Active Damping

Active Damping is an algorithm that reduces oscillation in a motion control system. It can help control systems prone to oscillation or with low damping, such as pneumatic cylinders. Active damping requires information on the accelerations or forces acting on the system. The active damping algorithm uses this information to affect the Control Output so that the accelerations or forces do not cause oscillations.

Active damping gives best results when a secondary input is used. Although in some cases, active damping can be achieved using only the primary position or velocity feedback.

Active Damping is especially useful for controlling fluid power systems with significant compression in the fluid. This includes:

Pneumatics
Due to the compressibility of air, pneumatic systems are notorious for oscillating. The active damping limits the oscillation, resulting in much better control.
Small Hydraulic Cylinders with Large Loads
In hydraulic cylinders with a large mass and a relatively small bore, the effects of the compressibility of the oil are the most pronounced, and can result in oscillation. Active damping limits the oscillation, resulting in much better control. In certain cases, the use of active damping can decrease the bore of the cylinder required, which can also reduce the size of the hydraulic power unit.

Deciding whether a System Needs Active Damping

To determine whether a system needs active damping, give it an open loop Control Output. If the system oscillates, especially after the initial start of motion, it could benefit from active damping.

Active Damping Options

Active damping requires information on the accelerations or forces acting on the system. This information can be obtained in a variety of ways. The table below lists the options available. Typically, using accelerometers or force feedback provides the best control. Position-based acceleration does not provide as good control.

Active Damping Type	Possible Sources	Required Axis Type
Force	Secondary Force Input (single- or dual-input)	Position-Force* or Velocity-Force*
Acceleration Input	Secondary Acceleration Input (single- or dual-input)	Position-Acceleration or Velocity-Acceleration
Position-based or or Velocity-based Acceleration	Position or Velocity Input (uses the Actual Acceleration of the feedback) Unfiltered Acceleration (not recommended) Filtered Acceleration (not recommended) Modeled Acceleration	Position or Velocity

*In rare cases, Active Damping with pressure feedback is possible on a position-pressure or velocity-pressure axis, requiring a secondary pressure input, but is generally not recommended.

Setting Up Active Damping

Pressure or Force Input

Define a position-pressure or position-force axis. On the RMC75, this secondary pressure or force input requires an AP2 module. On the RMC150, the CPU must have the pressure control option, designated as RMC151.
Set the High-Order Control parameter to Active Damping.

Acceleration Input

Define an axis with a secondary acceleration input. If you are using one accelerometer, choose Accel (single-input). If you are using two accelerometers, choose Accel (dual-input, diff.). On the RMC75, the secondary acceleration input(s) require an AP2 module. On the RMC150, the CPU must have the pressure control option, designated as RMC151.
Set the High-Order Control parameter to Active Damping.

Position-based

Set the High-Order Control parameter to Active Damping. This is only valid if the axis is position only. If the axis has a secondary feedback, the active damping will use the secondary feedback.
Set the Acceleration Filter Type parameter to Model or Low Pass. This is necessary to obtain usable acceleration readings. See the modeling and filtering topics for details.

Velocity-based

Set the High-Order Control parameter to Active Damping. This is only valid if the axis is position only. If the axis has a secondary feedback, the active damping will use the secondary feedback.
Set the Acceleration Filter Type parameter to Model or Low Pass. This is necessary to obtain usable acceleration readings. See the modeling and filtering topics for details.

Tuning Active Damping

Active Damping adds the Active Damping Proportional Gain and Active Damping Differential Gain to the PID or I-PD control algorithm. See the Tuning Active Damping and Acceleration Control topic for details on tuning these gains.