Dynamic force control using linear servo actuator + load cell

[fnaeem]

Hello,

I am designing a test fixture which will allow us to test the performance of actuators we install in our products. I need to apply a constant load to the actuator while it is extending/retracting. I would like to be able to use a linear servo actuator and a load cell on the same axis as the UUT to provide and monitor the load. The actuator under test does not travel at high speeds, its max speed is approximately 1in./sec.

I need to be able to maintain a constant load while the actuator is moving. Therefore  I'll need some kind of closed loop system to maintain a velocity such that the load being reported from the load cell is equal to the expected load as the actuator is moving.

I am trying to figure out how to best implement this in a cost effective and time-saving way. I have not purchased any hardware at the moment so am open to suggestions. We do have LabView 2009 with all the toolkits. Would it be best to implement this using a motion controller card or would I be able to get good results using a standard DAQ?

Message Edited by fnaeem on 02-05-2010 05:58 PM

[fnaeem]

Here is a drawing of the loading stand.

[Jochen]

As you have stated correctly, you will need a deterministic closed loop system to accomplish this task. The most cost efficient while reliable solution for this purpose is a motion control board like the PCI-7352. This board provides quadrature encoder inputs for positioning tasks and analog inputs for any other type of control. Torque/force control is a very typical task for this board and it provides a simple API and useful configuration tools.

If cost savings are a primary goal, you could also use a PCI-7342, but for your application I would recommend the PCI-7352, as the PCI-7342 provides only 12-bit ADCs, while the PCI-7352 provides 16-bit ADCs, resulting in a much better resolution of your force feedback signal.

I hope that helps,

Jochen Klier

National Instruments

[fnaeem]

Thanks for your feedback Jochen. I've been trying to figure out how complex the LabView software will be to perform such a task (applying a fixed load to an actuator under test while it's moving). In the NI-Motion User Manual there is a section that describes "Speed Control Based on Analog Value" (Section 13-14). The load cell amplifier I have outputs a 0-10V signal (5V = 0 lbs).

Is this the system I'll need to apply to get the desired result?

[Jochen]

As much as I understand your application, you don't need speed control, but force control. If you need to apply a constant force to a moving part, all you will have to do is to load a fixed setpoint and send the start command.

You can find details about using analog feedback for force control in this document. You can find more discussions about similar topics by clicking on the AnalogFeedback tag below.

Jochen