Current Limiter as a stop for linear actuator

Thread Starter

aidank

Joined Nov 22, 2023
15
Im relatively new to the world of control of linear actuators, im a mechanical engineer by profession.

I currently have an actuator which is 12V forwards and -12V reverse. Actuator is rated for 15amps. 300N push pull.

I want to limit the stroke length of the actuator via an adjustable mechanical stop. Not suitable to use a reedswitch on the actuator as the stop is 2ft away from the actuator. To keep the cost of the actuator down dont want any potentiometer or such in the actuator, I want to keep it simple.

What I would like is to be able to put a current limiter in the actuator circuit so that when the actuator pushes against the mechanical stop it just cuts out, autoresets and is ready to go again. this would protect both the actuator and the linkages and stops.

Any suggestions for either a nice easy circuit which i could build or even a off the shelf component for this application.
 

MisterBill2

Joined Jan 23, 2018
17,788
"Keep it simple" will certainly lead to a poor design with the linear actuator. Aside from that we have no information as to how the actuator is controlled, or what sort of power source is involved.
If the application is manually controlled by an operator using a spring-return -to-off switch, then possibly a simple stop could be adequate, but probably not.
Linear actuators are available with internal switches for end of stroke, which could be one option. The linear actuators that I worked with most recently were rather powerful, I think about 100 Newtons, 24Volts DC at about 18 amps. That model included a link designed to break if it was forced beyond the end of travel. So that application had limit switches to signal the automation to switch off the power relays at the end of the move. If this is an industrial machine you will need to use actual limit switches and not a reed switch. If it is a personal use, one-off product then a momentary switch can work.
The system that switches off automotive windows at fully open is another possible scheme, I have never studied how those work.
 

MisterBill2

Joined Jan 23, 2018
17,788
Use an industrial switch if this is an industrial application. It needs two relays or contactors anyway, might as well do the job right. Of course we have no hint as to what the job is.
To actually control a stop when it hits a barrier, you will need an electronic current monitor that will switch off at the current spike. That will cost more than a limit switch. And 15 amps is way too much for a reed switch anyway. And a hard stop into a barrier will lead to a shorter life for the whole motion setup. I have had to fix other people'sgoofs like that. Stuff jams, bends, or breaks.
The electrical engineer designing the controls will want to use a limit switch, but if it is an accountant designing the package, then hitting the stops will be OK.
 

Thread Starter

aidank

Joined Nov 22, 2023
15
here is a diagram

I want to replace the two hydraulic cylinders with electric and keep my manual stops. those cylinders are v small and currently have a restrictor and maybe even a pressure limiting valve fitted on the inlet port of the ram to prevent a huge force generated on all linkages and stops.
 

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AnalogKid

Joined Aug 1, 2013
10,943
What I would like is to be able to put a current limiter in the actuator circuit so that when the actuator pushes against the mechanical stop it just cuts out, autoresets and is ready to go again. this would protect both the actuator and the linkages and stops.
That's all good up to the "autoreset". How do you envision that?

What you are describing is a "foldback" current limiter. When the current exceeds a trip point, the circuit output goes to zero (or very little) current. It is a form of an electronic circuit breaker, and requires a reset of some kind. This can be a switch input, cycling the power, waiting for a prescribed amount of time, etc.

Your mechanics must be ablt to withstand the stall force of the actuator long enough for the monitor circuit to detect the higher current and act. Also, there must be a large enough difference between the run current and stall current to detect reliably.

ak
 

MisterBill2

Joined Jan 23, 2018
17,788
OK, I am guessing that is on a small agricultural tractor or landscape package. If the only control is manual and momentary then you may be able to make it work for this application. But I seriously suggest adding a bit of cushioning somewhere in the force linkage. You will also need two of the standard automotive cube relays to control each actuator. The application circuit for those has been posted in this forum many times.
Adding an automated return after hitting the stop is a very big deal, really. At the least, it will require a sensor switch for the returned position.
Switches are available that do not require any mechanical contact or motion, they are not reed switches at all.
OR, is this for something like a snow-plow blade? That is quite a different application yet.
 
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Ya’akov

Joined Jan 27, 2019
8,973
Aside from the scales, I assume the actuators are moving something else as well. Are you sure you have the headroom to ensure they will do the work they have to do and still be able to be stopped by the mechanical stop based on current consumption?

I think you are making problems for yourself by trying to avoid using absolutely standard industry practice of a limit switch. You are probably better of trying to make the limit switch implementation clever rather that trying to be more clever than the switch.

For example, if wires or galvanic signals are a problem, you could use a pair of fiber optic light pipes instead of a wire, and create use an optical shutter attached to the stop that works as a photo-interruptor. if it is space, could could use a flex PCB instead of wire.

Whatever the case, in the end depending on a sketchy analog signal to stop a 300N actuator sounds pretty dodgy and likely to be plagued with unexpected special cases.
 

MisterBill2

Joined Jan 23, 2018
17,788
Aside from the scales, I assume the actuators are moving something else as well. Are you sure you have the headroom to ensure they will do the work they have to do and still be able to be stopped by the mechanical stop based on current consumption?

I think you are making problems for yourself by trying to avoid using absolutely standard industry practice of a limit switch. You are probably better of trying to make the limit switch implementation clever rather that trying to be more clever than the switch.

