As soon as the measurement stops changing, whether or not it is at the set point, above or below it, the response due to derivative will cease. Among all brands of controllers, derivative response is commonly measured in minutes as indicated in figure 13.

 

The derivative time in minutes is the time that the open loop proportional plus derivative response is ahead of the response due to proportional alone. Thus, the greater the derivative number the greater the derivative response. Changes in the error are the result of changes in either the set point, or the measurement, or both. To avoid a large output spike caused by step changes in the set point, most modern controllers apply derivative action only to changes in the measurement. Derivative action in controllers helps to control processes with especially large time constants and significant dead time; derivative is unnecessary on those processes which respond fairly quickly to valve motion, and cannot be used at all on process with noise in the measurement signal, such as flow, since the derivative in the controller will respond to the very rapid changes in measurement which it sees in the noise. This will cause large and rapid variations in the controller output, which will keep the valve constantly moving up and down, wearing the valve and causing the measurement to cycle.

Figure 14 shows the combined proportional, reset, and derivative response to a simulated heat exchanger temperature measurement which deviates from the set point due to a load change. When the measurement begins to deviate from the set point, the first response from the controller is a derivative response proportional to the rate of change of measurement which opposes the movement of the measurement away from the set point. This derivative response is combined with the proportional response, and-in addition, as the reset in the controller sees the error increase, it drives the valve farther still. This action continues until the measurement stops changing, when derivative response ceases. Since there is still an error, the measurement continues to change due to reset, until the measurement begins to move back towards the set point. As soon as the measurement begins to move back toward the set point, there is a derivative response proportional to the rate of change in the measurement opposing the return of the measurement toward the set point. The reset response continues because there is still error, although its contribution decreases with the error. Also, the output due to proportional is changing. Thus, the measurement comes back towards the set point. As soon as the measurement reaches the set point and-stops changing, derivative response again ceases and the proportional output is back to 50%. With the measurement back at the set point, there is no longer any changing response due to reset. However, the output is at a new value. This new value is the result of the reset action during the time that the measurement was away from the set point, and compensates for the load change which caused the original upset.

CONCLUSION

This web article has described the responses of a three mode controller when it is used in the feedback control of industrial measurements. The reader should have a clear understanding of the following points.

1. In order to achieve automatic control, the control loop must be closed. -

2. In order to have a stable feedback control loop, the most important adjustment to the controller is the selection of the proper action, either reverse or direct, on the controller. Improper selection of this action will always cause the controller to be unstable. Proper selection of this action will cause the controller output to change in such a way that the movement of the valve will oppose any change in the measurement seen by the controller.

3. The proper value of the settings of proportional band, reset, and derivative time depend on the characteristics of the process. Proportional band is the basic tuning adjustment on the controller. The more narrow the proportional band, the more the controller reacts to changes in the measurement. If too narrow a proportional band is used, the measurement cycles excessively. If too wide a proportional band is used, the measurement will wander and the offset will be too large.

4. The function of the reset mode is to eliminate offset. If too much reset is used, the result will be an oscillation of the measurement as the controller drives the valve from one extreme to the other. If too little reset action is used, the result will be that the measurement returns to the set point more slowly than possible.

5. The derivative mode opposes any change in the measurement. Too little derivative action has no significant effect. Too much derivative action causes excessive response of the controller and cycling in the measurement.

3. The proper value of the settings of proportional band, reset, and derivative time depend on the characteristics of the process. Proportional band is the basic tuning adjustment on the controller. The more narrow the proportional band, the more the controller reacts to changes in the measurement. If too narrow a proportional band is used, the measurement cycles excessively. If too wide a proportional band is used, the measurement will wander and the offset will be too large.

4. The function of the reset mode is to eliminate offset. If too much reset is used, the result will be an oscillation of the measurement as the controller drives the valve from one extreme to the other. If too little reset action is used, the result will be that the measurement returns to the set point more slowly than possible.

5. The derivative mode opposes any change in the measurement. Too little derivative action has no significant effect. Too much derivative action causes excessive response of the controller and cycling in the measurement.

 

 

 

Introduction to Programmable Logic Controllers

The development of Programmable Logic Controllers (PLCs) was driven primarily by the requirements of automobile manufacturers who constantly changed their production line control systems to accommodate their new car models. In the past, this required extensive rewiring of banks of relays - a very expensive procedure. In the 1970s, with the emergence of solid-state electronic logic devices, several auto companies challenged control manufacturers to develop a means of changing control logic without the need to totally rewire the system. The Programmable Logic Controller (PLC) evolved from this requirement. (PLC™ is a registered trademark of the Allen-Bradley Co. but is now widely used as a generic term for programmable controllers.) A number of companies responded with various versions of this type of control.

