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Reviewing the features and specifications of commonly used discrete panel-mounted controllers.
The most common sizes by bezel format are, 96x96mm (called quarter DIN), 96x48mm (one eighth DIN) 48x48mm (one sixteenth DIN) and 48x24mm (one thirty second DIN).
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.
While DIN is useful shorthand one DIN (192x192mm) can be sliced many different ways in increments of 12mm and non-square models can be oriented landscape or portrait. I prefer to specify width and height.
Panel Cut-outs conform to the body width and height dimension (90, 45, 22.5mm etc) plus a small clearance for insertion. The front to back depth dimension depends on the packing density of the internal electronics. Consider depth when you want the most space-saving and cost effective enclosure.
|Size||1/32 DIN (W48mm H24mm)|
|Control Modes||On/Off or PID with auto-tune and overshoot inhibition|
|Outputs||Relay 250V 2A for heat, cool or alarm.|
Logic 9V 18mA dc for heat or cool.
|Cycle-times||0.2 – 100 sec Time-proportioning|
|Thermocouple Inputs||J K T L N Platinel II R S B C|
|RTD Inputs||DIN Pt 100 2-wire|
|Linear mV inputs||-12 to 80mV|
|Linear mA inputs||4 – 20mA|
|Display Ranges||-999 to 9999: up to 2 decimal places for linear inputs.|
º C or º F: Full useable range of temperature sensors
|Input Offset||User adjustable over the whole input range|
|Input Filter||1.0 to 99.9sec|
|Power supply||85 – 264V ac 2.5W 48 – 62 Hz|
|Set point range||High and low limits adjustable within full range of input.|
|Set point rate limit||0.01 to 99.99 º/ min or units/min|
|Display||Single 4-digit green LED 10mm high|
|Indicators||Output 1 and Output 2 Legends. Flashing display indicates alarm active|
|Sample rate||5Hz (5 A/D conversions /sec)|
|Calibration Accuracy||0.25% of reading ± 1LSD or ± 1º F or C|
|Cold Junction Compensation||> 15:1 rejection of ambient temperature change|
|Common Mode Rejection||140dB (107) Sensor tolerates 250V elevation to ground|
|Process Alarms||High, Low, Deviation or Deviation Band|
|Load Diagnostics||Alarms for heater circuit open, loss of heater supply and short circuitof solid state contactor (over temperature hazard)|
|Panel Sealing||IP65 NEMA 4X|
Fig 2 shows the wiring of a simple entry level 48x24mm controller.
Its sensor, (thermocouple or RTD) is shown connected to the input terminals. Line voltage can be any value from 85 to 264V. The controller’s switching power supply self-adjusts to accommodate this range. This design avoids the winding burnouts often suffered by misconnections on dual tapped transformer power supplies.
Fig 2. Rear Terminal Connections
The logic output is shown triggering a solid-state contactor with fast-cycling dc pulses. The contactor has internal diagnostics for load malfunction. A fault condition can be transmitted back on the logic wires to operate the relay provided that it has not been configured for cooling or some other alarm duty.
It is common practice to wire the relay for shutdown or audible or visible alarm.
With a 2-wire RTD connection the temperature will read high by about 1 º C per 0.4 ohm of cable resistance. You can trim this out by using the offset adjustment. This is just one of the many uses of the offset feature.
The internal input filter cuts down noise on input signals. Its effect is equivalent to a single RC low pass filter with the RC product expressed in seconds.
On entry-level instruments it is usual to have only one display and bump the process temperature temporarily in order to display the parameters one by one as you are setting or observing them.
A 7-bar digital LED display 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 set point without the need to touch any other control. The lower display can be toggled between set temperature and controller output, e.g. percentage, amps, valve position etc. Here it is important to distinguish between the output signal at the controller terminals and the actual state of the final control device that is supposed to obey the controller. You can be misled if this link is defective.
Two other buttons give auto/manual access and run/hold control for ramp and soak programs.
In the configuration mode the lower display is used to display 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.
The Page button takes you through various headings. Scroll takes you down a list of parameters under those headings. At this point you can configure or adjust the parameters using the UP/DOWN buttons. PASSWORDS??
The above is just one example of the user interface. Apart from the UP/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 from the same manufacturer.
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 more likely to be attributable to documentation and a non-intuitive HMI (human-machine interface).
Fig 4. Rear Terminals of a Mid-range Controller
Other Analog Inputs
Control Outputs, one Heat one Cool
Function Outputs. Several relay triac or logic type. Allocatable to:
Ramp and Soak Programming
Further yet upmarket