It used to be easy to select a temperature controller. You looked at a few of the popular vendors and chose the one that offered the correct input/output specifications and had a good reputation at the best price. In the past few years, this process has changed from a strictly technical selection to one of both technology and economy. Increasingly, engineers are asked to make selections using a much clearer economic justification. One of the drivers effecting this change is that the ancillary costs of purchasing a temperature controller easily outstrip the price of the controller itself -- in some cases by 10 to one. In Arthur Holland's article in this publication, Trends in Temperature Control Equipmenthe states that in 1980, it cost 14 hours of a tradesman's pay to purchase a controller. In 2004, it cost three hours pay. This article outlines several aspects that should be considered when selecting a temperature controller.
Get PhysicalThe first thing to consider is the size and mounting style. A study by Venture Development Corp., Natick, Mass., shows that 97 percent of temperature controllers purchased today are one of the panel-mount types in a DIN cutout size (1/32, 1/16, 1/8 and 1/4). The most popular size is 1/16 DIN -- small enough to be easily mounted yet large enough to have a usable display. Additionally, 1/16 DIN controllers are less expensive than the larger 1/8 and 1/4 DIN models. Microelectronics allow manufacturers to pack a lot of features into these compact packages. Figure 1 shows several panel-mount DIN cutout controllers.
As an alternative to panel-mount controllers, consider DIN rail (behind-the-panel) mounting models. DIN-rail-mounted controllers save valuable panel space while eliminating the need to cut a square hole in the panel and run wires to the front panel. The drawback to DIN-rail mounting is lack of a display or, if one is available, inaccessibility. For The first thing to consider is the size and mounting style. A studybyVenture Development Corp., Natick, Mass., shows that 97 percent of temperature controllers purchased today are one of the panel-mount types in a DIN cutout size (1/32, 1/16, 1/8 and 1/4). The most popular size is 1/16 DIN -- small enough to be easily mounted yet large enough to have a usable display. Additionally, 1/16 DIN controllers are less expensive than the larger 1/8 and 1/4 DIN models. Microelectronics allow manufacturers to pack a lot of features into these compact packages. Figure 1 shows several panel-mount DIN cutout controllers.
As an alternative to panel-mount controllers, consider DIN rail (behind-the-panel) mounting models. DIN-rail-mounted controllers save valuable panel space while eliminating the need to cut a square hole in the panel and run wires to the front panel. The drawback to DIN-rail mounting is lack of a display or, if one is available, inaccessibility. For DIN-rail-mounted controllers, interface choices are:
- Remote display.
- Display on the controller face.
- Communications interface to a human machine interface (HMI) or to a programmable controller with an HMI.
Having a display on the front of a DIN-rail controller is suited for applications that require infrequent interaction with the controller. It can offer a benefit if there have been problems caused by tinkering with the controllers. Figure 2 shows several types of DIN-rail-mount controllers.
Get ControlFor many applications, all that is needed is a basic PID controller with self-tuning. Self-tuning is no longer a product differentiator because virtually all models now have this feature. However, it may be worthwhile to look for some of the advanced functions offered such as cascade control, feedforward, override, ratio control, logic functions, and timers. If your application can take advantage of these features, they could save you time and money. These features may or may not add to the cost but can provide a better control system at lower overall cost by eliminating other equipment or simplifying operation.
For example, some time ago a manufacturer of ceramic capacitors was looking for a control system for his curing ovens. He was curing large batches of capacitors in multiple ovens but controlling the temperature in each oven with a single temperature controller. The oven mechanics required that he control the air temperature near the electric heater. If the sensor was moved very far from the heater, there was a risk of overheating the element. This configuration caused the top layer of capacitors to be ruined by scorching. He not only needed to scrap the top layer of capacitors, he also had to spend time inspecting much of the load for heat-related defects. This was the case for every run. His temperature controller was costing him wasted time and wasted product.
After assessing the application, the controls supplier recommended he install a cascade controller, which he did. The cascade controller cost nearly double his simple PID controller, but the cascade function performed exactly as he wanted without ruining any product. In this case, the best overall economic solution was the more expensive controller.
Get MoreSupplemental functions are things not related to the control loop that can add value to the process -- embedded timers, counters, limit alarms, logic functions, indication of noncontrolled parameters, etc. If the application calls for a simple temperature controller but also calls for an ammeter and a timer, there can be a substantial benefit if these are combined into the temperature controller, saving the cost of purchasing and installing the ammeter and timer. This approach also saves control panel real estate, which becomes more important as panels get smaller.
Programmable controllers are becoming less expensive, but if you are able to perform their function in your temperature controller, you will have a highly integrated system at a lower cost.
Display.Your panel-mount temperature controller with its HMI display is most likely mounted in a prominent place on your panel. Why not look for ways to use it? In the previous example in which the application called for a temperature controller, timer and ammeter for electric heating, the overall cost could be reduced by combining those functions into one instrument. To do this, you would need to select a controller that offers an indication of load current on its display as well as having a timer function. Figure 3 shows a controller that displays electric heater current by mimicking an analog ammeter on the front of the instrument.
Another use of the controller display is signaling or “messaging” the operator. All controllers have alarm indicator beacons, but what about a simple message to the operator? Some controllers have scrolling text messages that can be triggered by an alarm or digital input and used to send a message to the operator, such as the cycle is complete. A function like this can create significant cost savings over using another device to achieve the same result.
Digital Communications. Many applications call for a controller to be integrated with a programmable controller or SCADA (supervisory control and data acquisition) system via digital communications. The main point to remember about communications is that it is not always possible to retrofit the feature. For many lower cost controllers, the function must be fitted when the product is purchased. In higher-end controllers, it can sometimes be added later. If you think you will need communications in the future, it might be best to include it in your original purchase. There are many communications protocols from which to choose; the most popular are DeviceNet, Profibus, Ethernet and Modbus.
Configuration.Configuration is one of the most overlooked costs when making a controller purchasing decision. Today's controllers are considered to be “universal,” which means they can be configured for many different applications. The downside of this is that they need to be configured. Be sure you have a clear idea of what effort is required to configure a controller before making your selection.
Maintenance.Most engineers remember to consider maintenance costs when selecting control products. The points considered usually are ease of replacement, ease of troubleshooting and diagnostics. What is often overlooked is an evaluation of stocking spares.
Management says they do not want to pay to inventory spare parts, but this dictate must be balanced against the cost of the machine or process going down. The cost of downtime may offset the small insurance cost of stocking spare units. A low spare inventory cost can be engineered into the purchase by selecting a flexible controller that can be configured to replace most -- or all -- the various controllers in service. This means a single spare can provide security for the whole process or machine.
Engineers and technicians are continually pressured to lower the cost of the products they specify. Another way to achieve a lower cost is to exploit the extra functionality in a temperature controller. Looking beyond the purchase price of a controller should prove enlightening and fiscally healthy.
Sidebar: Single-Loop or Multiloop?If the application calls for more than one temperature controller and they are in close proximity, a multiloop controller may be a better choice. The single- or multiloop decision is so application dependent that it is impossible to address in general terms.
One scenario in which multiloop wins out is one in which the application calls for loops to interact. For example, when controlling an environmental test chamber, it is often a requirement to ramp both temperature and humidity in synchronization with one another on a single time base. This tight interaction is easier to achieve using one multiloop controller than two single-loop controllers. The figure addresses some of the application requirements to consider when choosing between single- and multiloop controllers.