error configuration

[CracKatoA]

I need technical support for the automation of a damper machine with PXI platform fitted with a 2-axis motion control board PXI-7352. This board is used to control the movement of the hydraulic actuator upon the servovalve. There is an analog feedback coming from an LVDT that measures position of the shock-absorber's shaft. The refresh rate for the axis (axis 1) is 62.5 ms.

The shock absorbers are tested with sinusoidal movements at different frequencies and amplitudes. For this movement the "contoured move" mode of the motion board is used, for this purpose a sample pattern for the sinusoidal signal is generated in the buffer with a sample period of 10 ms.

For amplitudes and frequencies ranging from 0 to 3 Hz and 50 mm of amplitude the system works fine, even though different PID configurations must be done. Once these values are exceeded, for instance, 3 Hz and 80 mm, the motion board stops controlling the system. It seems evident that there is a lack of efficiency of the hydraulic actuator.

Even though some better results can be achieved modifying the PID parameters, these changes become unmanageable for different testing conditions and different shock absorbers. It seems as if another type of control is required but, in order to solve these problems we need to know why the motion board behaves in this manner. It is seen that once the dynamic system cannot follow the generated trajectory the board stops the control. Is there any manner of increasing the maximum error arising from the difference between the actual position and the required trajectory in order to keep on using the board for control purposes ? Is there any manner of configuring the board for that?

Thanks

[Lorne H.]

CracKatoA,

"The refresh rate for the axis (axis 1) is 62.5 ms."

I am assuming you meant 62.5 us.

I need more information about your system.  I started jumping to conclusions regarding how solve your situation and then I realized I was making assumptions about your system.

Can you tell me more about your ServoValve and the ServoValve's driver/amplifier.

There might be some solutions to your problem.

Tell me if I am correct or not, as the frequency of your oscillation increases the torque required to put your shock-absorber in a specific position changes.  It sounds like there is some lag in your system that increases as velocity of your shaft increases.  You might need a control system that can compensate for this lag or remove the component in the system which is allowing it to lag.  Velocity feedforward gain (Vff) or Acceleration Feedforward gain (Aff) might help with this.  Or... I might be completely off base and this might not be what is happening at all.

Anyway, if you could give me more information about your system and comment about anything I said, it would help me help you.

Thanks,
Lorne Hengst
Application Engineer
National Instruments

[CracKatoA]

Thanks a lot Lorne for your kind reply.

Yes, you are right, the update rate is 62.5 us.

We have already used in our project the velocity and acceleration feedforward which did produce some better results in the overall performance of the damper machine. The real problem, which we are analyzing now before trying to keep pushing for additional betterments is that the hydraulic component of the machine itself seems too small for the required movements. The electronics cannot do magics in this case.

Our question about getting rid of the hardware control in the motion board was indeed somewhat silly since you may wonder why we are then using a dedicated board for that purpose if we do not want it to take control of the process. In this case, the problem was raised by our customer.

We will be back to you if we need to do so.

Nice technical support !