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Description

Figure 8: GUI for a typical PID loop, EV9178 in this case.

All the PID loop control GUIs look and behave the same. For the GeN target, all PID loops are used to control the position of a valve. Fig. 6.1 is the PID loop for buffer dewar pressure regulating valve EV9178. The name of the valve is given at the top of the GUI screen. In this example, the computer will open or close EV9178 in order to maintain a constant value of 1330 mbar for the buffer dewar pressure PI91104. Every 20 seconds (the Scan Time) the loop will compare the Current Input with the Set Value and decide how much to open or close the valve (determined by the gain settings). A negative gain setting means that the PID will produce a lower Current Output in order to increase the Current Input (eg the valve will close in order to increase the pressure in the dewar).

The user can set maximum and minimum positions for the valve, as well as the maximum change allowed in one Scan Time. If the PID calculates a change in valve position that is less than the Minimum Change, the PID does nothing. This minimizes wear and tear on the valve. A complete description of the PID settings is given below.

Parameter	Description
PID Name	Specifies which valve is to be controlled by the PID loop
Mode	Either Normal (under PID control) or Manual
Cur Pos	The current value of valve position (in % open or # turns open)
Cur Out	The current output of the PID (ie where it thinks the valve should be)
Max Pos	Maximum valve position allowed by the PID loop (set by the user)
Min Pos	Minimum valve position allowed by the PID loop (set by the user)
Max Chg	Maximum that the valve position can change in one move (set by user)
Min Chg	Minimum that the valve position can change in one move (set by user)
Man Mode	NO selects PID mode, YES selects PID mode
Input	Selects which variable the PID loop will try to keep constant
Cur Inp	Current value of the Input variable
Set Val	Value (of the Input Variable) that the PID loop will try to maintain
ST	Scan Time for the PID loop in seconds
Gp	Proportional Gain setting
Gi	Integral gain setting
Gd	Differential gain setting