The operation of these motors is governed by a speed-torque curve.
This means that the peak torque is achieved at no speed.
And as speed increases, the provided torque linearly decreases.
This can be modeled with a force damper system as seen in this image.
This force damper model is a useful abstraction of the speed torque curve.
This speed torque curve can be simply explained by a force damper model.
The force in this Model F is attenuated by a damper with constant beat.
Again, we see that at no speed, the full force of the actuator is available.
Whereas at larger speeds, the damper diminishes the output force.
Most electric motors are used in legged robots with the addition of a gear box.
The gearbox effectively increases the output torque and decreases the speed.
This allows the maximum power of the actuator to be achieved
at reasonable speeds.
While gears are very helpful to amplify torque they are also fragile and lossy.
In gear boxes there's considerable static, dry and viscous friction.
Furthermore the motor's inertia is reflected through the gear box so
that the inertia at the output is proportional to the gear ratio squared.
Gil Pratt made famous the notion of a series elastic actuator
in which a spring is placed between the load and the motor.
This is very helpful in that it protects the gears from shock loading
It also improves the compliance of the system and
it changes force control into a position control problem.
By changing the effective length of the spring
forces can be modulated when the actuator is in contact with the load.