Industrial Temperature Primer

Chapter 3.

Single Loop Temperature Controllers

A single loop temperature controller is an instrument that takes the signal from a sensor, compares it to a setpoint signal, and adjusts the output to the heating device to maintain, as close as possible, equilibrium between the measured temperature and the setpoint temperature. The key phrase here is "as close as possible". There are several types of control methods used to accomplish this. We will attempt to briefly explain the most common.

On-Off Control

Selection of the right temperature controller for the application depends on the degree of control required by the application. The simplest of applications may only require what is called "On-Off" control. On-Off control operates much in the same manner as the thermostat on our home heating systems. In other words, the output of the controller is either 100% on or 100% off. The sensitivity of the On-Off control (sometimes called "hysteresis" or "dead-band") is designed into the control action between the points at which the control output switches from "off" to "on". This designed in hysteresis prevents the output from switching from off to on too rapidly. If the hysteresis is set too "narrow", rapid switching will occur and often result in what is known as output "chattering". This "chattering" can result in poor lifetime of output relays and heating components. Therefore, the hysteresis should be set so that there is sufficient time delay between the "on" and "off" modes of the outputs. Due to the hysteresis needed in the output of the on-off controller, there will always be a certain "undershoot" and "overshoot" in the control action. The amount of under shoot and overshoot is dependent upon the characteristics of the entire thermal system of a particular application. (Figure 13A..)

Time Proportioning

Processes requiring a little more precise control than On-Off control usually require what is called Time Proportioning. A time proportioning control operates much the same way as an on-off control while the process temperature is outside of what is called the proportional band. The proportional band is that area around the setpoint in which time proportioning control takes places. When the process temperature enters the proportional band (approaching setpoint) the cycle time between time on and time off begins to vary. At the low end of the proportional band, the on time is much greater than the off time. As the process gets closer to the setpoint, the on time decreases and the off time increases. This changes the effective power to the heating load and causes a "throttling back" in the speed at which the temperature of the process is increased. This action continues until a stabilization takes place somewhere below the setpoint. At this point, control is achieved. The difference in the control point and the actual setpoint is called "droop". (Figure 13B.)

As long as there is no change in the process load, this condition will remain constant.

Integral or "Reset Action"

If the "droop" in the time proportioning form of control cannot be tolerated in the process, the Integral function of control must be added. The integral function found in "automatic reset" controllers uses a mathematical algorithm to calculate the amount of droop and then adjusts the output to "reset" the control result to setpoint. This is usually done by automatically shifting the proportion band slightly to compensate for the droop.

Automatic reset action can only take place within the proportional band. Should automatic reset be applied outside the proportional band, the result would be a condition of extreme overshoot of the setpoint. The process of eliminating the automatic reset outside of the proportional band is called "anti-reset windup" and is typically a standard feature of controls that include the automatic reset or "integrating" function. On many controls that do not offer "automatic" reset. This function is accomplished manually by a potentiometer adjustment that manually shifts the proportional band. (Figure 14A and 14B)

Derivative (Automatic Rate)

Temperature overshoot is when the process, during its cycling, exceeds setpoint. Overshoot can be small and insignificant or large enough to cause major problems with the process. In all the types of control discussed so far, overshoot occurs. Overshoot can be damaging in many processes and therefore must be avoided.

The derivative function (also called "automatic rate") can be used in control systems to prevent overshoot. The derivative function anticipates how quickly the setpoint will be reached. It does this by measuring the rate of change of process temperature and then by forcing the control into a proportioning action at a faster rate thereby slowing down the rate of process temperature change. This action allows the process temperature to "glide" into the setpoint and thereby prevent a large degree of overshoot on start-up and when system changes such as large load changes or the opening of a furnace door, etc. takes place.

Typically, the most precise of process control applications will require a control that has proportional, automatic reset, and automatic rate functions. This type of control is know as PID (Proportional, Integral, Derivative). (Figure 14C)

Control System Tuning

On-Off Control:

Tuning an On-Off control system is usually accomplished by one simple manual adjustment. This adjustment basically controls the switching hysteresis by adjusting the points at which the control turns on and turns off.

Proportional (P), Proportional plus Integral (PI), and Proportional plus Integral plus Derivative (PID).

There are several methods for the proper tuning of P, PI, and PID controls. Most methods require a considerable amount of trial and error as well as a technician endowed with a lot of patience! The following is one of those methods.

The first step is the tuning of the proportional band. If the controller contains Integral and Derivative adjustments, tune them to zero before adjusting the proportional band. The proportional band adjustment selects the response speed (sometimes called gain) a proportional controller requires to achieve stability in the system. The proportional band must be wider in degrees than the normal oscillations of the system but not too wide so as to dampen the system response. Start out with the narrowest setting for the proportional band. If there are oscillations, slowly increase the proportional band in small increments allowing the system to settle out for a few minutes after each step adjustment until the point at which the offset droop begins to increase. At this point the process variable should be in a state of equilibrium at some point under the setpoint.

The next step is to tune the Integral or reset action. If the controller has a manual reset adjustment, simply adjust the reset until the process droop is eliminated. The problem with manual reset adjustments is that once the setpoint is changed to a value other than the original, the droop will probably return and the reset will once again need to be adjusted.

If the control has automatic reset , the reset adjustment adjusts the auto reset time constant (repeats per minute). The initial setting should be at the lowest number of repeats per minutes to allow for equilibrium in the system. In other words, adjust the auto reset in small steps, allowing the system to settle after each step, until minor oscillations begin to occur. Then back off on the adjustment to the point at where the oscillations stop and the equilibrium is reestablished. The system will then automatically adjust for offset errors (droop).

The last control parameter to adjust is the Rate or Derivative function. Always adjust this function last. Always! The reason I am so emphatic on this point is that if the rate adjustment is turned on before the reset adjustment is made, the reset will be pulled out of adjustment when the rate adjustment is turned on. Then you just have to start your tuning procedure over!

The function of the rate adjustment is to reduce as much as possible any overshoot. The rate adjustment is a time based adjustment measured in minutes which is tuned to work with the overall system response time. The initial rate adjustment should be the minimum number of minutes possible. Increase the adjustment in very small increments. After each adjustment let it settle out a few minutes. Then increase the setpoint a moderate amount. Watch the control action as the setpoint is reached. If an overshoot occurs, increase the rate adjustment another small amount and repeat the procedure until the overshoot is eliminated. Sometimes the system will become "sluggish" and never reach setpoint at all. If this occurs, decrease the rate adjustment until the process reaches setpoint. There may still be a slight overshoot but this is a trade-off situation.

Autotune -

Tuning control parameters is no fun! Thank goodness for modern technology and the invention of "Autotune". Most of today's controller manufacturers offer single loop temperature controllers with the option of automatic parameter tuning which eliminates a lot of the drudgery of manual tuning. There are several methods of autotuning. Most operate on a system whereby the controller "looks" at the initial start-up cycle from start to the time the process reaches setpoint. Then by learning from the response characteristics of the first cycle it adjusts itself to optimum tuning parameters based on the history created in the first cycles. The auto-tune function continues to "learn" from subsequent cycles and readjusts parameters until the optimum settings for PID are reached. Since not all manufacturer's auto-tune controllers function the same, it is advisable to consult the instruction manual before attempting to use the auto-tune feature for the first time.

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