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Home   >   Products   >   Articles   >   Hysteresis with Temperature Controller

A temperature controller is a device that you can employ to provide a consistent temperature in your process. Panasonic temperature controllers have a PID function to calculate an appropriate counter response to fluctuations in the process.

Briefly, the PID algorithm involves three separate constant parameters: the proportional, the integral and derivative values, called P, I, and D. You can think of the algorithm as measuring present errors, accumulation of past errors and prediction of future errors based on the current rate of change. The weighted sum of these three measurements is used to adjust the process as necessary to reach and sustain a set value (SV) typically in temperature.

PID Block Diagram
Fig. 1 PID Block Diagram

Our temperature controllers provide heating and/or cooling action as well as an auto-tuning process. The auto-tune process will vary the output of the controller (on/off, or low to high) and monitor the change in response of the process (typically temperature). The function of the auto-tune is after monitoring is completed the controller has calculated an optimal set of PID values to reliably control the process.

Note: every process will have its own optimal PID constants. System or process stability is only as good as the PID constants and the PID algorithm used. In applications where stability based on a specific value (SV) is not critical a Hysteresis can be used instead. Hysteresis doesn't use a set point but instead a range for process control action. For example, instead of controlling the process precisely to a SV of 100°F we can have the controller respond when the present value (PV) is +/- 10°F from SV. This gives us a 20 degree band for hysteresis response.

Example Response of PID vs. Hysteresis
Fig. 2 Example Response of PID vs. Hysteresis

Above is a example of using setpoint control, like PID, verse band control, like hysteresis. The pink line is the setvalue at 100°F. This is the value at which our green wave PID is struggling to hold. This means the PID is constantly working to keep the process at 100°F. During this time the output is constantly oscillating. The hysteresis behaves quite differently however also keep temperature around the SV.

Notice after the first overshoot (see the asterix on the diagram in Fig. 2) the red line, hysteresis, will only turn on again until the temperature drops out of the lower constraint in purple. Hysteresis will constantly turn the output on until temperature rise above the upper constraint, represented by the blue line. Both of these controls have advantages and disadvantage.

  PROS CONS
Hysteresis
  • Good at holding process to a temperature band.
  • Better for relay type outputs.
  • No PID functions or auto-tuning needed.
  • Can not tune process to a specific value SV.
  • Overshoot can be larger than PID depending on process response.
  • Can not tune process to a specific value SV.
PID
  • Good at holding process to a specific temperature, SV.
  • Once tuned, overshoot is better than hysteresis.
  • Better for sensitive processes.
  • Oscillating output bad for relay type output (transistor or analog preferred)
  • PID constants must be tuned accurately
  • PID algorithm must be accurate.

In order to use the Hysteresis in the AKT4 you must set the device in the ON/OFF control action. The ON/OFF control action is active when the proportional band setting (P or P_b) is set to 0 or 0.0 degrees C (degrees F).

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Datalogic Controller

The Wide Range of Auxiliary Functions available completes the excellent performances offered by the controllers of the E series, with standard 48x96 mm format and single and double display versions. The main PID control action adjusts itself automatically to the optimum parameters with the AT initial autotuning, or with the ADT permanent autoadaptive function; while the auxiliary outputs can be programmed on 16 different alarm functions

Weidmuller Displays

The Current/Voltage Displays of the PMX420 series are available as a pure display unit or optionally with analogue outputs/4 alarm outputs. The basic model is suitable for displaying a wide range of bipolar mA or voltage signals. Inputs are isolated from the power supply. An integrated power source is available for supplying external sensors and transmitters. The PMX420 Plus adds four alarm channels and a fully isolated