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 ControlsThe 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.
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.
- 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.
- 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.
- 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.
- 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.
- 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.
- EIA485, Modbus Protocol.
- Precise, drift-free digital signals for sending and receiving remote setpoints and power demand to SCRs.
- 16 segment program.
- Multiple stored recipes.
- 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.