Velocity Mode Control for Servo Drives

In the context of servo drives, velocity mode refers to a control mode where the servo drive turns the motor at a velocity that is proportional to the command. Small commands will generate small velocities and large commands will generate large velocities. Velocity Mode requires the use of a velocity feedback device, such as an encoder, resolver, tachometer, or Hall Sensors.

Full Control Block Diagram Velocity Highlighted

A common misconception is that velocity mode is only useful for constant motion applications like fans, pumps, and conveyors. However, from the 1980’s to early 2000’s the standard configuration for position mode applications was to set the servo drives for velocity mode. This configuration can be descriptively called “position around velocity” where the motion controller closes the position loop by sending appropriate velocity commands to the servo drive. Examples of this control scheme would have been found on production line robots in the automotive industry and on CNC machines.

Velocity mode block diagram

The appeal for position around velocity was that it split the computational requirements between the controller and servo drives.  At the time, controllers weren’t quite powerful enough to handle position around torque and the servo drives weren’t quite ready to close the position loops on their own. These days both servo drives and controllers are more than capable to close any and all loops thus giving system designers the flexibility to use the control scheme that is best for the situation.

Other than position around velocity applications velocity mode can be used in stand-alone type applications that need either a constant set velocity or in situations where the velocity is only adjusted occasionally.  For example, a conveyor belt or a spindle motor.  The velocity could be set using the offset adjustment on the servo drive, using a dial connected to a potentiometer or by using a digital/analog converter on a microcontroller – as some examples.

In these applications, the servo drive receives a command signal specifying the desired velocity or speed at which the motor should operate. The servo drive then adjusts the motor's output to achieve and maintain the desired velocity. It continuously monitors the motor's actual velocity through sensor feedback, such as Hall sensors, encoders, or tachometers, and adjusts the motor's input power or voltage to match the desired speed.

Velocity Equation

In velocity mode, the motor speed is controlled by the amount of voltage sent to the motor. Changing the motor’s velocity (to accelerate or decelerate) requires an increase or decrease in motor torque, so a current control loop is also required in velocity mode.

When more than one control loop is used, the loops are cascaded, with current control being the innermost loop and the velocity control loop added “around” the current loop.

Unlike current mode, a servo drive operating in velocity mode will compensate with more power to maintain the target velocity if an outside force acts on the motor shaft. This makes the mode useful for motion control applications where something needs to maintain a target velocity in the face of interference. This acts very much like the cruise control function on many automobiles; even when traveling up and down hills, your speed remains the same while cruise control is active

Benefits

  • Excellent for applications that use simple controls to set a constant velocity or where the velocity doesn’t need to be adjusted frequently.
  • A good solution if a controller isn’t computationally powerful enough to control multiple axes using position around torque.
  • Hall Velocity Mode is an economical solution in applications where the majority of operation occurs above 500rpm and not external velocity feedback is available other than the Hall sensors built into the motor.

ADVANCED Motion Controls' Capabilities

  • Many AxCent® servo drives can operate in Velocity Mode
  • All DigiFlex® Performance™ and FlexPro® drives can operate in Velocity Mode
  • Certain DigiFlex® Performance drives can operate in Profile or Cyclic Synchronous Velocity Mode

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