Collecting data is a tedious and time-consuming component in many research projects
or experiments. Although necessary, it is a task that we would rather not spend
much time on.
Data acquisition and control allows us to automate the long hours of routine data
collection, freeing us for tasks that can make a difference.
How Does a Data Acquisition System Work?
Overview
A data acquisition and control system turns physical phenomena (temperature, pressure,
flow, position, speed) into data that can be stored, analyzed, displayed, or acted
on. DAC systems can be divided into 10 key parts for further discussion.
Transducers
Transducers are devices that transform physical phenomena (temperature, pressure,
flow, strain, position, speed, pH, acceleration, etc.) into electrical parameters
(voltage, current, resistance, pulses, etc.) that can be measured by instrumentation
Signal Conditioning
Signal conditioning is used in data acquisition systems to put the analog or digital
signal into a form that can be accurately measured by the equipment. The signal
may be too weak, too strong, too noisy, or in the wrong shape to be measured accurately.
The conditioning may be accomplished on the transducer end of the wiring or in the
measurement hardware.
Switching
A switch is a mechanical or electronic device that is used to make or break the
connections in an electrical circuit. Switches can be used to route the output signals
from multiple transducers to one measurement instrument (scanning). Switches are
also used as control devices to switch power to external fans, heaters, pumps, lights,
and motors.
Switches come in a variety of forms. A relay is an electrically-controlled device
where a mechanical part moves to make or break the circuit. Reed relays and armature
relays are two examples. A solid state switch is an electronic device that makes
or breaks the circuit. Field Effect Transistor (FET) switches are examples of solid
state switches. In general, solid state switches will outlast relays but can only
switch low voltages. The following table summarizes some common switch types.
Scanning
Multiplexers are electronic devices that are used to switch multiple channels
to a single measurement instrument, one at a time.They operate much the same way
as a television set, although several channels are available, only one channel is
viewed at a time.
Multiplexers are available in one-wire, two-wire, three-wire and even four-wire
varieties, each with its own special usage. One-wire (sometimes called single-ended)
multiplexers are used in applications where the measurement instrument is close
to the transducers and a common ground is practical. Two-wire (differential) multiplexers
are the most common type and are used with instruments that have a differential
(high and low) input.
Three-wire (guarded) multiplexers are used with high-end, guarded voltmeters for
the best accuracy in analog measurements (see the measurement hardware section on
A/D converters). Four-wire multiplexers are used in making four-wire measurements
on transducers such as RTDs (see Appendix C) that require a current source.
Some A/D converter cards come with a multiplexer on the card. In cardcage products,
the multiplexers are generally on a separate card from the A/D converter. In this
way, several multiplexers can be tied to one A/D converter. Some multiplexers require
the user to solder wires to the card while others supply screw-down terminals. For
convenience, some cardcage multiplexers supply a separate terminal block. Wires
from the transducers are attached to the terminal block without removing the multiplexer
from the cardcage. The terminal block simply attaches to the end of the multiplexer
card.
Control switching
Switching can also be used to supply power to external fans, pumps, motors, or lights
by completing a circuit. Power is normally supplied outside the DAC system. The
switch card acts much like a light switch, turning power on or off to the device.
In some cases, the DAC switch card supplies a small voltage signal to a larger external
relay which in turn supplies the power to the device.