For example, if wires or galvanic signals are a problem, you could use a pair of fiber optic light pipes instead of a wire, and create use an optical shutter attached to the stop that works as a photo-interruptor. if it is space, could could use a flex PCB instead of wire.

Whatever the case, in the end depending on a sketchy analog signal to stop a 300N actuator sounds pretty dodgy and likely to be plagued with unexpected special cases.
With manual control it may be possible to work fairly well. BUT a circuit breaker rated for less than full load current for each actuator.
 

nsaspook

Joined Aug 27, 2009
12,775
I've never had good results with current activated mechanical limits unless the force required was trivial. Most used some sort of mechanical safety backup like a sintered safety pin designed to snap at excessive stopping torque. Floor techs would get tired of replacing broken safety pins and replace one with a normal spring metal roll causing mechanical damage. Those designs were upgraded to use real limit switches with slowdown switches before hard stops for total travel calibration.
 
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crutschow

Joined Mar 14, 2008
34,037
Below is the LTspice sim of an example electronic fuse circuit:
The circuit shuts off when the voltage across the MOSFET exceeds about 0.4V
For the MOSFET shown, with an on-resistance of 20mΩ, the limit current is about 23A.
It is reset by removing the input power.


1700673662877.png
 
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LesJones

Joined Jan 8, 2017
4,172
How about mounting the actuator so that it can move a small distance in each direction. Have it held in the central position with suitable strength springs. Have the limit switches actuated when the force of hitting the stops moves the actuator body against the spring. With a diode in parallel with each limit switch it would only stop movement in the direction towards that limit switch.
I have used the current sense method for opening and closing curtains but I also added code to the microcontroller that would stop the motor if power was applies for more than a certain time. I don't think you will find a ready made product to do this.

Les.
 

MisterBill2

Joined Jan 23, 2018
17,788
How about mounting the actuator so that it can move a small distance in each direction. Have it held in the central position with suitable strength springs. Have the limit switches actuated when the force of hitting the stops moves the actuator body against the spring. With a diode in parallel with each limit switch it would only stop movement in the direction towards that limit switch.
I have used the current sense method for opening and closing curtains but I also added code to the microcontroller that would stop the motor if power was applies for more than a certain time. I don't think you will find a ready made product to do this.

Les.
I have seen that exact scheme used and it worked very well in that application. Not precision but rather close.
Of course while we have seen the hardware we still have no description of what is being operated or done, or how precise the control has to be. Or if the two actuators need to move at the same time.
Now here is a question: I know how well hydraulics handles shock loads, but leadscrew actuators are a whole different kind, they do not have built in shock absorbing ability. They may bend or suffer internal damage, such as flat spots on bellscrew balls, or even thread damage. So the TS needs to understand the difference before making the change.
 
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Thread Starter

aidank

Joined Nov 22, 2023
15
The picture i posted is the system controlling the dispensing system rate from a hopper. Actuators move the blue gates as shown in my picture attached. There is a linkage attached to the blue gates and this linkage brings the hard stop into the system.

Both hoppers need to open reasonably close together 0.2-0.3seconds will be fine. Just one feed line from hydraulic pump feeding both, thus both not opening at the same time at the min.
 

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Thread Starter

aidank

Joined Nov 22, 2023
15
OK, I am guessing that is on a small agricultural tractor or landscape package. If the only control is manual and momentary then you may be able to make it work for this application. But I seriously suggest adding a bit of cushioning somewhere in the force linkage. You will also need two of the standard automotive cube relays to control each actuator. The application circuit for those has been posted in this forum many times.
Adding an automated return after hitting the stop is a very big deal, really. At the least, it will require a sensor switch for the returned position.
Switches are available that do not require any mechanical contact or motion, they are not reed switches at all.
OR, is this for something like a snow-plow blade? That is quite a different application yet.
yes its an agricultural application, i am familiar with 5 pin relays, automotive cube relays. have you a link to the application circuit you speak of please, or what search term should i use.

yes the cushioning can be taken into consideration.
 

MisterBill2

Joined Jan 23, 2018
17,788
yes its an agricultural application, i am familiar with 5 pin relays, automotive cube relays. have you a link to the application circuit you speak of please, or what search term should i use.

yes the cushioning can be taken into consideration.
OK, the picture makes more sense with an explanation. Each actuator will need it's own circuit breaker even though they should be moving together.
I will need to search for a thread with that circuit. Or I can just give you the terminal numbers and what they connect to. I will try for that tomorrow.
 

Sensacell

Joined Jun 19, 2012
3,397
The thing to understand about this idea is the energy stored in the rotating inertia of the armature.
When you hit the end stop the current starts to rise as the motor slows, all the energy in the spinning motor dumps into the stop, which a can jam the mechanics, requiring a torque greater than the stall torque of the motor to un-jam. The mechanism is now stuck. This happens even if you cut the current to zero, it's too late already.
 

MisterBill2

Joined Jan 23, 2018
17,788
When the control scheme that I have been recommending is used, the motor terminals are shorted when the voltage is removed, which provides some serious dynamic braking. Also, in many heavily loaded screw-type mechanisms, friction brings a quick stop. Keep in mind that these are not AC powered induction motors.
 

LesJones

Joined Jan 8, 2017
4,172
Another point is that with the spring mounted actuator system if the motor does not stop instantly the spring holding the actuator in place will just be compressed a bit more. The positioning accuracy should be quite good as the spring compression will hold the pressure against the mechanical stop.

Les.
 
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