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13. CONTINUOUS SENSORS

•There are a number of defining properties of interest when considering sensors,

-Accuracy - a sensor will statistically vary about an exact reading. If we take a statistical range for all readings (e.g., ±3 standard deviations) this will be a reasonable accuracy. Accuracy can also be given as a relative value (e.g. percentage)

-Resolution - Typically used for systems that ‘step’ through readings. This is the smallest increment that the sensor can detect.

-Repeatability - When a single sensor condition is made and repeated, there will be a small variation for that particular reading. This is the repeatability.

-Range - Natural limits for the sensor

-Dynamic Response - the frequency range for regular operation of the sensor. Typically sensors will have an upper operation frequency, occasionally there will be lower frequency limits.

-Environmental - temperature and humidity limits exist for many sensors

-Calibration - most sensors require some degree of calibration, and their readings may drift over time.

-Cost

13.1 INPUT ISSUES

•Analog signals are more complicated to deal with than digital signals. This is primarily because electrical noise will degrade the quality of the signal quickly.

•To deal with this there are a number of measures to be taken,

shielding - shielding is used to reduce the effects of electromagnetic interference. single/double ended inputs - shared or isolated reference voltages (commons).

• A simple shielding example is shown below. A shielded cable has a metal sheath. This sheath needs to be connected to the measuring device to allow induced currents to be drained. This prevents electromagnetic waves to induce voltages in the signal wires.

Analog voltage source

+

-

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A Shield is a metal sheath that	Analog Input
surrounds the wires

IN1

REF1

SHLD

• The common voltage for each analog signal may be the same but more often it is different. If the commons are tied together we have a single ended system. If each signal is given its own common the connections are double ended. Most analog input cards allow a choice between one or the other.

device	+						device	+
					Ain 0						Ain 0
#1	-						#1	-
					Ain 1						Ain 0
device	+						device	+
					Ain 2						Ain 1
#1	-						#1	-
					Ain 3						Ain 1
					Ain 4						Ain 2
					Ain 5						Ain 2
					Ain 6						Ain 3
					Ain 7						Ain 3
					COM

Single ended - with this arrangement the signal quality can be poorer, but more inputs are available.

Double ended - with this arrangement the signal quality can be better, but fewer inputs are available.

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•Signals from transducers are normally small and cannot be directly input into an analog input. To make these signals more usable Signal Conditioners are used.

•Signal conditioners are normally amplifiers to increase the signal strength, but some will also change the signal (eg. conversion from current to voltage).

•An example of a simple single ended amplifier is given below,

+V
		offset		Rg
		Rf
-V				gain

-

+

Vin Ri

Vout

• An example of a differential amplifier with a current input is given below. Note that Rc converts a current to a voltage.

	Iin					-
					+

Vout

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• An example of a differential to single ended amplifier is given below.

-

+

Vin				-
		+

Vout

-

+

CMRR adjust

• The circuit below can be used to amplify the output of a resistive device.