PID Control Algorithm
The PID control algorithm is used for the control of almost all loops in the process industries, and is also the basis for many advanced control algorithms and strategies. In order for control loops to work properly, the PID loop must be properly tuned. Standard methods for tuning loops and criteria for judging the loop tuning have been used for many years, but should be reevaluated for use on modern digital control systems.
While the basic algorithm has been unchanged for many years and is used in all distributed control systems, the actual digital implementation of the algorithm has changed and differs from one system to another and from commercial equipment to academia.
We will discuss controller tuning methods and criteria. Also discussed will be the digital PID control algorithm, how it works, the various implementation methods and options, and how these affect the operation and tuning of the controller.
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December 2nd, 2006 at 11:40 am
Controller Operation
There are three common types of Temperature/process controllers: ON/OFF, PROPORTIONAL, and PID (PROPORTIONAL INTEGRAL DERIVATIVE).
On/Off CONTROL
An on-off controller is the simplest form of temperature control device. The output from the device is either on or off, with no middle state. An on/off controller will switch the output only when the temperature crosses the setpoint. For heating control, the output is on when the temperature is below the setpoint, and off above the setpoint.
Although capable of more complex control functions, the NEWPORT microprocessor based MICRO-INFINITY ® AUTOTUNE PID 1/16 DIN Controller can be operated as a simple On/Off Controller. The NEWPORT INFINITY ® series and INFINITY C ® series of highly accurate microprocessor based digital panel meters can all function as simple On/Off controllers.
With simple On/Off control, since the temperature crosses the setpoint to change the output state, the process temperature will be cycling continually, going from below setpoint to above, and back below. In cases where this cycling occurs rapidly, and to prevent damage to contactors and valves, an on-off differential, or “hysteresis,” is added to the controller operations. This differential requires that the temperature exceed setpoint by a certain amount before the output will turn off or on again. On-off differential prevents the output from “chattering” or fast, continual switching if the temperature cycling above and below setpoint occur very rapidly.
“On-Off” is the most commonly used form of control, and for most applications it is perfectly adequate. It’s used where a precise control is not necessary, in systems which cannot handle the energy being turned on and off frequently, and where the mass of the system is so great that temperatures change extremely slowly.
Backup alarms are typically controlled with “On-Off” relays. One special type of on-off control used for alarm is a limit controller. This controller uses a latching relay, which must be manually reset, and is used to shut down a process when a certain temperature is reached.
Proportional Control
Proportional control is designed to eliminate the cycling above and below the setpoints associated with On-Off control. A proportional controller decreases the average power being supplied to a heater for example, as the temperature approaches setpoint. This has the effect of slowing down the heater, so that it will not overshoot the setpoint, but will approach the setpoint and maintain a stable temperature.
This proportioning action can be accomplished by different methods. One method is with an analog control output such as a 4-20 mA output controlling a valve or motor for example. With this system, with a 4 mA signal from the controller, the valve would be fully closed, with 12 mA open halfway, and with 20 mA fully open.
Another method is “time proportioning” i.e. turning the output on and off for short intervals to vary the ratio of “on” time to “off” time to control the temperature or process.
With the analog output option, the NEWPORT INFINITY ® series and INFINITY C ® series of 1/8 DIN digital panel meters can function as proportional controllers. In addition, NEWPORT offers models of “INFINITY C” for thermocouple and RTD inputs featuring Time-Proportioning Control with its built in mechanical relays.
With proportional control, the proportioning action occurs within a “proportional band” around the setpoint temperature. Outside this band, the controller functions as an on-off unit, with the output either fully on (below the band) or fully off (above the band). However, within the band, the output is turned on and off in the ratio of the measurement difference from the setpoint. At the setpoint (the midpoint of the proportional band), the output on:off ratio is 1:1; that is, the on-time and off-time are equal. If the temperature is further from the setpoint, the on- and off-times vary in proportion to the temperature difference. If the temperature is below setpoint, the output will be on longer; if the temperature is too high, the output will be off longer.
