Sudden unexpected actuator movement when the HSM power is turned on.

PROBLEM

 

Actuator is suddenly moving when the HSM power is turned on.  

 

 

ANSWER

 

1) Zero or Insufficient I gain

 

If I gain is zero or too low and there is any imbalance in the system like a poorly balanced servovalve, the actuator can suddenly move when the HSM is turned on.  The algorithm that software uses to set the command to the current feedback position is reliant on I gain.  Make sure you have at least 1 or 1/10th of P gain for I gain; whichever is greater.  

 

Note: If you are using an older analog controller without automation software make sure you set the command to the current feedback position before applying hydraulic power.  Failure to do this will result in immediate actuator movement once the power is applied due to the error between the command and current feedback.  

 

 

2) Polarity Problem

 

 

Stop:  If you have a 3-stage servovalve scroll down to the section on 3-stage servovalve polarity.

 

 

CONDITIONER POLARITY

 

When calibrations are done the conditioner card polarity is set first to define tension either negative or positive then the servovalve polarity is set for control (Figure 1).  

 

 

Figure 1 – Location of the conditioner card polarity

 

 

Most often tension is defined positive resulting with normal polarity.  This is not always the case since it may depend on the polarity of the cable wiring as well. Sometimes tension defined positive may require an invert polarity setting.  Only the conditioner card polarity is recorded on the calibration report (Figure 2).

 

Figure 2 – Example of a Calibration Report Showing Polarity Extension (Compression) Defined as Positive

 

 It is important that you verify all conditioner card polarities (i.e. displacement, force, etc..) match the calibration reports first.  If you don’t have proper actuator movement after verifying the conditioner polarities are correct, you can change the servovalve driver polarity.  If no combination of conditioner and servovalve driver polarity works then there is a different problem.  

 

 

 

SERVOVALVE DRIVER POLARITY

 

Click on the Station Set-Up icon > Click on any of the control modes from your list of channel options > Click on the Valve Driver Icon  > Select the radio button for the desired polarity (Figure 3).  

 

Note: Actuator Movement is the result of the combination of the servovalve polarity and the transducer polarity (Figure 4). 

 

Figure 3 – Changing the servovalve polarity in Station Setup.

 

 

 

Figure 4 – Valve and Actuator Movement Based on Normal Servovalve and Conditioner Polarity with positive command and control cable wired normally

 

 

Note: If you cannot achieve control no matter what combination of polarity settings you are using it could be related to an incorrectly phased cable if you have dual servovalves.

 

Servo valve cables are produced with the first and second Servo valve either in Phase or out of Phase.

 

Check to see how the valves are mounted on the manifold, if they are mounted in the same plane, then the valves are in phase, However, if they are mounted 180 degrees from each other, then they are out of  phase.   

 

If they are out of phase you will need a servovalve driver cable that is wired for out of phase servovalves.  If they are in phase you will need a cable that is for servovalves that are in phase.   There is a part number on the cable.   It should be on a white label near either end of the cable. MTS can provide the cable pinout for the cable to show if it’s wired for in or out of phase.  

 

 

 

If you have a 3-stage servovalve follow the procedure below to determine the correct polarity settings.

 

1.  Switch to user level Calibration from the main Station Manager window.  The default password is Calibration with a capital C.  

 

Figure 1 – Calibration user level

 

2.  First set the Conditioner Polarity for the resource assigned to the channel signal to match the calibration report (Figure 2).

 

 

Figure 2 - Channel Conditioner Polarity

 

 

3.  Next, try to move the actuator in manual command.  If you can control the actuator then the polarity settings are correct and you are finished.  If you lose control or the actuator goes hard over to either end cap then the polarity is incorrect.

 

4.  Change both the 3-stage Valve Drive Polarity and the 3-stage Conditioner Polarity to their opposites.  

 

The 3-stage Valve Polarity can be found in station setup under Channel Drive > Valve tab > Valve Polarity (Figure 3). 

 

 

Figure 3 – 3-stage servovalve Valve Polarity

 

 

The 3-stage servovalve LVDT Conditioner Polarity can be found in station setup under Channel Drive > Valve tab > Conditioner (Figure 4). 

 

 

Figure 4 – 3-stage servovalve LVDT Conditioner Polarity

 

 

5.  Attempt to move the actuator in manual command again.  If you can control the actuator then the polarity is correct and you are finished.  If you lose control or the actuator goes hard over to either end cap then the polarity is incorrect.

 

6.  Change either the 3-stage servovalve Drive Polarity or the 3-stage servovalve LVDT Conditioner Polarity to its opposite.  It does not matter which one if you change only one of them.  

 

 

3) Unassigned HSM Power 

 

This can happen when there is a more than one channel in the station configuration sharing the same HSM power and the user forgets to assign the hydraulic power to one of them. The channel without the HSM power assigned will not be able to set the command to the current feedback when the HSM low button is pressed in the software because it’s only associated with the other channel.  This will result in instantaneous error and the actuator will move to zero the error.  

 

 

For example, you may have both an axial and torsional channel that are plumbed to HSM 1 for power but only the torsional channel has been assigned to HSM 1 for power in Station Builder (Figure 5).

 

Figure 5 – Channel Axial Missing Power Assignment

 

 

K
Keary is the author of this solution article.

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