for a current project, I need to imitate the movement of a measuring tape with a DC-motor.
A string is wrapped around a roll. Imagine it like on a garden hose.
The string can be pulled in both directions. Like with a measuring tape, when you pull on it, there is a light "force" opposing the pulling of the string.
When no one pulls on the string, the string should stay in the "reel"
When no one pulls on the string but it is already outside of the reel, it should retract with a constant velocity
External force applied:
When someone pulls on the string, there should be a light force opposing the pulling
When someone holds the string in place, there should also be a light force opposing it
In conclusion, it resembles a measuring tape. Everyone knows how it "behaves".
The Problem:
I have done some research. There are torque motors designed for this but for higher loads and higher torques. Also, they are quite big for my liking.
Applying a constant voltage should deliver a constant velocity, but too fast and i could imagine when using gears, it would be really hard to push against it.
I thought of something like a constant torque mode with a built in max velocity in one direction.
If anyone knows something, it would be a big help! Thank you!
Edit:
Please consider that the shaft of the motor should provide a constant torque. Always.
The shaft can be moved either way (freely), but the torque should still be there.
All motor devices use current ,not voltage. Current produces the magnetic field that all motor devices use. Your design also produces heat. How much heat can your device stand?
The tapes I have used speed up pretty violently, if I don't manage the speed with a thumb on the brake.
Constant voltage on a DC motor delivers constant torque, which will speed rotation up. Increasing speeds increase the back EMF and reduces the voltage. At slow speeds, the back EMF is negligible.
Numerically, How fast should the velocity be, how constant should it be, and how hard should a person have to pull to offset the winding torque? How much do the bits weigh?
That is exactly the Problem. I am unsure how this could be designed in order to decrease the heat output. Currently, i have no working prototype because I couldn't decide which method to use / which methods even exist.
I am unsure if i could just put a voltage across a motor and call it a day.
Wouldn't there be excessive heat generation at low speeds? Especially when external torque pulls against the motor or even stalls the motor completly?
Numerically, maybe a maximum velocity of around 0.5 m/s would be enough (This should be adjusted experimentally).
It does not have to be fast, it just needs to tighten the string between the reel and the users hand
(i want to avoid a loose string).
Also, the torque shouldnt be alot too. It just needs to tighten the string (when pulled) and retract it when nothing touches it.
My previous method was just to use a clock spring which is used in measuring tapes. Nevertheless, i need to design it electronically, so the mechanical method is obsolete.
So when someone pulls on it, a magnetic field opposes the movement? Are there small magnetic clutches available? Unfortunately, there is a size constraint.
Also, when would you retract?
Imagine i pull at the string and maybe go back a little and then pull it again.
How would you detect this and retract in order to tighten the string?
0.5m/sec is about 1RPS an 2cm axle, or 0.5rps on a 4cm axle.
Maybe look at the tricks in:
It suggests using small motors with cheap encoders to send varying PWM to control the speed at slow rates. 0.5RPS in the demo. With an encoder you could also detect if he motor is stopped.
Yes that right. But in my project, i need it to retract constantly to tighten the string.
I think the biggest problem is that i need to create a constant torque in any case. The torque is there for the acceleration to the max velocity (retraction) and to oppose the user that pulls just aaaaa little bit.
The standard method is to rapidly turn the current on and off using a MOSFET in series with the motor. Use a pwm voltage from your Arduino to turn the MOSFET on and off, thereby adjusting the average current to the motor.
The project mentioned is a good choice for a constant drive of the shaft. You shouldnt forget that i want to move the shaft of the motor freely, so there is no constant velocity basically. It just need to have it as its "goal" to reach. In order to reach this max velocity, it excerts a constant torque, which is always there, even when i turn the motor's shaft.
If the string went over a mechanical tensioner then if the tension was relaxed the feedback from the tensioner could be used to control the motor clutch combo
I have seen similar methods used with printing presses where a constant tension is required on the paper roll as the diameter diminishes
If you can drive the motor at a chosen speed and can know the rotational position, you can change the speed wit position.
Real tape measures use a coiling/uncoiling spring to provide the tenstion, with tension increasing as the tape is pulled out, and tension decreasing as the tape winds back into the housing.
Besides speed control during motion, PWM would let you put a fraction of the energy into the motor when stopped and reduce the wasted energy/heat.
