Above you can see 3d plots that help visualize what an actuator should be able to do. These plots are generated by plotting the torque/speed curve of the motor that’s being used and modifying it based on the gear ratio of the actuator. Now, sometimes, we make actuators that don’t have constant gear ratios. Examples of these are linkages, cam pulleys, etc. They can be very advantageous. In these particular plots the actuators had linkages that created different torque/speed capabilities at different angles. This is why I needed to generate a 3d plot that swept through joint angles as well.

Electric motors are limited by a few things. The first is where we usually fail and that’s in torque capacity. The motor’s torque constant called Kt [Nm/A] multiplied by the current rating of the motor driver will give you a peak torque assuming you don’t exceed the limits of the magnets. This sets the upper bound. There is also the motor constant Km [N*m/sqrt(W)] which dictates an upper power limit of the motor. Then there’s the peak voltage you can achieve with your motor driver. This voltage multiplied by your Kv [rad/s / V] will dictate how fast you can take the motor. If you use these constants and some information from your driver you can create a working envelope for your actuator. In the case where you’re evaluating an already existing system it is quite easy to overlay use data taken from that system on that plot. This helps tremendously to see if you are torque or speed limited.