Hybrid position/force control for a haptic interface

[phili]

I'm setting up a stewart-platform based haptic interface. The drive controllers I use require a +/-10V differential voltage to adjust the applied force.
On the platform I use a 6 DOF force and moment measurement system that a type of joystick is attached to.

At first stage I would like to realize a position control loop overlaid with a force control loop.

As long as no (or close to zero) forces are applied to my joystick I would like the position control loop to take care of a stable platform position and orientation.
Once forces are applied (to the joystick) the system should try to compensate these forces (at the 6DOF measurement box) by instantly tracing the direction of the force.

This would result in a joystick that could be moved through space "without" applying forces.
Without states that I will need to use a theshold that prevents unwanted motion.

Later I could use virtual friction and gravity just adding the appropriate force vectors.

The job of a haptic interface would be accompished overlaying the force vectors that should be feed back.

Is there a possibility to realize such a system with an NI Motion controller? I have to control up to 8 axes?

Currently I am looking at implementing the control loops directly on a PC using a counter for the A/B position signals of my drives and a DAQ card for the  joystick input forces.

Unfortunately I didn't find an analog output card allowing to apply +/-10V in differential mode.
Could anybody maybe give some hints?

Phili

[Jochen]

Phili,

as I wrote mulitple times in this forum there is no way to obtain stable control behavior on a Windows based PC platform. Every control algorithm relies on a stable loop timing with low jitter which can't be obtained on a non-deterministic OS like Windows, Linux or Mac OS.

Using an NI motion controller which runs the control algorithm onboard in real-time could be an option. I have already controlled a stewart-platform with an NI 7358 board and it worked quite fine but this was just a position control application.
The 7358 is capable of switching between position feedback mode and analog feedback mode but that's only one part of your requirements. As you need to be able to detect the direction of multiple forces you will need to develop your own control algorithm and run it on a real-time environment. A good solution could be an FPGA-based board like the PCI-7831R or a PCI-7833R (bigger FPGA for bigger applications) which can be programmed with LabVIEW FPGA. A good companion product could be the NI SoftMotion Development Module for LabVIEW which offers an easy to use hardware independant motion control API.

For the case that you need true galvanically isolated differential analog outputs you will have to use some external signal conditioning.

There are several other hardware options like a LabVIEW Real-Time based PXI system with DAQ boards or a cRIO system but I would have to write a whole novel to discuss all the advantages and use cases of each of these platforms. Please feel free to send me an email (jochen.klier@ni.com) how to contact you and I'll either call you or I will bring you in contact with an NI sales engineer close to your location.

Best regards,

Jochen Klier
National Instruments Germany

[phili]

Dear Jochen,
thank you very much for your answer.

Phili