Hello everyone, I am still fairly inexperienced with arduino and wanted to make sure I'm going to purchase the correct components for this project before I commit too much money to the wrong things so any help would be appreciated. I have an arduino uno and will be purchasing an A4988 stepper motor driver along with the stepper motor once I know a little more of what to look for.
I'm trying to build a device that uses a motor to move an arm in an arc to help pull a shirt on and off of someone who is unable to do it on their own. I am thinking that since the motor is turning the arm in a vertical arc and potentially pulling the shirt down onto someone, and the reverse, that it would need to be able to exert about 5 kg of force. The arm measures 482.6 mm from the point of rotation to the end on my full scale model. I'm not 100% sure that I did the calculations for the torque needed correctly but here is what I used, F = mg (5 kg * 9.81 N/kg = 19.05 N) and M = Fd (19.05 N * 482.6 mm = 23.67 N*m).
I was going to select a stepper motor because I would like the arm to stay in position when a button is released until the device is powered off or another button is held. If there is a better choice of motor for this application I would love to know though.
The program is going to be similar to Robin2's example "Simple Stepper Program" where 2 buttons control the direction of the motor rotation, but operating wirelessly through a bluetooth connection.
Does anyone have any suggestions or guidance that they could offer for this? Thank you.
torque = Fd (if the arm is perpendicular to the applied force) = 49 N*0.48 m = 102 N.m.
You would want a motor capable of exerting perhaps twice that torque.
Next consider rotational speed and power required: Torque (N.m) = 9.5488 x Power (kW) / Speed (RPM)
Sorry, I misread my notes when I typed up the calculations. I did have have 49.05 N as the force and ended with the torque value of about 24 N*m that I stated earlier.
I figured I should look for a motor that could produce more torque than that just for some factor of safety but am constrained by budget at the moment and haven't been able to find a stepper motor that fit my needs yet.
MarkT:
The OP can multiply better than you it seems! 24Nm is about it.
Clearly a lot of reduction gearing is needed, 100:1 or more, and its not clear a stepper is needed at all.
I know that gearing is potentially to way to make up for the torque but I would sacrifice speed. My only real need for the speed is that the whole arc of the arm/time it takes to have the shirt pulled over or on the individual is not absurdly long. So maybe a minute or two for the arc if the control button were to be held the whole time.
If a stepper motor may not be the way to go is there a specific type of motor that comes to mind for this application? I was thinking stepper motors because of their ability to maintain position as long as power is supplied. Thinking of if the shirt is partially on the person and they need to wiggle around to make things line up properly that it would pull. And that I may need to program in limits, thinking that I could potentially count the steps to where I need the limits and put them in that way.
edgemoron:
You're swinging a 19" radius arc of how many degrees in how much time (minimum)?
It needs to go 170 degrees and ideally it would take maybe two minutes. This is my first project working with motors so I don't know how realistic that is. I'm aiming for it to take as much time as it might for this person to have help from another person to put a shirt on.
cornwalllb:
I know that gearing is potentially to way to make up for the torque but I would sacrifice speed.
Yes, absolutely, but you have a vast amount of speed to spare. You may find a DC motor with
reduction gearing and a simple low-res encoder on the motor (not the gearbox output) is cheaper
option. Geared steppers tend to be high quality expensive components, cheap-and-cheerful
gearmotors are usually DC-brushed.
An geared 24Nm motor would weigh 10's of kilograms as motor torque is proportional to the volume
of the motor's rotor, roughly speaking, as its limited by the properties of iron and copper.
Well, 0.24 RPM at 24 Nm is only about 0.6 Watts of mechanical power, I would look for a 2 - 3 Watt worm gear type gearmotor so it won't need a brake to hold position, shaft RPM will depend on your mechanical arrangement, (direct to arm shaft, cog belt transmission, etc.) and suiting your planned power supply voltage.
EDIT: A brushless DC (BLDC) motor so no brushes & commutator to wear out.
cornwalllb:
... If a stepper motor may not be the way to go is there a specific type of motor that comes to mind for this application? ...
A stepper motor would work but it would seem (to me) to provide a level of precision that isn't really needed here. And it's not the cheapest option.
What you can find easily depends a lot on where you are located geographically.
Where I am (USA) one of the online surplus sites I use shows a 6" 665-lb 24v DC linear actuator for $36.
There are also smaller and cheaper screw-jack units for operating car power seats; the same place has a few different kinds for $15 (these are 12v dc, not 24v). Also with these you'd have more constructing to do, since they don't easily mount up to anything but a car seat as they are.
This also sounds a bit risky, if the person being helped is that physically unable... You could also have a foot-wheel they would spin, that would turn a chain to pull the arm up and down. This removes the need for the electrical motor entirely and makes it harder for them to get hurt (I'd think...).
A stepper motor could be dangerous as the available torque might be very large (static torque is
much larger than dynamic torque, so you have to live with excess low-speed torque). Given the application
I think you are forced to use a system which is physically unable to apply a large force that could strangle.
This means using a motor that will stall at a given torque even if the electronics goes nuts.
Thank you for the different suggestions of motors. Seems like DC may be the better choice instead of a stepper.
Doug384:
This also sounds a bit risky, if the person being helped is that physically unable... You could also have a foot-wheel they would spin, that would turn a chain to pull the arm up and down. This removes the need for the electrical motor entirely and makes it harder for them to get hurt (I'd think...).
Unfortunately the person doesn't have the range of motion or strength to operate anything with their legs, and has limited use of their upper body. This is why I was aiming for a wireless controller with buttons that they could press. So they can just focus on being lined up with the shirt as it is pulled over them and down their back.
MarkT:
A stepper motor could be dangerous as the available torque might be very large (static torque is
much larger than dynamic torque, so you have to live with excess low-speed torque). Given the application
I think you are forced to use a system which is physically unable to apply a large force that could strangle.
This means using a motor that will stall at a given torque even if the electronics goes nuts.
I have been trying to consider all of the different safety risks with this project as I go. I have a different disability than the person the project is for, but it is allowing me to be a guinea pig for some parts of it since we are a similar size and would be using this device while seated. I had planned to test the device thoroughly to make sure any kinks had been worked out, and someone else will review it when I am done with the project. But that is good to know about that risk.
I had concern over stalling the motor out because I didn't want to burn anything out, but that is definitely the better option if it comes down to risking injury to the person or damage to the device.