I am working on my B.Tech major project where I am building a Upper Limb Exoskeleton for rehabilitation of Stroke Patient. I intend to actuate the exoskeleton using Electric motors. However I cannot accurately determine the torque requirements of the motors at Shoulder joint and elbow joint. Please help
I don't think there's a single obvious answer, any more than there is for natural shoulders and elbows. I think you need to establish requirements, such as weight you want the arms to be able to lift, and then work backwards to calculate the torque required.
Torque = lever arm length * F * sin(theta)
where F is weight you must be able to lift, and theta is the angle between the lever arm and the force having to be applied. To make things simple, you might usually set up the problem with theta set to 90 degrees.
I would set up different scenarios. Such as, assume the arm is extended straight, elbow locked. A relatively long lever arm. Now lift the weight spec you have established (which is ultimately an arbitrary value, so you might want to consider what your own arm can do). That should be the worst case torque requirement for the shoulder motor.
Then you can do the same for the elbow. Keep the upper arm locked in place, and lift a weight using only the elbow.
Perhaps you want to require these motors to be capable of doing push-ups or pull-ups? That would make your torque requirements dependent on the weight of the person the exoskeleton is being designed for. Pull-ups would make a simple problem, to determine the torque of only the elbow motor. And so on.
Hi,
The approach proposed by Albert Manfredi is what we have applied on one of our exoskeleton project. I would like to add that you are talking about an electric "motor", you might also consider using an electric "joint" which includes a motor + a gear unit such as harmonic drives (most widely used in lower limb exoskeletons as far as I know). Thus, the required torque for the motor, if it seems too much, could be reduced by a gear ratio that you could apply. Note, that the gear is a source of loss and non-backdrivability but it may (the calculation has to be done) increase compactness of the total joint.
Regards
I was stuck in a similar kind of problem while designing a lower limb exoskeleton. I think that the solution for the upper arm is quite similar to that of the lower limb exoskeletons. This is how I calculated the specifications.
The biomechanical data for walking was obtained from already performed experiments. One can do his own experiments using the motion capture system as well. There are numerous sources and I decided to proceed with the data set provided by the Army Research Laboratory. The maximum joint moments for the knee, hip and ankle were obtained. Since I was focused on using Series Elastic Actuators (SEA) without any CVT (Continuous Vehicle Transmission), I looked for some existing models. I selected the Yobotics SEA with the minimum continuous force value at the maximum speed. Then, I computed the lever arm value from the following equation.
Maximum Torque Required (let's say for knee)= Continuous Force x Lever Arm
I believe my methodology is correct and this answer might help. Comments and constructive criticism are welcome.
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