Proportional servo valve & Force control

[Hreidmar]

Hello,

I have a Hydraulic system with one Ni PCI 7350 4 axis motion controller, two hydraulic cylinders, two 3-way Servo Hydraulic valves, two proportional servo hydraulic valves and two compression-tension Loadcells. Cylinders pull and push the test specimen.

Loadcells are mounted to the head of cylinder so we can measure pull and push force directly.

Proportional servo valves control the pressure applied to cylinders. 3-way servo hydraulic valves control the flow rate and direction of hydraulic going to cylinder.

I have to control the force applied by cylinders to the specimen. Loadcells have + - 10V amplified output and connected to motion controller's axis as analog feedback. Proportional valves have 0-10V input and connected axis's DAC. 3way servo hydraulic valves have +-10V and connected to unmapped DACs.

My problem is that, when the system is compressing something the forces coming from loadcell is positive so the onboard PID working great and output is perfect but when pulling something the forces coming from loadcell is negative  and the output of PID is negative. The output of PID has low limit at 0V because of valve can not take negative voltages.

I want to ask, is there a programmatic method to negate (for instance multiplying by "-1" ) the analog feedback which is entering the PID to have analog output always positive. Or may be an absolute function for output of onboard PID.

I have tried to define electronic gearing, on board programming to negate analog feedback. But with these methods I have failed.

to give a idea I attached  an application vi.

I need help. Thank you.

Message Edited by Hreidmar on 01-08-2009 03:55 PM

[Jochen]

Hreidmar,

yes, the 0-10 V spec for the proportional valves in combination with the bipolar output range of the loadcells smells like trouble. Electronic gearing with a ratio of -1:1 came to my mind first as a potential solution but this doesn't seem to work for you. Could you please elaborate a bit more about the issues that you were facing with this method?

The only other option I can think of is using external signal conditioning to rectify the signal of the loadcells as there is no way to do this in software on the board. Onboard programming is also not a solution, as this doesn't allow you to add calculations between the feedback acquisition and the PID algorithm.

If you need to make sure that the board doesn't output negative voltages, you can do this by setting the Negative Torque Limit (Control Loop Settings >> Torque Settings) to zero.

Regards,

Jochen Klier

National Instruments