Encoder accuracy

When mounted properly, the encoder allows for high accuracy which guarantees not only the precise positioning of the actuator but also reduces encoder-based distortions in the control loop and leads to vibration-free motion.

A precise encoder allows to achieve:

• faster reaction time,

• sharper position and velocity loops tuning,

• better tracking of the dynamic trajectories.

In this section the main encoder error types will be considered:

• Wide angle error

• Narrow angle error

The following figure shows a typical behavior before Wide angle calibration:

Wide angle error (long wave error, integral nonlinearity)

This type of error is mostly caused by imperfect mechanical assembly. If the disc rotates with some play or wobbling, this will be reflected in the measured velocity. Thus it is extremely important to produce encoder-relevant mechanical parts with the highest precision possible.

The effect of the wide angle error can be observed in the velocity signal: a sinusoidal component will be added to the velocity.

To inspect the effect of the wide angle error, rotate the encoder ring at a constant velocity that is a multitude of 60 RPM (60 RPM = 1 Hz).

Since the error has a peak once or more times per mechanical revolution, the frequency of the distortion will be a multiple of the mechanical rotation frequency (Rotations Per Second, RPS).

The multitude of this frequency indicates the origin of the error:

• 1 x RPS - Eccentricity of the magnetic ring axis

• 2 x RPS - Ovality of the magnetic ring

• 3-8 x RPS - mounting of the magnetic ring (for example amount of the screws used for fixing the ring)

Narrow angle error (short wave error, differential nonlinearity, Sub Divisional Error)

The error represents distortions of a quadrature sin/cos signals inside the encoder chip. This can be caused by improper magnetic field sensing and data processing, magnetic target imperfections and external disturbances. The positioning of the encoder ring changes the error pattern as well. Therefore, the analogue calibration should be repeated if the mechanical setup was changed.

A tool for calibrating the narrow angle is available in OBLAC Drives.

Important

it is of vital importance for good performance to calibrate the setup with the routine.