Automating Manufacturing Systems - Process Control and ...

plc analog - 22.20
Aside: A basic low pass RC filter is shown below. This circuit is suitable for an analog
output that does not draw much current. (drawing too much current will result in large
losses across the resistor.) The corner frequency can be easily found by looking at the
circuit as a voltage divider.
V PWM
R
C
V analog
1
--------- ⎛ ⎞
⎜ jωC ⎟
V analog
V PWM
--------------------
1
⎟
⎛ ⎞
= ⎜ = V
1 PWM
-----------------------
⎝ ⎠
R + ---------
jωCR ⎜ ⎟
+ 1
⎠ jωC ⎝
V
--------------- analog 1
= -----------------------
V PWM
jωCR + 1
corner frequency
1
ω = -------
CR
As an example consider that the PWM signal is used at a frequency of 100KHz, an it is to
be used with a system that has a response time (time constant) of 0.1seconds. Therefore
the corner frequency should be between 10Hz (1/0.1s) and 100KHz. This can be put at
the mid point of 1000Hz, or 6.2Krad/s. This system also requires the arbitrary selection
of a resistor or capacitor value. We will pick the capacitor value to be 0.1uF so that we
don’t need an electrolytic.
1
R ------- -------------------------
1 10 4
= = -------
Cω 10 – 7 2π10 3 = = 1.59KΩ
2π
Figure 22.16
Converting a PWM Signal to an Analog Voltage
In some cases the frequency of the output is not fixed, but the duty cycle of the output
is maintained.
22.3.3 Shielding
When a changing magnetic field cuts across a conductor, it will induce a current