Operating Description
This application contains a standard PID algorithm to control the temperature of a display case.
- The display case is heated using a light globe.
- The power to the globe is set via the 4-20mA output that in turn sets thyristor power control module.
- The temperature within the case is measured with a thermocouple.
- The brightness control (heat setting) is use to set the setpoint.
- The display switch controls whether CH1 or CH2 is sent to the access module display.
- Please note that the equation update time has been changed to 200ms because two analogue channels are in use and the thermocouple table is also in use.
- To tune the PID refer to the Ziegler Nichols tuning method contained in UAP0025.
Declarations
| Variable | Label | Description | Value |
|---|---|---|---|
| CH1 | SetPoint | Brightness control | 0-8Vdc = 80 to 130 deg F |
| CH1 | Measure | Thermocouple type J | 80 to 130 deg F |
| Con_a | PB(%) | Proportional band setting | (25) |
| Con_b | I time S | Integration setting | (1) |
| Con_c | D set | Differential setting | (0) |
| Con_d | EquTime | Set to same value as the equation | |
| evaluation timer. | (0.2) | ||
| Con_f | IntReset | Used to limit the integral value | |
| when control is not possible | (12.5) | ||
| Con_i | Val 2 | equation constant | 2 |
| Con_j | Val 100 | equation constant | 100 |
| Mem4 | Deg F | Value displayed on access module. When P1 = CH1, When NOT P1 = CH2 | |
| Mem6 | i | Integral error signal | |
| Mem7 | PiD | Controller output error signal | |
| Mem8 | delta | Difference between set point and process feedback (error) | |
| Mem9 | old delta | value of delta in previous sample. | |
| Timer enable: | Y | ||
| Timer sec: | 0.2 | ||
| Update on input: |
USC Programs
| File | Comment |  Help |
If unexpected results occur when loading the .usc file press back and click on help for instructions. |
|---|---|
UAP00341.usc |
First Issue. |