Selecting the proper temperature controller for an application can be a time-consuming and confusing task. There are many different manufacturers, each offering several different types of controllers. Following five simple steps will allow you to choose the controller that best fits your application.
1. Select the Desired Controller SizeControllers are available in different shapes and sizes. Size selection depends largely on user preference, as technology has been able to reduce the size of circuit components so that the new, smaller controllers pack more features than older controllers four times the size. The industry has standardized on four main sizes for the U.S. market; each has a standard panel cut-out that is universal across manufacturers. These sizes range from 1/32 DIN (48 x 24 mm) to 1/4 DIN (96 x 96 mm). The most popular size controller over the past decade has been the 1/16 DIN (48 x 48 mm) as it offers a larger display than the 1/32 DIN but takes up less panel space than a 1/4 DIN. Recently, for applications that have a central user interface for a panel, controllers that can mount on a DIN rail have been introduced. Some of these controllers do not have a display.
To choose the right size, consider how far the operator will be from the panel and how important it is for him or her to be able to see the current temperature from that location. Another consideration should be how much panel space is available for the controller, especially if more than one controller will be mounted in the panel. Keep in mind that smaller controllers also have smaller buttons; if the operator will be wearing gloves, the size of the controller may need to be larger.
2. Determine What Inputs Will Feed the ControllerMost controllers come with field-selectable inputs, but there are still some on the market that have to be specified when ordering. These lower-end controllers must be selected for thermocouples, RTDs or process inputs at the time of ordering. However, the majority of controllers now offer universal inputs. Universal inputs are selected via the programming and by which terminals the input is wired to. It is important that the controller be wired to the proper terminals for the input selected in the programming or the controllers will display an error message.
Be sure to check the manufacturer's specifications to see if the controller supports the preferred input type. When using a process input, check to see if external resistors are required for current inputs. For RTD inputs, users will want to know if the controller accepts two- or three-wire RTDs. It may be necessary to jumper one leg of the RTD to the extra terminal on the controller to avoid an error message.
3. Decide Which Control Operation Is RequiredTemperature controllers are normally offered with four main control operations (PID, on-off, ramp-and-soak, and manual). For many applications, simple on-off control is sufficient. The controller latches the relay either on a temperature fall or rise once the process temperature reaches the setpoint value. Whether the controller switches on a rise or fall is based on whether the controller is programmed for heating or cooling. For testing a system, or when an operator is watching the process continuously, manual control is available to allow the operator to adjust the output of the controller directly. In order to more tightly control a process, PID (which stands for proportional- integral-derivative) control can be used with user-supplied values or values that are learned by the controller’s observation of the process. To provide even better control, higher-end controllers offer fuzzy logic, which gives the controller a higher level of precision. Lastly, ramp-and-soak control modes are helpful for applications that require different temperatures for set amounts of time.
The decision of which control operation is required is based on tolerance for temperature overshoot and undershoot in the process. Temperature controllers that offer features such as fuzzy logic and ramp-and-soak modes usually are more expensive and have more parameters to program. It is best to go with the simplest control operation that will meet the needs of your application.
4. Ensuring You Have Enough OutputsControllers typically have up to two process control outputs and may have additional alarm outputs. When having two process outputs, one is programmed to provide heating control and the other for cooling control. The two outputs work together to help a process quickly reach and maintain the desired setpoint. For building-automation systems that have both a chilled water loop and a hot water loop, this type of control is ideal. If the application requires only heat or cooling control, then a lower cost controller with one process output will suffice. The control outputs can be relays, solid-state relays, pulsed voltage, linear voltage or linear current. For applications that cycle often, it usually is best to get a switched-voltage or solid-state relay output to extend controller life. As the relay is usually the first part of the controller to fail, plug-and-play outputs are becoming popular.
Alarm outputs are used to monitor the temperature and whether it is exceeding its acceptable limits. The alarms can be either deviation or absolute. Deviation alarms measure how far the temperature is from the setpoint; absolute alarms notify users when a specific temperature threshold (high or low) has been breached. The number of alarm outputs that are needed depends on whether the same function is acceptable for both high and low alarms. For example, if there is a single light or horn that sounds when the temperature breaches the high or low alarm setting, then the alarm can share that contact and be set as a high/low alarm.
Evaluate the application to decide how many control outputs and alarms outputs are required. If using a 1/32 DIN controller, users should verify with the manufacturer if the alarm output is separate from the second control output. Very few suppliers offer additional alarm values on this size control. Investigate the alarm operations that are offered by a controller. Some manufacturers have the flexibility of a hysteresis alarm, which would allow the alarm to be used as a second or third control output. Consider whether the alarm outputs are standard and whether the control only has a light or internal buzzer for the alarm. Lastly, alarm outputs typically have lower electrical ratings, so it is important to ensure that the ratings meet the application requirements.
5. Getting Through the ProgrammingOf all of the steps, this is the most crucial. Because the order and even the names of the parameters vary among controller manufacturers, many operators prefer to specify a single brand. The base set of parameters can be found in every controller, and once the operator gains experience with a new controller, he will find that the programming of that controller becomes easier. In the early stages of owning a temperature controller, the manufacturer’s technical support staff is an important resource that can help quickly steer the operator to the proper menu in the controller and answer questions about parameter operation.
Programming tools can be useful for larger equipment manufacturers that do not want to use valuable labor hours programming controllers. Typically, the best method is to have controllers equipped with a RS485 communications option, so a set of saved parameter settings can be downloaded into the control in a matter of seconds. Some manufacturers provide free software while others provide a register list and require the user to develop the software interface.
Following these five steps to evaluate your process and understand your control needs before selecting a controller will help ease the job of specifying or purchasing of new single-loop temperature controllers. Getting the proper size reduces cost by maximizing panel space while not compromising the operator or increasing the risk of error. Proper wiring and setting of the inputs eliminates costly troubleshooting. Choosing the most efficient method of control operation reduces controller cost and the time for setting up unneeded parameters. Having enough control and alarm outputs eliminates the need for extra components or controllers. Lastly, adjusting to a new set of parameters can be time-consuming initially, but good technical support and experience will compensate for any difficulties that might be experienced.