Linear actuator with force sensor? - Page 1
Forum section: electronics / projects

author: Cretel
author_rank: Contributor
timestamp: January 07, 2016, 02:02:03 am
content: Is there a linear actuator that can detect the force it is applying and stop when it reaches a predetermined force?I want to use an actuator to hold down objects in my device, so far I have used a pneumatic which just extends all the way when triggered.It works fine until I tried softer/fragile items. The minimum pressure for the pneumatic to move puts too much force onto the items and breaks them.Also, I was under the impression that pneumatics run even at lower than rated pressures, the one I use (500mm extension, quite large) requires at least 0.2Mpa to move.That's pretty high isn't it? Could mine be leaky or have too much friction inside?

author: retrolefty
author_rank: Super Contributor
timestamp: January 07, 2016, 02:35:45 am
content: Quote from: Cretel on January 07, 2016, 02:02:03 amWell an actuator is just an mechanical output device, it can move until told to stop. To create a controllable feedback loop a sensor is required and control electronics and/or software to tie them together.I know there are tiny strain gauges of very good sensitivity and repeatably. First step is getting to know your requirements. How sensitive must it be, stated in a number value?

author: qwaarjet
author_rank: Regular Contributor
timestamp: January 07, 2016, 02:36:47 am
content: Whenever I've had to do force limit I installed a load cell on the actuator and used that in my control loop. for a cruder measurement, current in an electric actuator will track with applied force (ignoring thermal effects). Precision control of pneumatics is doable but difficult, air is very springy, I tend to use hydraulics or electric drives.

author: AhmedAbuShamla
author_rank: Contributor
timestamp: January 07, 2016, 02:38:25 am
content: i have almost the same question lolbut i think if the actuation is done by air pressure , the force is controlled by how much PSI u appliedor am i completely wrong in that regard ?

author: qwaarjet
author_rank: Regular Contributor
timestamp: January 07, 2016, 02:42:12 am
content: Force will be the pressure of the working fluid * area of the piston P*A=Fand if you have a moving system there will be inertia to compensate for in the control systemthere is also static and a dynamic friction

author: AhmedAbuShamla
author_rank: Contributor
timestamp: January 07, 2016, 02:46:53 am
content: ignoring the dynamic friction if P is constant in P*A=F and A is const then F is const right ?

author: qwaarjet
author_rank: Regular Contributor
timestamp: January 07, 2016, 02:48:50 am
content: yep

author: retrolefty
author_rank: Super Contributor
timestamp: January 07, 2016, 02:59:08 am
content: Quote from: AhmedAbuShamla on January 07, 2016, 02:38:25 amAt the refinery I worked at we had many examples of hydraulics, pneumatic, and electric drive actuators of lots of different sizes. It all depended on the maximum force required, travel distance, travel speed required as to which works best for a given cost. And all required at least some degree of preventive maintenance to run reliably long term. Hydraulics was by far the most expensive to purchase, install, and maintain, but could work where no other actuator type worked as well. Again the feedback sensor used is just as important and is a separate topic but essential for precise control objective.

author: AhmedAbuShamla
author_rank: Contributor
timestamp: January 07, 2016, 03:15:21 am
content: i don't wanna imagine an open loop systemsome kind of feedback / closed loop system is alwaysbest to implement

author: bxh
author_rank: Contributor
timestamp: January 07, 2016, 09:42:19 pm
content: Quote from: Cretel on January 07, 2016, 02:02:03 amDoesn't that pretty much mean that there's significant static friction or equivalent with your pneumatic system?

author: Paul Moir
author_rank: Frequent Contributor
timestamp: January 07, 2016, 09:53:33 pm
content: Yes, 0.2MPa to start moving sounds crazy high to me, unless the diameter is tiny. You say it's half a meter long but what is the diameter?

author: Cretel
author_rank: Contributor
timestamp: January 11, 2016, 05:18:23 am
content: Quote from: Paul Moir on January 07, 2016, 09:53:33 pmDiameter is 10mm.I'm trying to think of a suitable feedback system but the head that presses onto the object is free spinning, kind of like a lathe.Just the thought of having to use a slip ring system for any head mounted sensors is causing me a headache

author: Pack34
author_rank: Frequent Contributor
timestamp: January 11, 2016, 06:22:16 pm
content: For electric linear actuators you could just measure the current being applied to it and then extrapolate. Throw a PID in there for control and it should work fine. You'll just need to do a lot of testing to tune it in there right.

author: Cretel
author_rank: Contributor
timestamp: January 12, 2016, 04:32:54 am
content: Quote from: Pack34 on January 11, 2016, 06:22:16 pmShould I use series current detection or EMI? I am not sure which will be more suitable. I want the actuator to make contact then apply about 10-50N more force then stop, do I need coarse or fine sensitivity for that?http://www.sgbotic.com/index.php?dispatch=products.view&product_id=278http://www.sgbotic.com/index.php?dispatch=products.view&product_id=1851

author: bxh
author_rank: Contributor
timestamp: January 12, 2016, 08:22:26 am
content: If you're about to drop some cash on sensors for this, why not just get a load cell instead? Something like the FX1901 will save you from having to infer force from current measurements, compensating for temperature and all that other guesswork.

