Do hobby servos respond to step inputs with a step in torque?

Question

I'm designing a piston that I need to expand a few cm in <100ms to load energy into a spring-mass system with around a pound of force (as shown below). Based on power specs alone, high-end hobby servos are light, strong and cheap enough for my needs: they simulate well when I'm able to push max voltage into them right away (i.e. operating them on the max torque-speed curve).

What I'm concerned about is the torque behavior in real-world servos. I'm interested in digital high-torque, high-frequency servos, typically used for steering in RC vehicles, that are either coreless or brushless. As a representative example, consider something like the Savox SV1272 or SB2274 (resp.), with refresh rates of 333Hz.

Is anyone aware if servos like this hold back on their power output with some kind of integrating control/loop delay in response to a step function? The tight timing of the stroke is what really concerns me: even if they wait for ~50ms I could lose most of the energy.

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Looking around for clues, I've seen this comment here that links to this linearized position model of a Futaba servo as an example. This model is a PID controller in position rather than torque, so I imagine this is the response you get from operating below the torque-speed ceiling, while I really want to know the delta → torque behavior along that ceiling.

Answer 1

Generally, for small movements (or steps), most servos are torque limited to prevent overshoot and oscillations. If you change the input pulse timing by 20uS, the motor will not reach full torque before it starts braking the motor to stop. The motor torque is also throttled at the end of the desired motion to prevent overshoot of the target position. Even when moving about 10 degrees (100uS change in pulse width) the motor will not reach full torque before it starts to get throttled to stop. At movements of about 15 degrees the motor will reach maximum speed in about 20-40 mS (depends on load), but then is throttled to stop within 10-20 mS. Greater movements sustain maximum speed for longer, of course.

So, If 50 mS is problematic in your system, you will likely have problems with the servo response.

One additional note is that servos can behave poorly with elastic loads.

As far as the specific servos that you reference, I don't have experience with those. The second one claims 12 bit resolution, I'm guessing this is the encoder resolution that is probably a magnetic sensor rather than a potentiometer like the old-school servos. If these servos are very high accuracy, this definitely requires some advanced motor current control schemes that will limit torque for low angle moves and at the end of the movement as the angle approaches the set point.

Some scope traces measuring motor current @300mA/V Ch1 and motor voltage (across motor terminals) Ch2. Servo is JX 5521 digital with a constant elastic load. Notice that the rise time on motor current stays the same no matter the step size of the setpoint. Also, you can see by the commutation glitch frequency that the speed is still increasing even after the current reaches its maximum. Time constant on this servo is about 28 mS for max current and >70 mS for max speed.

Image 1 - 100 uS step from 1500 uS to 1600 uS.

Image 2 - 200 uS step from 1400 uS to 1600 uS.

Image 3 - 400 uS step from 1300 uS to 1700 uS.