A valuable contribution to this industry is putting together a tabular presentation of controllers and their main features from all the first-rank manufacturers. You are able to take in a great variety of information at one glance. Here begins your quest for the simplest product that meets your application. It’s not quite so satisfying as looking and handling, but the many features listed give you a good look in-depth at the specifications and features that matter to you.
Shape and Size. Of the formats in common use, the largest size is 96 x 96 mm -- called 1/4 DIN. Three chops yield three successively smaller sizes.
DIN means Deutsches Institut für Normung (German Institute for Standardisation). DIN has pioneered many industrial standards -- and those for rectangular panel instruments have gained worldwide adoption, aiding interchangeability and reducing panel-fabrication costs. Don’t expect plug-in interchangeability between manufacturers.
Take account of the front-to-back depth of enclosure needed. This could be a significant cost issue.
Another format comes as DIN-rail-mounted modules. They often are inside an enclosure, or they could be distributed round the plant, usually in multiloop applications. Having no display or buttons, the human-machine interface (HMI) for all loops is normally done at an operator panel or computer; reducing panel space and fabrication costs even more.
More Functional and Performance Features. In addition to the usual features listed in surveys, a deeper search into manufacturers’ information will reveal features and techniques such as:
- Overshoot inhibition.
- Access passwords.
- Feedforward control.
- Adjustable input offset.
- User-configurable linearization of inputs.
- Input filter to deal with noisy signals.
- Setpoint rate limit to avoid temperature shock.
- Indicator lights to denote state of outputs, alarms, autotune in progress, second/remote setpoint, etc.
- Sample rate (A/D conversions/sec). Control response speed depends on this.
- Calibration accuracy. Don’t pay for more than your process needs.
- Common mode rejection. Look at this item if your sensor could be at some potential above ground.
- Load diagnostics such as alarms for partial heater failure, loss of heater supply, heater power or final control element not obeying the control signal.
- Analog command inputs such as for remote setpoint, external command of power limit and feedback to denote valve position.
- Logic inputs for external control commands and interventions.
- Logic outputs. Possible types include contact, triac or DC logic type, allocable to alarms; manual mode; sensor break; input out-of-range; load fail; tuning in progress; DC output open circuit; new alarm and program segment.
- Digital communications protocols.
- Multiloop controller/programmers for cascade, ratio and temperature/humidity.
Surveys are but an entry door to making informed choices. Keep the above list in mind and extend your reading to manufacturers' catalogs, operation manuals and web sites.
Terminology. Unfamiliar terms can stop your studies in mid-sentence. Here are two sites showing useful glossaries of process control terms.
To refresh your memory on control usage, terms and techniques, you might look back into the archived “Heating Highlights” columns at www.process-heating.com.
Informed technical reps and sales engineers who want your business can be an excellent source of information. You can benefit from all the troubles they've seen. Helplines are useful but more often than not, they become overloaded and inaccessible.
Finding the best buy is one thing. Putting the product to use has now become a challenge. You study the manual, do some wiring and poke a lot of buttons. The ease of installation, matching product to process, and finding help when you need it can be much bigger factors than the price of the product. By this time -- preferably before now -- you are able to judge your supplier. You are on trial too. Your part in this is diligent study and practice of your trade. You too, an equal partner, have to be a well-read, well-trained part of the process. PH