Bosch 6000 User's Guide download pdf (Page 229)

214

6000 Series Programmer's Guide

Master Position Prediction

Master Position Prediction is a technique used to compensate for the fact a slave's position

command cannot be calculated and implemented infinitely fast.

The master position prediction mode is enabled by default (FPPEN1) in the Following

algorithm, but can be turned off as desired with the FPPENØ command.

Position Sampling Period

Steppers – 2 milliseconds.

Servos –

system update

period

(depends on SSFR

and INDAX command

values – see SSFR

command description).

The 6000 controller measures master position once per position sampling period, and

calculates a corresponding slave position command. This calculation and achieving the

subsequent slave commanded position requires 2 sample periods.

If master position prediction mode is disabled (FPPENØ), waiting 2 sample periods results in a

slave position lag. That is, by the time the slave reaches the position that corresponds to the

sampled master position, 2 sample periods have gone by, and the master may be at a new

position. Measured in time, the lag is 2 sample periods. Measured in position, the lag is 2

sample periods ∗ current slave velocity.

For example (stepper controller), suppose the slave is traveling at a speed of 25000 counts per

second. If master position prediction mode is disabled (FPPENØ), the slave will lag the

master by 100 counts (25000 counts/sec ∗ 4 ms = 100 counts).

By measuring the change in master position over sequential sample periods, the master's

present velocity is calculated. The present master velocity and position are used to predict

future master position. If master position prediction mode is enabled (FPPEN1, the predicted

future master position is used to determine the slave's position command. In this case the

slave has no velocity-dependent phase delay. The slave's velocity for a given sample will

always be the velocity required to move from its current position to the next calculated

position command.

If the master motion is fairly smooth and velocity is not very slow, the measurement of its

recent velocity will be very accurate, and a good way of predicting future position. But the

master motion may be rough, or the measurements may be inaccurate if there is no filtering

(see Master Position Filtering below). In this case, the predicted master position and the

corresponding slave position command will have some error, which may vary in sign and

magnitude from one sample to the next. This random variation in slave position command

error results in rough motion. The problem is particularly pronounced if there is vibration on

the master.

It may be desirable to disable the master position prediction mode (FPPENØ) when maximum

slave smoothness is important and minor phase delays can be accommodated.

If master filtering is enabled (FFILT≠Ø), then the prediction algorithm would be used on the

filtered master position, resulting in a smoother slave position command. However, due to the

delay introduced by the filtering, the prediction algorithm would not compensate for the total

delay in the slave's tracking command. (See also Master Position Filtering below.)

Master Position Filtering

Position Sample Period

Steppers – 2 milliseconds.

Servos –

system update

period

(depends on SSFR

and INDAX command

values – see SSFR

command description).

The slave axis' position command is calculated at each position sample period. This calculation

is a function of the master position and the master velocity estimated from the change in master

position over 2 position sample periods.

The Master Position Filter feature allows you to apply a low-pass filter to the measurement of

master position. Master position filtering is used in these situations:

• Measurement of master position is contaminated by either electrical noise (when analog

input is the master) or mechanical vibration.

• Measurement noise is minimal, but the motion that occurs on the master input is

oscillatory. In this case, using the filter can prevent the oscillatory signal from

propagating into the slave axis (i.e., ensuring smoother motion on the slave axis).

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Bosch 6000 User's Guide Page 229

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Bosch 6000 User's Guide Page 229

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