What I can do to minimise my offset in velocity in PXI 7344?

Question

Hi,
I am using Motion control Card (PXI 7344) to control both position and linear velocity of the cylinder shaft. I am reading analog voltage from potentiometer(input range = -5V to +5V corresponding to positions 100mm and 0mm respectively). I have used DAC output (+10 v to -10V) of the motion control card as in input to Servo valve (Note: i have not used any amplifiers inbetween) which controls the hydraulic flow which inturns controls the linear movement of the cylinder shaft. In the MAX i have used Axis 1 and configured it to velocity mode selecting only the primary feedback and primary output (I haven't used secondary one) and set PID update period to 62.5 microseconds.
The problems I am facting righ
t now are
1. In the 1D interactive control of the MAX I found the current trajectory data (position and velocity) keeps on changing before starting the test (when the station if OFF) after intialising the card. I have check the Analog feedback signal from the potentiometer. It remained constant without fluctuating.
2. I can't able to achieve the set point velocity (i.e., there is always an offset and it varies with increasing setpoint). So I have used Auto-tuning option in the MAX. It automatically sets some PID parameters but it remains changing when i do it again and again.Eventhough i make changes in the PID parameters, my offset remains the same.
Whether I have to use Encoders as secondary feedback device or else what I can do to minimise my offset in velocity?

Answer 1

Project Support,
After reading over your description, I am still not sure that I entirely understand what is happening. Some of the questions that I have are:
-What type of motor is hooked to Axis 1?
-What type of feedback is Axis 1 using?
-Does the linear movement have feedback?
If you post back the answers to some of those questions, this forum may be a little bit better equipped to help answer your questions.
As for the questions that you posed, if you are using analog feedback, and the system issuing that feedback is turned off, then you may be seeing fluctuations from noise in the environment. This is my best guess about what is happening with problem #1. As for problem #2, I don't fully understand what you are asking. If you could post some a
dditional details about this problem, it may help to resolve the problem.
I wish you the best of luck with your application.
Regards,
Jed R.
Applications Engineer
National Instruments