Controlling Linear Actuator via Load Cell

Hello,
I need to control the position of a linear actuator based on the force feedback from a load cell (rated up to 100 ft/lbs) connected to the tip of the linear actuator. What I want to be able to do is to set a force and have the linear actuator (driven by a stepper motor) move until it is within 0.1 ft/lb of the force setpoint. Once within 0.1 ft/lb of the setpoint, I would like to hold and control to the setpoint based on a user configured hold time.
I have the following hardware:
PXI-1033 chassis
PXI-7340 Motion controller card
UMI-7764 Terminal Block
P7000 Drive
Omega Engineering Load Cell model number LC201-100
Omega CCT-80 Load Cell Signal Conditioner
Omega FAR-1 Power Supply
I have the + output of the CCT-80 connected to the Analog Input (AIN1) and the - output connected to the Analog Input Ground (AIGND). This output signal is -10V to +10V (verified by DMM)
I have the ADC range set to -10 to +10
I'm reading the ADC as fast as possible (get about 700 samples/second) and this just isn't fast enough to control to the precision that I need. Also, it seems like there is a substantial amount of noise on this line.
Does anyone have any recommendations on improving the speed of the ADC read, or how to reduce the noise on the line?
If my requirement of holding to 0.1ft/lb is too strict for this hardware (7340 motion card) is there something else that I should try?
If I'm missing any other info that is needed to help me out, let me know and I'll update this post.
Thanks!

Hello AMac,
I have a setup similar to yours with a load cell attached to a linear stage. I want to use position control of the stage to apply a set load on the load cell (position-based explicit force control). Do you have a example VI I could look at if yours was successful?
Thanks,
Kyle

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Sorry, I have been extremely busy the last couple of days (months?). I still haven't had much time to ponder the problem, but one thought comes to mind, at what frequencies are you running this and what is the load cell, and its supporting electronics, frequency response. You mention that the amplitude decreases as the frequency increases, but that you increase the gain of the driver amplifier. It (the system) is most likely non-linear in its response, so how much you increase the gain will vary. The nonlinearity of the response is one of the reasons there is so much published about control systems, PID controllers, etc.
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Dynamic force control using linear servo actuator + load cell

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

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

How to finetune the force control of an AC Motor Linear Actuator?

Hello,
I
am hoping to find some help here with the following problem, which I am trying
to solve for several days now....
I
am using the UMI-7774 and an AC servo-motor-driven linear actuator to do
force control. Basically, I need the actuator to apply a constant force and
react quickly to force changes. My system setup is as follows: I am using
a PCI-7344 Motion Control Card to provide the analogue command signal
(0-10V). This signal is supplied to the actuator controller (through a
UMI-7774), which transforms the command signal to an amperage command signal
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(1)
The results so far show that it works in principle, but I believe my system is
not yet tuned optimally. However, the best step response results I have
obtained and the PID parameters I used are shown in the attached files. As I am
relatively new to this area, could you please advise me on how I could further
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observed during undershoot is something I would like to remove.
(2)
I have tried increasing Kp and Kd values. Increased Kd does not seem to correct
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Derivative Sampling Period but that gave even worse results (System Inertia: J
= 1.8629*10^-5 Kgm^2).
At
Td=2 the actuator was retracting on its own to its home position when I set Ki>=8.
I got the most stable behaviour using Td=4. Is there any other way I could
improve the actuator’s response?
(3)
I have seen that in some cases people have used a “double PID loop” one
for the encoder feedback and one for the force feedback. Do you believe that
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I implement two PID loops? I mean, MAX only allows me to enter
one set of PIDs....or is there a possibility to set up a second PID-loop
somewhere?....I have seen that the "Force Control Using Monitoring
Force" example essentially works with two Closed-loops, but one of them is
implemented in the application software and this would result in a
non-deterministic and comparatively slow response of the system, right??
Sorry for the long post and thanks
a lot in advance for help and advice.
Attachments:
Force Conrtol Loop Settings Actuator-BEST.JPG ‏41 KB
Step Responce Actuator-Force Feedback-BEST.JPG ‏58 KB

