Basic Information For Open Loop Position Control Using Steppers Or Servos - Siemens SIMATIC S7-200 System Manual

Basic Information for Open Loop Position Control Using
Steppers or Servos
Both the PTO built-in to the S7-200 PLC and the EM 253 Position Module use a pulse train output
to control both the speed and position of a stepper motor or a servo motor.
Using the PTO or the module for open loop position control requires expertise in the field of motion
control. This chapter is not meant to educate the novice in this subject. However, it provides
fundamental information that will help as you use the Position Control wizard to configure the PTO
or module for your application.
Maximum and Start/Stop Speeds
The wizard will prompt you for the maximum speed (MAX_SPEED) and Start/Stop Speed
(SS_SPEED) for your application. See Figure 9-3.
MAX_SPEED: Enter the value for the optimum operating speed of your application within
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the torque capability of your motor. The torque required to drive the load is determined by
friction, inertia, and the acceleration/deceleration times.
The Position Control wizard calculates and displays the minimum speed that can be
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controlled by the Position module based on the MAX_SPEED you specify.
For the PTO output you must specify the desired start/stop speed. Since at least one cycle
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at the start/stop speed is generated each time a move is executed, use a start/stop speed
whose period is less than the acceleration/deceleration time.
SS_SPEED: Enter a value within
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the capability of your motor to drive
your load at low speeds. If the
SS_SPEED value is too low, the
motor and load could vibrate or
move in short jumps at the
beginning and end of travel. If the
SS_SPEED value is too high, the
motor could lose pulses on start
up, and the load could overdrive up and the load could overdrive
the motor when attempting to stop.
Open Loop Motion Control with the S7-200
Speed
MAX_SPEED
SS_SPEED
Figure 9-3 Maximum Speed and Start/Stop Speed
Chapter 9
Distance
255