I want to do something that is "not allowed". I want to stall a motor and keep it stalled for a amount of time, exceeding what might be good for the motor! I am aware of the fact that the motor can pull very high amounts of current (up to and sometimes over 8X nom current). The thing is that i need to apply a force to an object and if the object moves, the force on the object has to remain constant hereby dictating that the motor must move as well. My plan is to measure the current in the motor and hereby limit it to a value well under the nominal continuous current - on top of that i will be cooling the motor with a fan. Is there any type of motor that can withstand keeping it at stall better then others? I know that your first response might be that i should use other means like pneumatic or hydraulics but that is not an option!
If you have any kind of input i will be happy for the response.
I assume we are talking about a brush DC motor. Obviously you can stall it all day at 0v power. As you raise the voltage current will flow according to the coil resistance. Up to some voltage stalling will be harmless to the motor. The question is how high can you go and is the safe voltage adequate for you purpose. Cautious testing is your best bet.
Do not ignore the gear box. Well lubricated metal gears may be necessary.
Another approach employs a good quality servo with a strain gauge in front of the load. Some of the better servos will report motor current and EMF.
You should give more mechanical details of your application.
The thing is that i need to apply a force to an object and if the object moves, the force on the object has to remain constant hereby dictating that the motor must move as well. My plan is to measure the current in the motor and hereby limit it to a value well under the nominal continuous current - on top of that i will be cooling the motor with a fan. Is there any type of motor that can withstand keeping it at stall better then others?
Sure, use a geared motor such that the motor can supply the desired force at much less than at its stall current.
I stalled it out (accidentally) and broke one of the metal gears inside the motor. To replace the gear, I contacted the manufacturer - they wanted me to purchase 50 of them, at $7.00 a pop! I had only spent $10.00 on the motor (I ended up buying another surplus motor on Ebay).
So - just be aware that even metal gears might not save you, if the torque is high enough; the motor will just keep going!
It might help the understanding if i included you in the project - i am making a steer-by wire prototype (if you want to read: http://steer-by-wire.info/) that works by applying a force to the wheels hereby making them turn (just as when you turn the steering wheel of your car) until the force from the road is equal to the force applied to the wheels, hereby making the motor stall. As it requires more force the further the wheels have to turn. So what i need to do is steer the wheels with an electric motor where i can control the torque delivered.
I currently have a Brushed DC motor like this one:
Gearing-wise i have made a 4:1 reduction to a rack and pinion gear that converts my circular motion to a linear motion. Using a gear has to be done with some care because if the ratio of the gear is to high, an outside force will have a hard time overcoming the friction and the large reduction.
The reason for the choice of motor was that pancake-motors have very little inertia hereby making them very quick to accelerate and decelerate. It must be a DC motor and type-wise i think brushed is a good choice. BLDC motors are subjected to torque ripple especially at low RPM where i mostly will be using it. A stepper motor also has disadvantages (please correct me if i am wrong) because they are made to move to a specific position not to deliver a certain speed or torque.
Looks like you need a brushed DC motor controller, with an adjustable current limit that you can set to 8A or lower (or current sense, and use it to implement your own current limiting). Try Pololu.
Well i have a motor controller lying around. an OSMC (you probably know it, http://www.robotpower.com/downloads/OSMC_project_documentation_V4_25.pdf) which can supply more than enough power. To sense the current (current is directly proportional with torque, linked by: T=A*k_t where k_t can be found as 6 in the datasheet for the motor) i use a hall-sensor with a lowpassfilter (5kOhm and 10nF) which gives me a slower but readeble filter. I am not sure that it is the correct filter for my application but i will attend to that when fine tweaking the system.
Steer-by-wire! How big is this prototype? This still sounds to me like a possible servo application. You should still consider high end servos that supply current and EMF feedback. Don't you also need some feedback torque applied to the steering wheel?
Mr. Bear sounds like he is on the right track in using motor current as a feedback signal to get constant torque. Just have to impose a maximum current limit to ensure that you can not exceed the maximum continuous dissipation of the motor; very often that limit is imposed by brush heating, but the motor manufacturer should specify a Watt rating which is based on heat load (generally, this is mechanical power produced, not power consumed, and efficiency is always well less than 100% so power consumed is much greater). If the manufacturer supplies characteristic curves for the motors, you can see what current gives that Watt output (and calculate Watts input, efficiency etc.). If you stay below that, using a controller that can have a current limit set, the motor should not overheat.
If you'd like to see a pretty clear description of torque-mode control, you can go to the Roboteq web site and download the user manual for their NextGen controllers.
Hi.
I have now made a schematic of what i have setup (criticism is welcomed, it is for a school project). @ joe mcd, as you can see i have a motor controlling the steering wheel - this is basically a servo with 2 potentiometers as encoders.
I am almost done with wiring up the system so now i have quite a bit of work cut out for me in regulating the damn thing. The issue is that the driver does not have any control of the steering wheel!!! the driver can apply a force on the steering wheel but it is locked in place due to the gear (it is meant to be like that) - the only thing that can rotate the steering wheel is the motor connected to the gear. So when a force is applied to the steering wheel the motors on the (road)wheels will start to turn because an equivalent force is "sent" to the motor. When the steering wheels turn, an error between the two potentiometers (one to measuring the angle on steering wheel and one measuring the traveled distance on the (road)wheels) is calculated - this error is used in my PID loop to move the steering wheel to a position in conjunction with the (road)wheels.
The very tricky part is that because a force in the steering wheel is measured it is very hard to make the system work fluently and smooth. Thats because, the moment the motor starts to turn the steering wheel, the force is reduced hereby making the motor on the (road)wheels produce less torque, making them go back towards the center..... if you run this loop through a few times at high speed you will get a oscillating system..... and i am not quite sure how to go about making it better. My plan is to start very slowly (turning the steering wheel) and then (while tuning the PID and the code) try to increase the speed of which i apply force on the steering wheel. If you have ANY idea, please share, because i am a little bit out of my league by now.
