Process disasters, often put down to operator error, are just as likely to be attributable to complex documentation and a non-intuitive human-machine interface (HMI). Understand your controller and you can better understand your process.

Figure 1 shows the face of a typical mid range 1/4 DIN model with the controls and displays identified. An 1/8 DIN model could provide about the same range of features with smaller display but better space usage for multiple-loop panels such as extruders.

Continuing my discussion of how to purchase a temperature controller -- and the importance of understanding the specification before you buy -- in this issue I will look at how spending a little more can benefit your process.

Displays and Controls

The seven-bar digital LED display (figure 1) in red or green is the most common for process temperature (upper display) and set temperature (lower display). In the operation mode, touching the up/down buttons will change the main setpoint without the need to touch any other control. The lower display can be toggled between set temperature and something that represents controller output. When you have to depend on knowledge of the final output, make sure that it is a feedback signal truly representing the critical status, say heater current, valve position, fuel flow etc. A third button suitable for operator use could toggle between auto and manual access, and the fourth button could toggle between run and hold control for ramp and soak programs.

Figure 2 show the rear terminals of a typical mid-range controller. Some 36 spare terminals are show in this example; available for analog or digital inputs or outputs and interventions between process and controller.

In the configuration mode, the lower display is used to show the short name (mnemonic) of any one of the many control, alarm or configuration parameters that you might be adjusting. Their values will be shown in the upper display and the "page" button takes you through various headings. The "scroll" button takes you down a list of parameters under those headings. At this point, you can configure or adjust the parameters using the "up" and "down" buttons. Access to parameter changes is usually by password known by a trained setup technician.

This is just one example of a user interface. Apart from the up and down buttons, there is no consensus on the names, functions and keystroke sequences of the other controls or buttons. This applies between manufacturers and even between different models of controller from the same manufacturer (figure 2).

Additional features when you move up from entry level include the following.

Control Modes
  • Reverse or direct-acting.
  • Self and adaptive tune.
  • Dual heat/cool outputs.
  • Auto/manual with bumpless transfer.
  • Ramp/soak programming with stored programs.
  • Feed-forward control.

Process Inputs

  • Higher calibrated accuracy, stability and fast cold junction compensation.
  • More sensor types. User-defined nonlinear inputs.
  • 10 Hz sampling rate.
  • Three-wire RTD connection.

Other Analog Inputs

  • Input voltage or current for remote, setpoint, setpoint trim, external power limit, valve position feedback, heater current and load diagnostics.

Other Analog Outputs

  • Retransmission of setpoint, process temperature or deviation from setpoint.

Logic Inputs

  • Up to 11 available inputs can implement any 11 out of some 30 commands.

Control Outputs: One Heat, One Cool

  • Analog current or voltage.
  • Form C relay for heat, cool or valve position motor.
  • 10 A relay.
  • 2 A triac for heat, cool or valve position motor.

Alarm Modes

  • High; low; deviation high or low band; rate of change.
  • Blocking alarms -- used on low alarms, becoming active at startup only after the temperature has reached the safe state.
  • Latching -- must be reset manually or by a signal input when the temperature returns to the safe state.
Function Outputs
  • Relay, triac or logic type. Allocatable to alarms, manual mode, sensor break, out of range, load fail, load on when not commanded, self-tuning in progress, DC output open circuit, new alarm, end of program, specified segment of program running.
Digital Communications
  • EIA485, Modbus Protocol.
Controller-to-Controller Communications
  • Precise, drift-free digital signals for sending and receiving remote setpoints and power demand to SCRs.
Ramp-and-Soak Programming
  • 16 segment program.
  • Multiple stored recipes.
  Further yet upmarket, the following features are offered.
  • Multiloop controller/programmers for cascade, ratio and temperature/humidity.
  • Profibus and Modbus communications for integrating into supervisory systems.
  • Real-time clock.
  • Tool-kit blocks for internally wiring analog and digital functions together without taking up external terminals.
  • PC configuration software.
  • Dot matrix displays to enable scaleable and versatile alphanumeric and graphical displays to be shown.
  • Bargraphs showing, for example, load current, valve position, deviation from setpoint and controller output signals.
  • Zirconia oxygen probe inputs.
Understanding a controller and applying it to the process demands deep study of the manual and practice at the displays and controls. Process disasters, often put down to operator error, are just as likely to be attributable to complex documentation and a non-intuitive human-machine interface (HMI). As you approach challenging levels of versatility, you would be well advised to home in on your preferred supplier and find the person with the best combination of process and product knowledge.