Bosch 6000 User's Guide download pdf (Page 231)

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6000 Series Programmer's Guide

Maximum Velocity and Acceleration (Steppers Only)

The slave's attempt to faithfully follow the master may command velocities and accelerations

that the slave axis is physically not able to complete. Therefore, the FMAXV and FMAXA

commands are provided to set the maximum velocity and acceleration at which the slave will

be allowed to move.

If the slave is commanded to move at rates beyond the defined maximums, the slave will begin

falling behind it's commanded position. If this happens, a correction velocity will be applied

to correct the position error as soon as the commanded velocity and acceleration fall within the

limitation of FMAXV and FMAXA.

The FMAXA and FMAXV commands should be used only to protect against worst case

conditions, and should be avoided altogether if they are not needed. If an axis is not able to

follow its profile because of limitations imposed by these commands, some correction motion

will occur. This is due to the Following error and the resulting Dynamic Position

Maintenance (see below). If the maximum acceleration (FMAXA value) is set very low, some

oscillation about the commanded position may occur because the slave is not allowed to

decelerate fast enough to prevent overshoot.

Dynamic Position Maintenance (Steppers Only)

Even while following a master, a slave axis can be in encoder mode with stall detection,

position maintenance, and/or deadband wait enabled. In this mode of operation, a difference

between actual slave position and the desired slave position may arise just as it may with

normal encoder mode moves. Systems with an encoder mounted on a load where mechanical

backlash or product stretching is present will be most prone to these desired vs. actual position

differences.

An axis becomes a slave by specifying a master with the FOLMAS command. When an axis

in encoder step positioning mode (ENC1) becomes a slave axis, it automatically begins the

equivalent of position maintenance. This remains true even if the EPMØ (disable position

maintenance mode) command is given, and even while the axis is in motion.

There is a very important reason for the continuous dynamic position maintenance. In regular

time based moves, the axis has no defined position relationship with anything while it is

moving, but it does have a position goal while at rest. Therefore, position maintenance is

only meaningful when the axis is not moving. When a slave axis is following a master, there

is always a defined position command, calculated from the position of the master. Therefore,

position maintenance occurs even while the axis is moving.

The position error in each sample period is usually very small, so only a small correction

velocity is added to that required for the slave to follow the master at the commanded ratio. The

6000 product limits the maximum correction velocity to that specified with the EPMV command.

Factors Affecting Following Accuracy

There are additional accuracy requirements of Following applications beyond those of standard

positioning. The slave must maintain positioning accuracy while in motion, not just at the

end of moves, because it is trying to stay synchronized with the master.

Assuming parameters such as master and slave scaling and ratios have been specified correctly,

the overall positioning accuracy for an application depends on several factors:

• Resolution of the master

• Resolution of the slave

• Position sampling accuracy

• Accuracy of the slave motor and drive

• Accuracy of load mechanics

• Master position prediction

• Master velocity relative to master position prediction & master position filtering

• Tuning (servos only)

• Dynamic position maintenance (steppers only)

• Repeatability of the trigger inputs and sensors

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Bosch 6000 User's Guide Page 231

Comments to this Manuals

Bosch 6000 User's Guide Page 231

Comments to this Manuals

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