Automating Manufacturing Systems - Process Control and ...

continuous sensors - 23.32
23.2.9 Others
A number of other detectors/sensors are listed below,
Combustion - gases such as CO2 can be an indicator of combustion
Humidity - normally in gases
Dew Point - to determine when condensation will form
23.3 INPUT ISSUES
Signals from sensors are often not in a form that can be directly input to a controller.
In these cases it may be necessary to buy or build signal conditioners. Normally, a signal
conditioner is an amplifier, but it may also include noise filters, and circuitry to
convert from current to voltage. This section will discuss the electrical and electronic
interfaces between sensors and controllers.
Analog signal are prone to electrical noise problems. This is often caused by electromagnetic
fields on the factory floor inducing currents in exposed conductors. Some of
the techniques for dealing with electrical noise include;
twisted pairs - the wires are twisted to reduce the noise induced by magnetic fields.
shielding - shielding is used to reduce the effects of electromagnetic interference.
single/double ended inputs - shared or isolated reference voltages (commons).
When a signal is transmitted through a wire, it must return along another path. If
the wires have an area between them the magnetic flux enclosed in the loop can induce
current flow and voltages. If the wires are twisted, a few times per inch, then the amount
of noise induced is reduced. This technique is common in signal wires and network cables.
A shielded cable has a metal sheath, as shown in Figure 23.32. This sheath needs
to be connected to the measuring device to allow induced currents to be passed to ground.
This prevents electromagnetic waves to induce voltages in the signal wires.