Hello Jochen and thank you for your very nice reply.
The past few days I have been trying to implement your advice, but unfortunately I did not have much success. Let me start by saying that explanations to my problem, the 2nd one looked more probable, as I always make sure that there is contact between my actuator and the workpiece I am pushing against. So, I unmapped my DAC by setting up the axis as a stepper axis with no feedback and I used the Load DAC to apply voltage and see when my actuator starts moving. That gave me the upper and lower limits for the Static Friction Compensation filter (please refer to attached image). I then implemented this filter and tried to retune the PID parameters, but the results I got were poor.
I can now adjust the parameters so that there is no initial spike and undershoot, but now the force ramps up fast to a specific value and then keeps rising slowly. Vibration is observed at the point of transition from fast to slow ramp-up (ref. Step Responce Actuator-Force Feedback-WITH FILTER_v2).
I could not eliminate the steady state error. When I tried to eliminate the steady state error by increasing Ki, or tried to create an initial overshoot by increasing Kp, I always ended up with an unstable system or a system with a response similar to the one shown in attached figure “Step Responce Actuator-Force Feedback-WITH FILTER_v1”.
Do you perhaps have an explanation or comments for this behaviour? As for your other questions:
Sensor Bandwidth: N/A, Charge Amplifier Bandwidth: 200 kHz, Low pass filter: No (Amplifier has the capability though)
PID Update Period: 250 microseconds (fastest available)
NI Motion Version 2.3
Labview Version 8.5.1
Thank you,
Thomas
Attachments:
Step Responce Actuator-Force Feedback-Filter Settings.JPG ‏28 KB
Step Responce Actuator-Force Feedback-WITH FILTER_v2.JPG ‏69 KB
Step Responce Actuator-Force Feedback-Filter Settings.JPG ‏28 KB

Is it possible to condition a load cell signal with a 1520 SCXI and then use it as the primary analog feedback for a 735X motion control board?

My system
PXI 1052 chassis
PXI 8196 RT controller
Flexmotion 7356 motion controller
Flexmotion 7354 motion controller
6259 M series DAQ
1520 SCXI
1520 SCXI
1314 SCXI frount mounting terminal block
DSM strain gage load cells
Exlar SR Brushless PM motors
Kollmorgen servo star CD series 5 drives
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The motion controller boards (7356 and 7354) can be configured for force feedback control via analog feedback from a load cell. However the boards implicitly expect the load cell signal to be amplified and conditioned with and external signal conditioner rather then using an NI SCXI module. Currently I’m am reading the SCXI signals via the 6259 M seriers DAQ and consequentially have no way to route the signal into the 735X boards for force feedback control. I can use softmotion to control the motors but then I loose the fast PID update rates possible with the 735X boards.
The question:
Is it possible to condition a load cell signal with a 1520 SCXI and then use it as the primary analog feedback for a 735X motion control board?
Thanks
Patrick Aubin
Ph.D Candidate
University of Washington &
VA MedicalCenter of Excellence for
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Hi GG,
Thank you for your feedback.
>create your own external circuitry to amplify the signal.>
True a few op-amps could amplify and filter the signal externally but the performance of such an amateur built system will never match the accuracy and functionality of the 1520 SCXI. That’s why we spent the $2500 for the professional version.
>One thing that you could do is to use a UMI with your motion controller and then wire your signal from your SCXI to the UMI.>
As I understand it the UMI is basically a break out connectivity interface for third party drives. After conditioning and amplifying the analog force signal with the SCXI module the analog signal isn’t available to wire into a UMI. To my knowledge the conditioned signal of the SCXI resides only in the PXI backplane (i.e. there is no physical access to the conditioned analog signal). Can you explain how the UMI could be help here.
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Servohydraulic control with load cell feedback

