Using The Pwm (Pulse Width Modulation) Output - Siemens SIMATIC S7-200 System Manual

Using the PWM (Pulse Width Modulation) Output

PWM provides a fixed cycle time output with a variable duty cycle. The PWM output runs
continuously after being started at the specified frequence (cycle time). The pulse width is varied
as required to effect the desired control. Duty cycle can be expressed as a percentage of the
cycle time or as a time value corresponding to pulse width. The pulse width can vary from 0% (no
pulse, always off) to 100% (no pulse, always on). See Figure 9-1.
Since the PWM output can be varied from 0% to
100%, it provides a digital output that in many ways
is analogous to an analog output. For example the
PWM output can be used to control the speed of a
motor from stop to full speed or it can be used to
control position of a valve from closed to full open. control position of a valve from closed to full open.
Configuring the PWM Output
To configure one of the built-in outputs for PWM control, use the Position Control wizard. To start
the Position Control wizard, either click the Tools icon in the navigation bar and then double-click
the Position Control Wizard icon, or select the Tools> Position Control Wizard menu command.
See Figure 9-2
1. Select the option to configure the
onboard PTO/PWM operation for
the S7-200 PLC.
2. Choose the output Q0.0 or Q0.1
that you wish to configure as a
PWM output.
3. Next select Pulse Width
Modulation (PWM) from the drop
down dialog box, select the time
base of microseconds or
milliseconds and specify the cycle
time.
4 4. Select Finish to complete the Select Finish to complete the
wizard.
The wizard will generate one instruction for you to use to control the duty cycle of the the PWM
output.
Open Loop Motion Control with the S7-200
Cycle Time
Pulse Width
Time
Figure 9-1 Pulse Width Modulation (PWM)
Figure 9-2 Configuring the PWM Output
Chapter 9
Pulse Width
Time
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