A sensor measures a variable by converting information about that variable
into a dependent signal of either electrical or pneumatic nature. Cadmium sulfide
resistance varies inversely and nonlinearly with light intensity and we can employ this
device for light measurement. Analog signal conditioning provides the operations
necessary to transform a sensor output into a form necessary to interface with other
elements of the process-control loop.
We often describe the effect of the signal conditioning by the term transfer
function. By this term we mean the effect of the signal conditioning on the input
signal. Thus, a simple voltage amplifier has a transfer function of some constant that,
when multiplied by the input voltage, gives the output voltage. Signal conditioning
can be categorized into the following types:
1. Signal-Level Changes
The simplest method of signal conditioning is to change the level of a signal.
The most common example is the necessity to either amplify or attenuate a voltage
level. Generally, process-control applications result in slowly varying signals where
dc or low-frequency response amplifiers can be employed. An important factor in the
selection of an amplifier is the input impedance that the amplifier offers to the
sensor (or any other element that serves as an input). In process control, the signals
are always representative of a process variable. In accelerometers and optical
detectors, the frequency response of the amplifier is very important.
The process-control designer has little choice of the characteristics of a sensor
output versus process variable. Often, the dependence that exists between input and
output is nonlinear. Even those devices that are approximately linear may present
problems when precise measurements of the variable are required. Specialized
analog circuits were devised to linearize signals. For example, suppose a sensor
output varied nonlinearly with a process variable, as shown in Figure-1a. A
linearization circuit, indicated symbolically in Figure-1b, would condition the sensor
output to produce voltage signal linear with the process variable, as shown in Figure –
1c. The modern approach to this problem is to provide the nonlinear signal as
input to a computer and perform the linearization using software.
systems understand only discrete on/off information, conversion of analog signals to
digital representations is necessary. In analog signal transmission the wiring system
can effectively reduce noise interference. Analog signal transmission employs two-
wire signal leads or three-wire signal leads for high precision and accuracy. The third
signal lead, or shield, is grounded at the signal source to reduce noise. There are many
different wiring options that are available to reduce unwanted noise pickup from
entering the line. Four types of wires are fundamental in data acquisition-plain pair,
shielded pair, twisted pair, and coaxial cable.
1. Plain wire is not very reliable in screening out noise and is not suggested. A
shielded pair is a pair of wires surrounded by a conductor that does not carry current.
The shield blocks the interfering current and directs it to the ground. When using
shielded pair, it is very important to follow the rules in grounding. Again, the shield
must only be grounded at one source, eliminating the possibility of ground-loop
2. Twisted-pairs help in elimination of noise due to electromagnetic fields by twisting
the two signal leads at regular intervals. Any induced disturbance in the wire will
have the same magnitude and result in error cancellation.
3. A coaxial cable is another alternative for protecting data from noise. A coaxial
cable consists of a central conducting wire separated from an outer conducting
cylinder by an insulator. The central conductor is positive with respect to the outer
conductor and carries a current. Coaxial cables do not produce external electric and
magnetic fields and are not affected by them. This makes them ideally suited,
although more expensive, for transmitting signals.
Coaxial Cable Construction
Both types of modulation can incorporate error detecting and error correcting
information to the transmitted signal. However, the latest trend in signal transmission
is forward error correcting (FEC). This scheme, which uses binary numbers, is suited
to digital transmission. Extra bits of
information are incorporated into the
signal, allowing any transmission errors to be corrected at the receiver end.
Analog Signal Transmission
Analog transmission inserts signals of varying frequency or amplitude on
carrier waves with a given frequency to produce a continuous wave. In a telephone
system, an electric current or the reproduction of patterned sound waves are
transmitted through a wire and into the telephone receiver. Once this is completed,
they are then converted back into sound waves.
In digital transmission, the signals are converted into a binary code, which
consists of two elements—positive (1) and non-positive (0). Every digit in a binary
number is referred to as a bit and represents a power of two. As an example of digital
transmission, in a type of digital telephone system, coded light signals travel through
optical fibers and are then decoded by the receiver. When transmitting a telephone
conversation, the light flashes on and off about 450 million times per second. This
high rate enables two optical fibers to carry about 15,000 conversations
simultaneously. Digital format is ideal for electronic communication as the string of
1s and 0s can be transmitted by a series of “on/off” signals represented by pulses of
electricity or light. A pulse “on” can represent a 1, and the lack of a pulse “off” can
represent a 0. Information in this form is very much easier to store electronically.
Furthermore, digital transmission is usually faster and involves less noise and
disturbances as compared to analog data transmission.
Analog signal transmission uses direct current (dc) variations in current or
voltage to represent a data value used to communicate information. Most data
acquisition signals can be described as analog, digital or pulse. While analog signals
typically vary smoothly and continuously over time, digital signals are present at
discrete points in time. Analog signals represent continuously variable entities such as
temperatures, pressures, or flow rates. Because computer-based controllers and