LOGO! functions
4.4.29
Pulse Width Modulator (PWM)
Short description
The Pulse Width Modulator (PWM) instruction modulates the analog input value Ax
to a pulsed digital output signal. The pulse width is proportional to the analog value
Ax.
Symbol in LOGO!
Wiring
Description
Input Ax
Analog signal to be modulated to a pulsed
digital output signal.
Parameter
A:
B:
T:
p:
Output Q
Gain
Range of values: +-- 10.00
Zero offset
Range of values: +-- 10,000
Periodic time over which the digital output
is modulated
Number of decimals
Range of values: 0, 1, 2, 3
Q is set or reset for the proporition of each time
period according to the proportion of the
standardized value Ax to the analog value
range.
Parameter T
Note the defaults of the T parameters listed in Chapter 4.3.2.
The periodic time T can be provided by the actual value of another
already--programmed function. You can use the actual value of the following
functions:
• Analog comparator (actual value Ax - Ay, see Chapter 4.4.18)
• Analog threshold trigger (actual value Ax, see Chapter 4.4.16)
• Analog amplifier (actual value Ax, see Chapter 4.4.20)
• Analog multiplexer (actual value AQ, see Chapter 4.4.26)
• Analog ramp (actual value AQ, see Chapter 4.4.27)
• Analog math (actual value AQ, see Chapter 4.4.30)
• PI controller (actual value AQ, see Chapter 4.4.28)
• Up/down counter (actual value Cnt, see Chapter 4.4.13).
Select the required function by the block number. The timebase is configurable. For
information on valid ranges and parameter defaults, refer to Chapter 4.4.1.
Parameters p (number of decimals)
Parameter p applies only to the display of the Ax value in a message text
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Functional description
The function reads the value of the signal at the analog input Ax.
This value is multiplied by the value of parameter A (gain). Parameter B (offset) is
added to the product, as follows:
(Ax * Gain) + Offset = Actual value Ax
The function block calculates the proportion of the actual value Ax to the range.
The block sets the digital output Q high for the same proportion of the T (periodic
time) parameter, and sets Q low for the remainder of the time period.
Examples with Timing Diagrams
The following examples show how the PWM instruction modulates a digital output
signal from the analog input value:
1. An analog value of 500 (range 0...1000) as the value for Ax must be modulated
to a digital signal string. The user--defined T (periodic time) parameter is 4
seconds. At the digital output of the PWM function the digital signal string is 2
seconds high, 2 seconds low, 2 seconds high, 2 seconds low and continues in
that pattern as long as parameter En = high.
2. An analog value of 300 (range 0...1000) as the value for Ax must be modulated
to a digital signal string. The user--defined T (periodic time) parameter is 10
seconds. At the digital output of the PWM function the digital signal string is 3
seconds high, 7 seconds low, 3 seconds high, 7 seconds low and continues in
that pattern as long as parameter ”En” = high.
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Calculation rule
Q = 1, for (Ax -- Min)/ (Max -- Min) of time period T, when Min < Ax < Max
Q = 0, for PT -- [ (Ax -- Min) / (Max -- Min) ] of time period T.
Note: Ax in this calculation refers to the actual value Ax as calculated using the
Gain and Offset.
Setting the Par Parameter
The following illustration shows the view in programming mode that corresponds to
the first example:
B1
1+/
Min=+00000
Max=+01000
A=+01.00
Press
A "
B1
2+/
B=+00000
T=00:04s
P=2
Use the and keys to navigate to the Min, Max, A, B, T and P parameters. For
each digit of a value, use the and keys to scroll through value choices. Use
the key to navigate to the second screen from the last line of the first screen,
and the key to navigate from the top line of the second screen to the first screen.
Use the OK key to accept changes.
View in parameter assignment mode:
B1
1
Min=+00000
Max=+01000
A = 1.00
208
Press
A "
B1
2
B =+00000
T =00:04s
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