The proportional band is usually expressed as a percent of full scale, or degrees. It may also be referred to as gain, which is the reciprocal of the band. Note, that in time proportioning control, full power is applied to the heater, but cycled on and off, so the average time is varied. In most units, the cycle time and/or proportional band are adjustable, so that the controller may be better matched to a particular process.
One of the advantages of proportional control is the simplicity of operation. However, the proportional controller will generally require the operator to manually “tune” the process, i.e. to make a small adjustment (manual reset) to bring the temperature to setpoint on initial startup, or if the process conditions change significantly.
Systems that are subject to wide temperature cycling need proportional control. Depending on the precision required, some processes may require full “PID” control.
PID (Proportional Integral Derivative)
Processes with long time lags and large maximum rate of rise (e.g., a heat exchanger), require wide proportional bands to eliminate oscillation. The wide band can result in large offsets with changes in the load. To eliminate these offsets, automatic reset (integral) can be used. Derivative (rate) action can be used on processes with long time delays, to speed recovery after a process disturbance.
The most sophisticated form of discrete control available today combines PROPORTIONAL with INTEGRAL and DERIVATIVE or PID .
The NEWPORT MICRO-INFINITY® is a full function “Autotune” (or self-tuning) PID controller which combines proportional control with two additional adjustments, which help the unit automatically compensate to changes in the system. These adjustments, integral and derivative, are expressed in time-based units; they are also referred to by their reciprocals, RESET and RATE, respectively.
The proportional, integral and derivative terms must be individually adjusted or “tuned” to a particular system.
It provides the most accurate and stable control of the three controller types, and is best used in systems which have a relatively small mass, those which react quickly to changes in energy added to the process. It is recommended in systems where the load changes often, and the controller is expected to compensate automatically due to frequent changes in setpoint, the amount of energy available, or the mass to be controlled.
The “autotune” or self-tuning function means that the MICRO-INFINITY will automatically calculate the proper proportional band, rate and reset values for precise control.
May 26th, 2007 at 4:56 pm
The new Sitrans TH line of head-mounted temperature transmitters from Siemens Automation and Drives ( A&D ) offers fieldbus interfaces now. The new Sitrans TH400 is compatible with either Profibus PA or Foundation Fieldbus. The Foundation Fieldbus version includes LAS ( Link Active Scheduler ) capability to perform the backup bus master function. It provides two analog input blocks and one PID ( Proportional-Integral-Differential ) function block. Thus the Sitrans TH400 can be used for simple closed-loop control tasks within a Foundation Fieldbus segment. Due to its compact size and design the temperature transmitter is suitable for small Form B connection heads and also direct mounting on top of the sensor unit. The new fieldbus transmitter supplements the already existing series of Sitrans TH head-mounted transmitters which include the universal Sitrans TH100, Sitrans TH200 and Sitrans TH300.
May 26th, 2007 at 4:57 pm
`Leave PID loops out of the PLC network`
PID (proportional integral derivative) loops should be handled by dedicated hardware devices rather than being built into PLC networks, argues West Instruments.
The company, part of the Danaher Corporation, has just launched a multi-loop PID controller designed to remove the burden of processing PID parameters from PLC and SCADA networks.
Traditionally, PLCs have had limited analogue-handling capabilities and have not been particularly user-friendly. They are especially complex to program for loop control, West contends.
Also, the performance of most PLCs suffers when they have to handle high-density analogue inputs; most will only support simple floating-point PID terms. Furthermore, variable scanning speeds can affect their PID performance and prioritisation requires complex programming.
“This is the PLC dilemma - when loop meets logic,” suggests Robert Jelski, managing director of West Instruments. “PLCs are the first choice for automation, but analogue loop control is not their strength, because logic deals with Boolean on/off states, whereas loop control deals with continuously variable analogue parameters.”
“PID loop control demands direct connection of sensors, continuous PID calculations and single loop integrity,” Jelski argues.
West Instruments` MLC 9000 controller allows up to eight DIN-rail-mounted PID loop modules to run off a single bus communications module. The modules can be “hot swapped” without powering down or interrupting the process. Up to 32 of the modules can be linked in a Modbus network.