National Instruments NI-Motion User Manual - Pid Loop Descriptions

Chapter 3

Tuning Servo Systems

NI-Motion User Manual

3-4

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PID Loop Descriptions

The following are common variables relating to the PID control loop.

Kp (Proportional Gain)

The proportional gain (Kp) determines the contribution of restoring force 

that is directly proportional to the position error. This restoring force 

functions in much the same way as a spring in a mechanical system.

Each sample period, the PID loop calculates the position error, which is the 

difference between the instantaneous trajectory position and the primary 

feedback position, and multiplies the position error by Kp to produce the 

proportional component of the 16-bit DAC command output.

An axis with too small a value of Kp is unable to hold the axis in position 

and is very soft. Increasing Kp stiffens the axis and improves its disturbance 

torque rejection. However, too large a value of Kp often results in 

instability.

Ki (Integral Gain)

The integral gain (Ki) determines the contribution of restoring force that 

increases with time, ensuring that the static position error in the servo loop 

is forced to zero. This restoring force works against constant torque loads 

to help achieve zero position error when the axis is stopped.

Each sample period, the position error is added to the accumulation of 

previous position errors to form an integration sum. This integration sum is 

scaled by dividing by 256 prior to being multiplied by Ki. 

In applications with small static torque loads, this value can be left at its 

default value of zero (0). For systems having high static torque loads, this 

value should be tuned to minimize position error when the axis is stopped.

Although non-zero values of Ki cause reduced static position error, they 

tend to cause increased position error during acceleration and deceleration. 

This effect can be mitigated through the use of the Integration Limit 

parameter. Too high a value of Ki often results in servo loop instability. 

National Instruments therefore recommends that you leave Ki at its default 

value of zero until the servo system operation is stable. Then you can add a 

small amount of Ki to minimize static position errors.