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BECKHOFF
3.3
Limits and Ranges of Operation
Overview
Another important task in achieving best system performance is setting certain motion limits and ranges of
operation to protect equipment from damage and to optimize operational efficiency.
Two Types of Settings
There are two types of settings for motion limits and ranges of operation:
l
l
Type of Setting
Fault limit
Tolerance band
3.4
Acceleration and Deceleration
Overview
If the servo amplifier is operated with motion tasks under position control, different acceleration/deceleration
profiles can be chosen. It depends on the mechanical structure of the machine and the required dynamical
quality, which profile should be chosen. If the machine tends to sway (e.g. robot arm), sine² would be the best
choice. Here the torque is altered linear and the velocity characteristic becomes square. This reduces the
excitation to sway. Disadvantage of this profile is the double up of the acceleration/deceleration time.
If the machine is mechanically stiff and there are high requirements in dynamics, the linear profile should be
chosen. This leads to a torque step at the beginning and the end of each acceleration/deceleration ramp.
Two Types of Acceleration and Deceleration
The following table describes the two fundamental acceleration and deceleration types, linear and square.
A motion profile may accommodate a combination of these two types.
Accel / Decel Type
Linear
sine²
Setup Software
Fault limits
Tolerance bands
Meaning
Fault limits are settings that signal errors when certain limits on motor movement, such as
speed and position, as well as electrical current, are exceeded. Fault limits are designed
to protect equipment from damage and can cause the drive and motor to shut down.
For example, every motion control system has hardware limit switches, which are used in
the position loop to set a limit on how far the actual motor position can deviate from the
commanded position before a fault is signaled. You may also program software limits via
DRIVE.EXE parameters. The difference, or gap, between commanded position and
actual position is known as following error. Such a limit protects against motor runaway
and stalling.
Tolerance bands are set and specify the safe, efficient physical ranges for the equipment.
Some of these tolerance bands do the following:
—
In the current loop, set a limit on the amount of electrical current to the drive and
motor. This protects the motor from damage that would be caused by excessive
current.
—
In the position loop, place a limit on how far the motor can travel in a positive or
negative direction.
—
Set a range of positions that are considered to be In Position. That is, this range
specifies how far the motor can deviate from its commanded position and still be
considered in the correct position.
Description
Linear is a rate of acceleration and deceleration that theoretically represents a steady
speed-up and slow-down.
To limit any jolting, the drive is accelerated/decelerated within the acceleration time
along an acceleration ramp without any discontinuities. The resulting speed character-
istic corresponds to a sine² curve.
07/03
Servo System Overview
11
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