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A Study On PID Temperature Control For ENGR 315, Control Systems
T. VanDerPuy, Non-IEEE Member
Abstract—This paper focuses on an in depth study of industrial temperature controllers, their use in a workplace environment, some terms and options associated with them, and a physical testing thermal system. The testing was done in a controlled environment using a Watlow Series 935A temperature controller a 7.5-watt heater, and a half-gallon fish tank. Some other topics included are inputs and outputs of temperature controllers, PID, and its function associated with temperature controllers, and some other functions available for use with industrial temperature controllers.
Index Terms—Proportional Control, Temperature Control, Temperature Measurement, Thermoresistivity, Thermal Variables
I. Introduction
T
emperature control in industrial applications is an old science, taking off mostly during the industrial revolution, and coming into its own in the United States early in the Twentieth Century. This control was very simple, mechanical control that did not go beyond turning a heater or cooling device on or off. PID control, however is a fairly new concept that was immediately accepted into use for temperature control applications, and gave way to an entire line of PID temperature controllers, including the entirely digital units seen at work in most applications today.
II. PID Control, and its use with temperature
PID control stands for, and consists of three distinct feedback and control areas. A block diagram of the control system can be found in the appendix.
A. Proportional
The first of these areas is proportional. The output of the proportional controller is relative to the difference between the temperature that is present and the set point. An adjustable proportional band is set up as either a range of temperatures, or a percentage of the set point temperature, and is located below the set point. The Proportional band is good for reducing the rise time of a process, and reduces, but never erases the steady-state error.
B. Integral
The second system in a PID controller is the integral control. The integral control eliminates the steady-error, but makes the transient response worse. The integral eliminates the “droop” caused by the proportional band. Since the power level at set point is zero, and near zero right before it, the temperature settles at a point slightly below the set point using just proportional control, this results in a “droop” down from the set point. The integral part of PID control eliminates this. The controller output is proportional to the amount of time the error is present.
C. Derivative
The third system working in a PID controller is the derivative control. The derivative control affects the system by increasing stability, and by reducing the overshoot and undershoot of the function, and improving transient response. The output under derivative control is proportional to the rate of change of the error over time. This part of the control system is critical because in some processes, an overshoot in temperature might cause a part or machine to be damaged.
III. TYPES OF TEMPERATURE CONTROLLERS
Temperature controllers are usually characterized by the type of control they provide, and therefore, the type of outputs that are available on each unit.
A. On/Off
Temperature controller’s outputs generally come in one of three varieties. The first of these is called an ON/OFF output.
figure 1
Time vs. Temperature Graph of ON/OFF Control
This was the first type to arrive into production, and is the simplest of the three. An ON/OFF output controller simply turns the power on or off to a heater or cooling device depending on which side of the setpoint that the temperature is on. The controllers that use this sort of output are very economical, however they have many problems, depending on the system you are controlling with it. The main problem with this is that the ON/OFF control does not provide a steady temperature, and can be off on either side of the setpoint by a certain magnitude depending on the heater or cooling device that is attached.
One other problem is that this output is usually attached to a relay or other device to transmit the signal to power the heater or cooler. A mechanical relay, or even a transistor-based device only has a certain amount of switches until it needs to be replaced. If the temperature gets around the setpoint, and stays around there, the device will constantly be switching back and forth since it is so close to the setpoint. This puts a lot of stress on the switching device, and shortens its lifespan enormously.
B. Time Proportioning
The second variety is Time Proportioning Output.
figure 2
Time vs. Temperature Graph of Time Proportioning Control
The output on this controller is switched on at intervals, which depend on where the temperature is in the proportional band; it is on more when it just enters the proportional band, and off more as it reaches the setpoint. Figures 3 and 4 show how the device reacts at different required power loads.
figure 3
Time vs. Power Graph of Time Proportioning Control At 50% Power
figure 4
Time vs. Power Graph of Time Proportioning Control At 75% Power
The benefit of this kind of controller is that the overshoot is greatly reduced due to the slowed down operation approaching the set point. A proportional or full PID controller must be used for this output.
C. Process Control Output
The final type of output used in controllers today is the Process Control output. This output gives a steady signal instead of an on or off command, as shown in figure 5.
figure 5
Proportional Control Output (0-5V)
Proportional Output control is used to turn a heater or cooling device on, but only with partial power depending on how much it determines the heater needs to be actuated. This output is used exclusively on full PID controllers. The output is an analog signal, usually a voltage from 0-5 VDC or a current from 4-20 ma.
As shown in the diagram above, the controller is telling the heater to give 25% power. This voltage or current is converted into a percentage of power needed to send to a heater or cooling device, usually by an analog PLC input. Another way that this output reaches the device to be controlled is by an SSR or SCR control device that rapidly switches the power on and off to the heater, some switch many times a second. The benefits of this output are a completely smooth operation without any of the on/off jumps the other outputs have. This helps the process level out more evenly, reducing overshoot. It also extends heater life because it is getting a more even flow of power than with the other two devices.
IV. INPUTS
Another area of options available in choosing the correct PID controller are inputs.
A. Contact Sensors
Contact Sensors are sensors that are actually touching or immersed in the material whose temperature they are measuring. This means that their temperature range is limited because of the degradation of the material that can occur due to high temperatures. The two most distinct types of contact sensors commercially available are thermocouples and RTDs.
