# Detecting an external resistance on a motor

Discussion in 'Robotics & Mechatronics' started by mvoltin, Dec 18, 2015.

1. ### mvoltinNew Member

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We are building a very cool project for school (x-wing fighter) with functional cockpit canopy that opens and closes by linear actuator (Arduino, relay, limit switches, etc.). We cannot figure out an efficient way to detect resistance/obstruction while the canopy is closing but before it reaches the limit switch (i.e. kids hand or leg is in the way). the actuator can impose 250lbs force and definitely do not want a child getting squashed in there.
it should be fairly easy solution since all automatic trunks for vans have this feature - door stops as soon as it detects an extra resistance. One guess is that it detects when motor starts drawing too much current? If this is correct, how does it handle the fact that the motor naturally would draw much more current when the door is horizontal (extended) as opposed to vertical (close to being closed)?
Or is there another way an external resistance is detected? thanks in advance,
Mike

2. ### MaxHeadRoom78Active Member

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Automobiles on doors and windows usually have an electronic breaker or over load, just senses a certain current level over the norm.
Often done in motor drives as the small volt drop across a motor series resistor.
Max.

3. ### mvoltinNew Member

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Thank you, Max. Two follow up questions:
1) how is the "norm" determined? when door starts opening it probably draws very little current because the door is still vertical (relatively), as opposed to when it extends up towards horizontal position it would impose much heavier load to the motor. We would have a similar situation (canopy moves from horizontal to a vertical position) where the load/current will vary across the motion range.
2) Any suggestions about how to implement this? what the schematic may look like for this type of circuit and what components to use? I am relatively newbie at this and any guidance would help.

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5. ### alec_tWell-Known MemberMost Helpful Member

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Cool it may be, but you are right to be concerned. Do you really need 250lb of force? It would be safer to use the smallest force necessary. Can you use counter-balance weights or springs to offset the canopy weight so that a less powerful actuator is needed?
One safety mechanism could be a pivoted guard strip on any canopy edge which could trap a body part. Displacement of the strip would actuate a microswitch to cut the actuator power.
In my view there should also be a secondary safety mechanism.
Whether or not monitoring the actuator current would be effective would depend on the actuator design, since a damagingly large counter-force might make little difference to the current with a highly-geared mechanism such as a worm drive. This is something you should research.

6. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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Since you guys designed it, how about optical detection. e.g. A modulated IR LED. Possibly with prisms to go around the corners?

If you have position feedback, you could "possibly" check the rate of descent or MAP the current vs position.

A mechanical limit bar similar to an elevator.

Not sure how the elevator is now done with an optical system.

A FSR or force sensitive resistor.

Some examples:

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7. ### Tony StewartWell-Known MemberMost Helpful Member

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1) Torque = Current * k

Max torque = max current , occurs at start or braking stop = stall Current rating or "locked rotor " =5x~8x rated current
Motion induced back EMF voltage counteracts stall current
Rated voltage/ rated current = Zo. = 5~8x Coil Resistance which determines start/stop current peak.

2) motor sensing current shunts are chosen to drop very little voltage e.g. <100mV Max
Motor protected polyfuse PTC do the same and are cheap protection.
Normally current shunts with Differential amplifiers can be made or combined in an IC.

Normally current * time limit by timer is measured to detect a fault condition, so an RC value is typical method with a threshold to detect the criteria.

Recovery can be many choices, like toggle reverse door.

As always , write a spec with testable values before design is started to be part of your Design Verification Test (DVT) plan , point form is OK, but list as much as you expect in function and parameters. This is key to any good design, even DIY.

Last edited: Jan 5, 2016
8. ### Little GhostmanWell-Known MemberMost Helpful Member

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Do you have a pic or drawing of the cockpit? and an indication of size?

I have an idea but it would be limited to smaller sizes probably, I have a project that is using cap touch sensing over a fairly big panel. It might be possible to do cap sensing on the canopy, that way the moment the rim was touched it would stop closing and open.