Pour les anglophone, voici une explication simple trouver sur CNC Zone
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Velocity mode servos have lower performance and are more difficult to tune than torque mode servos.This is because a velocity mode servo has 90 degrees less phase margin than a torque mode servo (velocity is the first integral of acceleration).Torque mode controls acceleration, velocity mode controls speed.
Sounds complicated but it's not.
This car gas pedal analogy should help:
1) The gas pedal on your car acts as a torque mode input and you personally complete the PID feedback loop when you drive.The gas pedal modulates acceleration;press a little and you accelerate slowly, press hard and you accelerate quickly.
Your goal is to maintain a freeway speed of 80 MPH (it's a California freeway).You press the pedal hard on the freeway on-ramp and accelerate quickly, say 10 MPH/sec.As your speed builds and approaches 80 MPH, you begin to ease off on the pedal, lessening your acceleration, say 1 MPH/sec.You smoothly round off acceleration to zero as you hit 80 MPH.Once you are at speed you easily maintain it by making small adjustments in pressure on the pedal (small accel, decel).
2) What if the gas pedal was a speed control (velocity mode)?Press it to the floor and the car will accelerate like a bat from Hades to 120 MPH.Release the pressure and it will panic-brake to 0 MPH. Acceleration and deceleration is either zero or neck-snapping;there is nothing in between.
You reach 80.001 MPH and you ease the pedal pressure the tiniest bit.The result is full panic braking to 79 MPH.You minutely increase pressure;the result is full-tilt acceleration to 81 MPH if you are the least bit late in releasing pressure.Repeat the process and you get an ever increasing oscillation;75/85, 70/90, 60/100, etc. The only way you could maintain 80 MPH is by making the tiniest micrometer adjustments to the pedal and by making them VERY slowly.
Most people prefer car (1) to car (2).
Mariss
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Puis une réponse de la Ste Galil qui confirme le choix d'une utilisation en Mode Couple pour les CNC.
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Torque mode is ideal for positioning movements. The advantage of running an amplifier in velocity mode is when your application calls for a constant speed with low velocity ripple. Typically during point to point moves, or positioning movements, holding a constant and perfect velocity through the move is not critical. What is critical is maintaining and holding position at the end points. In most applications this is easier to tune with an amplifier in Torque mode.
For a CNC machine, holding position under a load is typically the most critical aspect. With a properly sized amplifier this can be obtained regardless if it is a torque or velocity mode amp.
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Ok, le mode de sera en Couple, prochaine étape....