Hello,
We are planning to build a tie rod fatigue test system with Labview and NI PCI-7344 4-Axis Motion Control Board. We have two linear servohydraulic actuators for applying linear force to tie rod and a servo motor for steering wheel movement. Tie rod must be vibrated by linear servo hydraulic actuators with constant force (+/- 50N , compression and tension direction) as 20 Hz while the steering turns (this is for road simulation) .
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Best Regards
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Steward,
I think you should be able to use the analog feedback feature of the 7344 to accomplish your task. Please refer to this link where you can find an attachment that explains the details of using analog feedback.
I don't think that you will need any onboard programming for the control task but depending on your security needs you might want to use it for emergency stop actions (e. g. when something goes wrong and the speed or the force become too high).
Please note that the 7344 does provide 12 bit ADCs for analog feedback. If the accurracy of these ADCs is not enough for you you should consider to use a 7356 instead which provides 16 bit ADCs.
Please read the document linked above. If you find that for some reason analog feedback is not the solution for your task we can talk about other options.
Best regards,
Jochen Klier
National Instruments Germany

Pulsatile Linear Actuator Control - Help Needed

Hi,
I have a VI that controls the speed of a linear actuator, which is placed within a case structure. A limit switch input switches
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I have tried various approaches with the elapsed time function, alternating wait times but just can't seem to get this, any help
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Thanks
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Solved!
Go to Solution.
Attachments:
Pulsatile Linear Actuator.vi ‏95 KB

A few things:
Are you using the Run Continuously button?
If so, stop and put this into a proper loop or state machine.
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1. The VISA Clear and VISA Close should be outside the loop. You do not want to be re-initializing the VISA session each time through the loop.
2. You have the port initialized to use line feed as a termination character. When using termination character, you should not use Bytes at Port. Just set the byte count input of VISA Read to a constant larger than the longest message you expect to receive.
3. With the termination character you do not need the waits between or before the reads.
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A state machine architecture is more flexible and more robust.
As to why it stops in a particular place, you have not given us nearly enough information to even guess. What is the relationship between motor speed and displacement? Where do you do anything with the encoder? What is the format of the encoder output? How far does the motor move at various speed settings during one iteration of the outer loop? Where is the command to stop the motor when it gets to where you want it?
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Closed loop velocity control based on load cell force

Hello,
My application is for a drill, that drills into rocks of various densities for the purpose of collecting rock core samples.
My setup has 2 motors which get controlled, one spins the drill bit at a constant velocity, the other moves the drill mechanism along a Z axis.
For efficient cutting, it is desired to apply a constant force between the drill bit and the rock. I have a load cell which measures the force being exerted by the drill bit on the rock surface, and this force can be adjusted by changing the velocity of the Z axis. So I would like to employ closed loop control to adjust the Z axis velocity to maintain a constant force on the rock.
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1. Your hardware and software look appropriate assuming the motors are compatible with the 9505s. You didn't mention the NI SoftMotion module in your software list which is something you will need.
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Assuming you don't need any trajectory generation, you can scrap the entire RT portion of the NI 9505 example and just create the necessary FPGA code. On the FPGA, you won't need the Spline or Synchronization code either because this information would no longer be coming from RT. You could take these out and replace the position loop with a force loop and possibly a velocity loop and your FPGA program would basically be finished. In fact the only real motion IP that you will need is for the encoders (assuming you want velocity control) and PID. Then you would need to create an RT VI that allows you to send down enable, disable, PID gains, and setpoints.
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Regards,
Burt S

Control a linear actuator

I need to be able to control a linear actuator from labview. Can someone show me an example of how I might do this? Also, I have no idea how I should wire the actuall motor up so that it can go forwards and reverse.
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Simple control of a linear actuator through LabVIEW advice

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http://www.firgelli.com/Uploads/LAC_Datasheet.pdf

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http://zone.ni.com/devzone/cda/epd/p/id/4953
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http://www.ni.com/white-paper/6931/en#toc1
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A/C three Phase motor control using 1/4 bridge Load Cell output and a USB 9172 Daq

NI Assistant,
I am tasked with setting up a Motion Control using two each OMEGA LC712-100K, -50K and -10K load cells to monitor the cable tension on six A/C motor driven winches. I also need to Monitor and calculate a Moment as well as the load slope for Load verses Time.
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Layout for Testing Spars.docx ‏15 KB

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Controlling the displacement of 2 linear actuator

im using arduino to work the linear actuator
1) im not able to sync the 2 linear actuators togethers
2) stop at same distance with each other
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pls check the file below and offer any suggestion in doing so.
Attachments:
woirking linear actuator.vi ‏26 KB

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Controlling Linear Actuator via Load Cell

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