Servo stiffness

Servo stiffness, a.k.a. control stiffness, describes how well a servo motor resists deviation from a setpoint.

For example servo stiffness can be easily visualized with position control servo: stiffness is motor's ability to counter any attempt to displace the axis from the setpoint position.

Stiffness has two easily distinguishable properties:

• static stiffness - how well the controller resists static loading of the actuator
• dynamic stiffness - how fast and strong controller resists dynamic loading of the actuator (i.e. instantaneous change of load)

Examples

Position control mode example

In this example we have a servo motor capable of producing 5 Nm torque.

Servo motor setpoint is constantly zero, so motor ideally holds static position even if its being forced to rotate. Then motor being loaded by intantenous 1 Nm disturbance torque (i.e. someone grabs the shaft ant turns it). If motor shaft turns 30 degress before it returns to zero, it may be considered low stiffness. But if motor turns only 3 degres, it may be considered high stiffness.

Velocity control mode example

In this example we have a servo motor capable of producing 5 Nm torque.

Servo motor setpoint is constantly at 1000 rpm, so motor ideally keeps 1000 rpm speed when its being loaded or spinning without load. Then motor being loaded by intantenous 1 Nm disturbance torque (i.e. motor is being braked). If motor shaft speed dips to 500 rpm, it may be considered low stiffness. But if motor speed drops only to 990 rpm, it may be considered high stiffness.

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