author: Cretel
author_rank: Contributor
timestamp: January 12, 2016, 01:08:40 pm
content: Quote from: bxh on January 12, 2016, 08:22:26 amWell, because I can't think of a good way to mount the load cell on a rotary shaft.

author: Pack34
author_rank: Frequent Contributor
timestamp: January 12, 2016, 02:40:23 pm
content: The ACS712 should work fine. Have you done current measurements to verify what you're working with?Also, I would disagree with the load cell unless you really need precision in the force exerted. It would be simpler and mechanically cleaner to implement a PID based a basic current monitor for this project. It'll keep everything nicely tucked away.

author: Ian.M
author_rank: Super Contributor
timestamp: January 12, 2016, 03:26:03 pm
content: Quote from: Cretel on January 12, 2016, 01:08:40 pmDUH!You mount the rear end of the actuator body to the load cell. The forward end will need a low friction roller slide to locate it without diverting thrust from the load cell. It wont be as accurate as contact pressure sensors on the head, because you'll have a bit of stiction in the slide, but should be good enough.

author: Cretel
author_rank: Contributor
timestamp: January 13, 2016, 06:27:12 am
content: Quote from: Ian.M on January 12, 2016, 03:26:03 pmMy god.Something like this?

author: bxh
author_rank: Contributor
timestamp: January 14, 2016, 05:35:00 am
content: I'm not sure I understand what's going on in that drawing...

author: Pack34
author_rank: Frequent Contributor
timestamp: January 14, 2016, 02:33:36 pm
content: Quote from: bxh on January 14, 2016, 05:35:00 amIt looks like the linear actuator is attached using a slotted bracket instead of a single mounting point. This would provide the linear actuator room to push down on the load cell directly.

author: Cretel
author_rank: Contributor
timestamp: January 15, 2016, 02:22:08 am
content: Quote from: bxh on January 14, 2016, 05:35:00 amSorry, I draw like a 4th grader

author: Ian.M
author_rank: Super Contributor
timestamp: January 15, 2016, 02:38:12 am
content: You'd want a front guide on the body as well. Since the load cell deflection is negligible, you could also use a long arm at right angles to the body with a pivot on the far end, which might be mechanically preferable. Add a spring acting on the actuator body to keep the actuator in contact with the load cell and preload the load cell to get it into its linear region.

author: alho
author_rank: Contributor
timestamp: January 15, 2016, 03:23:05 pm
content: Can you put load cell under object that you need to hold down? Probably won't work with "springy" objects.

author: bxh
author_rank: Contributor
timestamp: January 19, 2016, 02:45:39 am
content: The springy-ness of the object will not make a difference to the force measured. If the arm only needs to work in a limited area and with one object at a time, it might be a simpler approach!

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 19, 2016, 04:50:33 am
content: There are two approaches to designing a product. One is to think a lot, and then build a prototype and find out what you didn't think about. The other is to build a prototype and find out what else you need to think about. Both work, and both have their advantages. You seem to be on the second approach. It can be expensive and time consuming to use this path when you are using actuators, load cells, closed loop feedback on top of whatever you are doing that requires a rotating front end.I would recommend a little time on the other approach before you proceed. First step is to understand the range of forces, speeds and distances you need and can live with. How much force can the fragile items stand. How much force do you need (at the effect end) to accomplish your task. How fast must you be there, and how fast do you need to return. Are the distances always the same? Now you have some numbers to help generate ideas and test solutions against. For example, if the distances are always the same a simple limit switch may be all you need. Then some time thinking about how your solution works. This can keep you from implementation mistakes. For example a load cell or other force sensor or limit switch will need to do more than close the air input, as this will not immediately stop or reduce the force. You are now talking about a proportioning valve, an expensive bit that can be tricky, particularly in a system with a lot of stiction. Might have to use dither or some other technique to linearize. Maybe a brake is a better idea.

author: Cretel
author_rank: Contributor
timestamp: January 20, 2016, 05:18:23 am
content: I decided to go with an electric actuator with a stepper motor w/ brake for that.Problem now is, there were two air cylinders initially, now it doesn't make sense to have all the filters and regulators around just for one stupid little pneumatic cylinder to eject objects off the spindle.Tempting to go full electric, but the cost of another electrical actuator is massive (stepper motor and stepper motor driver)...I wanted to install a foot pump operated hydraulic cylinder for ejection since the force needed is not high. Something with a 1 to 1 compression to extension length.But that flies smack in the spirit of automation and I can't find oneI thought for sure such a thing exists.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 20, 2016, 05:55:14 am
content: A solenoid won't work? If the part ejected can stand an impulse load you can even store some kinetic energy in the free stroke of the solenoid before it strikes.

author: walkeryyj
author_rank: Newbie
timestamp: October 18, 2023, 10:51:09 pm
content: I know this is an ancient thread, AND I'm with this company so I'm hoping this isn't out of line... However, should anyone be looking to answer this question TODAY, here is a somewhat easier way of approaching it:We build integrated force sensing/controlled/back-drivable linear motors with no need for an external load cell/etc.Happy to answer any questions as appropriate!https://irisdynamics.com/