I have a 12v linear actuator pulling on a brake cable. I’m assuming the amperage draw of the actuator will increase as the pulling force increases. I need a device to turn the power off to the actuator when the pull force ( and thus amperage) increases to a certain point. Any recommendations?
What do you want to happen in the moments after the high pull occurs? Do you expect it to occur often? Why not just a fuse or circuit breaker? Current limiting power supply?
I realize that the term can mean a lot of different things outside vehicle-stopping in machine tools and controls, firearms, etc but do you really want a brake releasing without warning? Or do you want to reduce the pull a bit?
Keep in mind that brushed DC motors (usually used in inexpensive linear actuators) briefly draw the full stall current every time they start moving. So your idea won’t work.
I suggest to use a motor driver with current sense capabilities, and a microprocessor to control the driver and keep track of motor state. The uP should ignore the high current draw during startup.
I’m using a linear actuator to pull a cable connected to a parking brake on a car. Ideally, I’d like to push a button and the actuator then starts retracting and pulling on the cable. When the amperage draw of the actuator increases to a certain level, I know the cable is pulling hard enough on the parking brake mechanism. I want the device to sense the amperage draw and in turn, cut off power to the actuator which would then hold its retracted position and keep the parking brake engaged. I would then like to press the button again, and the actuator extends out and thus reduces tension on the cable and releases the parking brake.
Most people use a limit switch to cut the power to the actuator when it reaches a certain extension.
A diode added in parallel to the switch will allow reverse current to flow and retract the arm.
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Limit switches and diodes would be your best bet but if you want to monitor current as well, an amp meter shunt resistor will change a current into a small voltage. Monitor that and you can accurately calculate current and have your control system cut power in an overcurrent situation.
This and all the tense cables reminds me of another part of the car, the electric window controls and especially ones with Auto up/down. If you roll the window up on your own orbital buffing wheel or younger brother’s neck, it doesn’t guillotine clean off like we might hope but does detect the obstruction with minimal damage to the buffer.
I don’t think limit switches will work since the mechanical position is not directly related to force because the cable will stretch over time and I don’t want to have to adjust the position of the limit switch. How is it done for a garage door which will reverse direction if encountering an obstruction? I don’t want it to reverse, just stop when the motor in the actuator is about to stall out. I was hoping for a single device and not fiddle with individual components.
My garage door has an inductive speed sensor on the motor shaft. When the speed drops below some percentage, the motor shuts off or reverses if the door is not fully closed.
The old opener had limit switches at the open and closed positions, I think the new one learns how many motor rotations it takes to get from open to closed and counts how many have gone by to determine door position. When it detects the door isn’t where it thinks it should be, it shuts down and flashes the lights.
In any case, you’re not likely to find an off the shelf part that does what you’re looking for. You will probably end up construting something that monitors multiple sensors (or switches) and then decides to cut power or not.
I explained above why this won’t work, and it is a bad idea for several other reasons as well, one of which is that the actuator motor life would be dramatically shortened.
The rule of thumb for typical brushed DC motors is that the current draw under load during normal operation should not exceed 50% of the stall current (except for the brief startup value), and on average, 25% of stall.
I suggest to find some other method to detect the end condition, and/or come up with a better mechanical design.
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Your suggestion
“the amperage draw of the actuator increases to a certain level, I know the cable is pulling hard enough on the parking brake mechanism. I want the device to sense the amperage draw and in turn, cut off power to the actuator which would then hold its retracted position and keep the parking brake engaged. I would then like to press the button again, and the actuator extends out and thus reduces tension on the cable and releases the parking brake.”
Is likely to create a dangerous and unsafe condition. If the linear motor must remain engaged to hold the parking break in place - a power failure, low battery, defective electronics or blown fuse could release the parking brake. If the car is on a hill - it could roll away.
Linear motors come in all sorts, screw type motor - in which the rack remains locked at last position. Or a gear on a rack - that can move when the power is off. It’s also possible to have a linear motor which is entirely just a solenoid (iffy at best).
The effort to engage a parking brake depends on the brake system design and possibly the brake pad or shoe wear. Limit switches themselves might not work because the distance to travel changes. The motor current will also change depending on hot/cold weather conditions. If the brake cable runs outside of the car and it ices up - it may seem like the brake is engaged when it isn’t. Or if iced up it might not correctly release (I’ve had that happen).
You might want to consider some kind of force measurement (strain gauge) that can report back a big change in pull/release force and used that to help detect faults. Looking at current alone is likely iffy.
For vehicles - that people ride in - redundant systems are almost always a requirement.
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[I assume OP will/I direct OP to] be building in a ratchet or similar tensioning device, the type already familiar in automotive manual parking brake applications. The electric brake motor proposed is to take the place of the motorist’s own arm or foot when ‘jacking’ what’s sometimes called the E-brake, the emergency brake, and, of course, the brake remains engaged while the motorist is away.