1. Thermocouples
The thermocouple is the most prevalent, consisting of a two-strand wire of dissimilar metals. Some examples of these types of wires are the K type thermocouples, whose wires are nickel and nickel chromium. All metals change their electrical EMF potential as temperature increases or decreases. In an environment where these two wires are exposed to the same temperature, they experience different changes in electrical potential because they are made of dissimilar elements.
When joined together, the difference in electrical potential between these two wires can be measured by the temperature controller, and this analog signal in turn can be turned into a digital signal that is sent to the controllers brain to be converted into a number (temperature) based on what type of thermocouple is connected. The controller must be told which type of thermocouple is connected in order to make this computation.
There are many different kinds of thermocouples for use in different environments, so it is key that the temperature controller is computing results for the right thermocouple. Usually a controller can accept a number of different types, and some can even tell what type is being used as soon as it is plugged in.
2. Resistance Temperature Detectors
Resistance Temperature Detectors, or RTDs as they are known detect temperature with a single element wire, instead of the two element wire of the thermocouple. They consist of a length of fine wire wrapped around a glass or ceramic core. The temperature controller reads the resistance of the wire, and correlates it to a temperature listed for that resistance that is programmed into the controller.
3. Comparing RTDs and Thermocouples
RTDs are generally better performing than thermocouples in almost every area, except for a few. The first is that they are not as robust as thermocouples, and so usually come in a sheath or jacket to protect them from the environments they will be introduced to. Even so, there are many environments that they cannot go in, especially ones where they will be jolted or shocked by the machine operation. They are also slightly more expensive, but not so much to justify buying them over a thermocouple because of their numerous advantages.
The first of these advantages is that they are more accurate than thermocouples, and more repeatable as well. A thermocouple can get worn out more easily because it is usually not jacketed as well as an RTD. Another advantage that RTDs have is that they have a higher immunity to electrical noise from equipment. This means that they can be placed next to generators, transformers, or motors in a workplace environment, and give a more accurate temperature than a thermocouple.
One real advantage that thermocouples have is that they have a good point reading. What this means is that the analog signal that is sent to the temperature controller is from a single point, where an RTD’s resistance is read along the entire coil. The advantage here is that the thermocouple can be placed in a small area, or placed so that it reads a temperature on a minute area of a part or machine.
Another advantage that thermocouples hold over RTDs is that they can be used over a broader temperature range than RTDs. RTDs are more sensitive to extreme temperatures as well, and have a more limited range of operation. A full comparison of temperature controllers versus RTDs can be found in the appendix.
B. Non-Contact Sensors
Most commercially available non-contact sensors are devices that detect radiation heat energy from a target that the sensor is pointed towards. These are used in rugged environments where it is physically impossible or impractical due to machine movement or temperature to place a thermocouple or RTD. These sensors are usually connected to more complicated temperature computers that are not in the scope of this paper.
V. TUNING A TEMPERATURE CONTROLLER
Tuning the PID system on a temperature controller was not an easy task by any means in the earlier years of PID temperature control. It involved setting up the system, configuring it to how you best thought it might need, and guessing what parameters the PID system should use for the operation. This was more than tedious for a worker to do, and sometimes took days on more complicated temperature systems.
This was because a temperature system is one of the more complicated systems to model for PID control. There are numerous variables that are nearly impossible to simulate versus how they behave in the real world. For this reason, manufacturers of temperature controllers soon started producing units that auto-tuned. This was an incredible time and labor saving creation because all it took to tune the new controllers was to set it up in the environment and let it run and decide the right PID variables for the process by itself. The way it does this is shown in figure 6 below.
figure 6
Temperature vs. Time Auto tuning
The temperature controller starts out by putting the heater or cooling device on full power until it reaches 90% of the set point. It does this to determine how fast the heating or cooling device works so that it does not overshoot the set point. As soon as it reaches 90%, it begins to back off the power proportionally to what it has learned about the heating or cooling device, and watches how fast the temperature drops when it shuts off the device. This is important for it to decide when and how much power to cut when the process gets near the set point.
After this the auto tuning is complete, and the temperature controller decides on reasonable values for the PID. This is usually not the end of the process, however, as an operator still has to come in and fine-tune the PID values to make sure the process is operating at an optimal level. Other situations, however, are less demanding, and require only the auto tuning of the controller for safe and optimal operation.
Most of the time the only problem with the temperature controllers auto tuning settings is a slight overshoot in the final temperature. This can be adjusted down by simply changing the value of the integral in the PID control.
Most contemporary PID controllers come with an easy to use interface that can be learned in a couple of hours. The earlier models were not so easy to use, and tuning a temperature controller usually involved bringing in a representative from the company for a day to teach a few maintenance and engineering workers how to set and adjust the controllers.
VI. TYPES OF TEMERATURE CONTROLLERS
Temperature controllers come equipped with a number of different options. Deciding on a temperature controller has a lot to do with the inputs and outputs, but there are also other features that they can utilize that are not necessary for all operations.
Temperature controllers can be used in either a stand-alone operation, or can be run with a programmable logic controller or PLC. The more complicated operations usually have the temperature controllers hooked up to a main communications bus that can be monitored from any part of the installation.