MOOG P-I Servoamplifier G122-829A001

  When 4-20mA is selected, the output V/I switches must be in I and the output current SW1 must have switch 3 selected for 20mA. Maximum load for 4-20mA output is 500 Ohm. The output amplifier is limited to approximately 105% of the selected full scale output. If both the proportional and integrator stages are saturated, the output will not be twice the selected full scale but still only 105% of full scale.

8 Step push button The step push button (SW3) injects -50% valve drive disturbance into the output. When released, the valve drive reverts to its original level. This feature is useful for closed loop gain optimisation.

9 P-I selection 14 Dither For position closed loops, initially select only P (SW6:2). For pressure or velocity loops select I (SW6:4) initially and then P. See paragraph 12 below for more detail. For a complete discussion of P and I control, see the G122-202 servoamplifier Application Notes (part no C31015).

10 Integrator input The servoamplifier has a unity gain input error amplifier followed by two parallel stages, one a proportional amplifier and the other an integrator. The outputs of these two stages can be switched to the output power amplifier (see paragraph 7 above) which then drives the valve. The input to the integrator stage can be switch selected (SW4:1) from either the output of the error amplifier, I in = E, or the output of the proportional stage, I in = P. The latter arrangement is used in the G122-202. It is beyond the scope of these Application Notes to detail the benefits of each arrangement. If you have experience with the G122-202, I in = P would seem to be an easy choice.

11 P only gain For position loops select only P control (SW6:2). Input a step disturbance of 50% valve current with the step push button (SW3). Adjust the P gain for the required stability, while monitoring the front panel valve test point, or the feedback signal. The gain range of the proportional amplifier can be moved by changing the plug-in resistor R17. The value loaded when shipped is 100k Ohms, which gives a 1 to 20 range. Selecting 200k Ohms will give 2 to 40. The circuit will function correctly with the value of R17 between 100k Ohms and 10M Ohms. Note that as P gain is increased, the movement due to the step push button decreases.

12 P and I gains together If you are inexperienced with integral control the following set-up method is a good starting point. 

  I in = E: Initially select only I (SW6:4). Press the step push button (SW3). Increase I gain until one overshoot in the feedback signal is observed. Next select P (SW6:2) and I (SW6:4) together and increase the P gain to reduce the overshoot. For the I in = E arrangement the P and I sequence could be reversed. i.e.: adjust P first, followed by I. 

   I in = P: For an I in = P arrangement, only the “P followed by I” sequence of adjustment can be used. For a more thorough discussion see G122-202 Application Notes (part no C31015)

13 I limit The contribution from the integrator to the output amplifier can be reduced by selecting I limit on (SW6:3). When this switch is on the integrator contribution is reduced to approximately 15% of the level when it is off. This feature is useful in a position loop that may require integral control to achieve the required steady state accuracy. The limited integral control removes valve null error when the final position is reached. It is also useful in a pressure loop to limit overshoot, if the valve drive saturates.