The PLCs are designed to be relatively "user-friendly" so that electricians can easily make the transition from all-relay control to electronic systems. They give users the capability of displaying and trouble-shooting ladder logic on a cathode ray tube (CRT) that showed the logic in real time. The logic can be "rewired" (programmed) on the CRT screen, and tested, without the need to assemble and rewire banks of relays.

The existing push-buttons, limit switches, and other command components continue to be used, and become input devices to the PLC. In like manner, the contactors, auxiliary relays, solenoids, indicating lamps, etc., become output devices controlled by the PLC. The ladder logic is contained as software (memory) in the PLC, replacing the inter-wiring previously required between the banks of relays. If one understands the interface between the hardware and the software, the transition to PLCs is relatively easy to accomplish. This approach to control allows "laymen" to use the control without necessarily being expert computer programmers.

The following introduction to PLCs should be considered generic in content. While each PLC manufacturer may have unique addressing systems, or varying instruction sets, you will find that the similarities will outnumber the differences. A typical program appears on the CRT as a ladder diagram, with contacts, coils, and circuit branching, very similar to that which appears on an equivalent schematic for relay logic.

This page is a help to try  to understand the transition from relays to PLC control, rather than trying to teach the details of designing and programming a specific brand of equipment.

Manufacturers have numerous programming schools, from basic to advanced training, and you should consider attending them if you plan to become a proficient programmer.

PLC HARDWARE

Programmable controllers have a modular construction. They require a power supply, control processor unit (CPU), input/output rack (I/O), and assorted input and output modules. Systems range in size from a compact "shoe-box" design with limited memory and I/O points, to systems that can handle thousands of I/O, and multiple, inter-connected CPUs. A separate programming device is required, which is usually an industrial computer terminal, a personal computer, or a dedicated programmer.

Industrial controllers market :

Power Supply

The internal logic and communication circuitry usually operates on 5 and 15 volt DC power. The power supply provides filtering and isolation of the low voltage power from the AC power line. Power supply assemblies may be separate modules, or in some cases, plug-in modules in the I/O racks. Separate control transformers are often used to isolate inputs and CPU from output devices. The purpose is to isolate this sensitive circuitry from transient disturbances produced by any highly inductive output devices.

CPU

This unit contains the "brains" of the PLC. It is often referred to as a microprocessor or sequencer. The basic instruction set is a high level program, installed in Read Only  Memory (ROM). The programmed logic is usually stored in Electrically Erasable Permanent Read Only Memory (EEPROM). The CPU will save everything in memory, even after a power loss. Since it is "electrically erasable',' the logic can be edited or changed as the need arises. The programming device is connected to the CPU whenever the operator needs to monitor, trouble-shoot, edit, or program the system, but is not required during the normal running operations.

I/O rack

This assembly contains slots to receive various input and output modules. The rack can be local, combined with the CPU and power supply, or remote. Each rack is given a unique address so that the CPU can recognize it. Within each rack, the slots have unique addresses. Power and communication cables are required for remote installations. The replaceable I/O modules plug into a back-plane that communicates directly with the CPU or through the cable assembly. Field wiring terminates on "swing arms" that plug into the face of the I/O modules. This allows a quick change of I/O modules without disconnecting the field wiring. Every module terminal also has a unique address.

I/O modules are available in many different configurations, and voltages, (AC and DC). Special modules are available to read analog signals and produce analog outputs, provide communication capabilities, interface with motion control systems, etc. The input modules provide isolation from the "real world" control voltages, and give the CPU a continuous indication of the on/off status of each input termination. Inputs sense the presence of voltages at their terminals, and therefore usually have very low current requirements.

Output modules receive commands from the CPU and switch isolated power on and off at the output terminals. Output modules must be capable of switching currents required by the load connected to each terminal, so more attention must be given to current capacity of output modules and their power supply.

Programming devices

Every brand of PLC has its own programming hardware. Sometimes it is a small hand-held device that resembles an oversized calculator with a liquid crystal display (LCD).

Computer-based programmers typically use a special communication board, installed in an industrial terminal or personal computer, with the appropriate software program installed.

Computer-based programming allows "off-line" programming, where the programmer develops his logic, stores it on a disk, and then "down-loads" the program to the CPU at his convenience. In fact, it allows more than one programmer to develop different modules of the program.

Programming can be done directly to the CPU if desired. When connected to the CPU the programmer can test the system, and watch the logic operate as each element is intensified in sequence on the CRT when the system is running. Since a PLC can operate without having the programming device attached, one device can be used to service many separate PLC systems. The programmer can edit or change the logic "on-line" in many cases.

Trouble shooting is greatly simplified, once you understand the addressing system. Every I/O point has a corresponding address in the CPU memory.

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Home

Function of automatic control .  - The feedback loop. - The measurement .  - The process .  

 The automatic controller .  -  Controlling the process .  -  Selecting controller action - Upsets . 

 Process characteristics and controllability . - Controller responses . - Proportional action . - Integral action (reset ).

 Derivative action . - Conclusion .