Simple to monitor motor current for stall condition with the right drive, where current fold back is used.
BLDC motors and solid state control produces no 'sparks'

 

There is nothing in this thread to suggest that the linear actuators would have BLDC motors. This sort of application points much more toward PMDC brushed motors. Especially the size, shape, and maximum starting torque requirements..

 

The reference to using BLDC was your post regarding the negative side to brushed motors producing sparks, My thread was simply to offer an alternative that prevents this!!

 

There is nothing in this thread to suggest that the linear actuators would have BLDC motors. This sort of application points much more toward PMDC brushed motors. Especially the size, shape, and maximum starting torque requirements..

Hi,

Just to note, I did not read his post like that. I just read it as another suggestion.
For example, "I would use 100000 mice running in 100000 wheels to generate the torque."
That reminds me, I better check the trap, we have mice come in here when the weather starts to get cold outside when winter hits.

 

That reminds me, I better check the trap, we have mice come in here when the weather starts to get cold outside when winter hits.

I walk the trap line every morning!

 

Operating hopper discharge gates is not such a high accuracy application, and with preset hard stops would not require position feedback for adequate control.
So now I have more questions for the TS, as nobody else is familiar with the operation. #1 How are the gate operations presently powered? #2 How many operations are anticipated at each location the hopper is placed? #3 Already, DC power is required for operating the control valves, how is this power provided for the current scheme that uses hydraulic power.? #4. As currently an automatic return function exists with the hydraulic system, does that mean that the opening is initiated by an operator who then simply releases a control lever to have the discharge gate close with no additional operator actions?? It seems to me that there is a lot more involved here than was originally described.

If there were only a very limited number of operations required, then possibly a correctly sized hydraulic accumulator could supply all of the required power and not require modifications at all. That could be another option to consider.

1 gates currently operated by hydraulic cylinders
2 could be idle for weeks at a time, then the day its running, once every 10mins for 4-5hrs
3 Hydraulic is open centre, pull a lever to activate. there will be a source of electric power present, the battery on the engine which powers the hydraulic pump which feeds the hydraulic cylinder.
4 pull the lever in the opposite direction to reverse the hydraulic flow to close.

Thats a small ram, its the extend size which closes the hopper and that hard stops on itself.
opening the hopper- its the weak side of the ram which does the opening, to stop it opening too fast and hammering the stop limit there is a disc in the inlet hydraulic hose with a small hole in it to limit the flowrate of oil.

All controlled by human using your eyes as the sensors.

 

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.

indeed fertilizer and seed spreading is where the inspiration came from, using it to dispense powder.

dont need automated return,

 

OK, if an automated return is not required, then the control scheme gets back to two relays, one for open and one for close. And a semi-automated close can be achieved with one limit switch to open the relay control circuit at the fully closed position. The first benefit is zero power consumed unless moving, the second benefit is a simpler power system as only battery power is needed, with no hydraulic power unit. (I happen to like hydraulic power for some applications, this is not one of them. And no nasty hydraulic oil spills when things go wrong.
Older school technology would use a longer lever with an integral serrated quadrant latch. That sort of system will easily last 100 years with only occasional grease application. And never need battery or oil changes.

 

Hi there,

This is interesting because I was recently think about this kind of solution also with my relatively new CNC machine.

The key point is accuracy. That is, how accurate the stop point has to be. For low accuracy, almost anything would work. For high accuracy you could pay a lot for a limit switch. There are also optical limit switches.

Some systems would have a built in current limit so if the movement hit any kind of mechanical stop that was strong enough to prevent it from moving without breaking the stop mechanism, then it would surely stop because there would be limited current to drive the motor. It would not stop trying to drive farther forward however without a detection mechanism as you proposed.
For my system I suspect it is driven with drive circuits that are somewhat normal, and that means that there is already current limit built in. For your system it could blow out the driver if the movement hits a stop. If you could detect the current (as regular stepper motor drivers do every day in millions of places in the world) then you could make a very fast stop. Once it stops, you could ensure that the next movement operation is only allowed to move it in the opposite direction.

Now accuracy comes to mind because there are problems with low cost limit switches and also with current measurement circuits. Again, if you need high accuracy then more attention to details in either the switches or the current measurement circuits. For low accuracy you can get away with a lot. This means you should try to figure out just how much accuracy you need. This would partly depend on the kind of drive and how it works. If while the motor turns the logic thinks the stage is still moving, then it could result in a positioning error. This would mean the accuracy of the stop would have to be good enough to prevent a position error more than whatever tolerance you are working with.

So I guess the first question is, does the control logic think the actuator is still moving just because the motor is turning, and if so, would that cause a significant positioning error? The best system would be able to detect the increase in current and possibly the first derivative of the increase in current so that it could stop it very quickly. This of course may or may not require a braking system.

Give this a little thought as we zero in on the nuances of this kind of electro-mechanical system.

dont have any control logic put together yet, starting from the point of wanting to replace hydraulic actuators with electric while keeping the hard stop arrangement

 

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

I don't think there has been any details yet on the 12v actuators quoted yet.?
It would be interesting to see product detail & more of what is in mind?
I don't see why a system such as this would not work?

 

OK, if an automated return is not required, then the control scheme gets back to two relays, one for open and one for close. And a semi-automated close can be achieved with one limit switch to open the relay control circuit at the fully closed position. The first benefit is zero power consumed unless moving, the second benefit is a simpler power system as only battery power is needed, with no hydraulic power unit. (I happen to like hydraulic power for some applications, this is not one of them. And no nasty hydraulic oil spills when things go wrong.
Older school technology would use a longer lever with an integral serrated quadrant latch. That sort of system will easily last 100 years with only occasional grease application. And never need battery or oil changes.

i like this idea, BUT if i keep my finger on the momentary switch for too long every time I open the shutter and the actuator pushes against the hard stop will i not burn the actuator motors in a short space of time ? - this was my original thinking. Hence me thinking i needed a current limiter circuit to protect the actuators

 

I don't think there has been any details yet on the 12v actuators quoted yet.?
It would be interesting to see product detail & more of what is in mind?
I don't see why a system such as this would not work?

nothing finalized yet, but have seen this

https://www.firgelliauto.com/products/programmable-over-current-cut-off-module

and a youtube vid of it in action here, i dont like the time delay for it to reset itself when the overload is tripped however

 

thanks to everyone who has contributed so far some electrical system experts here

 

i like this idea, BUT if i keep my finger on the momentary switch for too long every time I open the shutter and the actuator pushes against the hard stop will i not burn the actuator motors in a short space of time ? - this was my original thinking. Hence me thinking i needed a current limiter circuit to protect the actuators

Certainly, if adequate overload protection is not included, it will be quite possible to burn out the motors. No question about that. Now I have a couple of questions: Have you located an actuator that will fit the available space and provide enough force? AND, are those actuators adequately weather proof enough to survive a rainy season?
And I have been wondering about the actual benefit of using an electrical actuator instead of a hydraulic device. Or is the issue the power for the hydraulic motion??

 

Certainly, if adequate overload protection is not included, it will be quite possible to burn out the motors. No question about that. Now I have a couple of questions: Have you located an actuator that will fit the available space and provide enough force? AND, are those actuators adequately weather proof enough to survive a rainy season?
And I have been wondering about the actual benefit of using an electrical actuator instead of a hydraulic device. Or is the issue the power for the hydraulic motion??

this looks like it would be suitable based on my quick measurement with a spring balance to measure the force

progressive automations do some good stuff when it comes to waterproof

Waterproof Electric 12 Volt Linear Actuator - Used in Marine Industry - Progressive Automations not going to do much better on the waterproof rating too

I want to be able to flick my momentary switch from the next room rather than having to go outside to move hydraulic levers

 

Could you just use the actuator to move the hydraulic levers or replce the levers with solenoid valves ?

Les.

 

Could you just use the actuator to move the hydraulic levers or replce the levers with solenoid valves ?

Les.

i could but i want to get away from hydraulics

 

Here is the scematic of the system that I use for controlling curtains that comes close to your original request. It is similar to your requirement as it controlls a pair of curtains
View attachment 308285
It senses the motor current by comparing the voltage developed across the 2.7 ohm resistors with an adjustable voltage of between 0 and 05 volts. The 220 uF capacitors and 2.2K resistors slow down the response to prevent false triggering due to the starting current of the motors. (Which is about the same as the stall current.) To use it with your actuators which take 15 amps you would need to use 0.033 ohm current sense resistors.
You could probably get rid of the comparator IC as more modern microcontrollers have build in comparators.

Les.

i must study this, im not too well up on this, thanks for posting

 

The only part of the schematic that I thought would be of interest to you is the use of the comparator IC for detecting the current level when you considder the motor to be stalled. (I noticed that I had made a typing error.
When I wrote 0 to 05 volts it should have been 0.5 volts.) You could use the output of the comparator to latch or unlatch a small relay, a D type latch IC or even a small SCR. I used the 16C84 microcontroller as it was simpler than implementing the other functions in discrete logic but the logic you require is simpler than the curtain controller. Also the microcontroller I used is very old. It was even very old in 2006 when the unit was built.

Les.