Module 3. Characteristics of instruments and measurement systems
Lesson 8
Static characteristics of measuring instruments - III
8.1 Hysteresis
Hysteresis is a phenomenon under which the measuring instrument shows different output effects during loading and unloading. Hysteresis results from the inelastic quality of an element or device. It is the maximum difference between corresponding upscale and downscale outputs for any single test cycle, that is, the maximum difference in output at any given value of the measured variable within the specified range when approaching that point first with increasing the input and then decreasing the input. Figure 8.1 illustrates the hysteresis phenomenon.
Fig. 8.1 Input and output hysteresis
The numerical value of hysteresis is specified in terms of output or input values. The sum of the two effects of dead band and the hysteresis is determined directly from the deviation values of a number of test cycles. Hysteresis is determined by subtracting the value of dead band from the corresponding value of hysteresis plus dead band for a given input. The maximum difference in the readings is reported as hysteresis. The difference may be expressed as a percent of ideal output span. For example, the hysteresis is reported as 0.2% of output span. Hysteresis results due to the presence of several factors, such as, mechanical friction, motion in bearings, magnetic and thermal effects. This could also be due to a free play or looseness in the mechanisms.
8.2 Threshold and Resolution
8.2.1 Threshold
Threshold of a measuring instrument is the minimum value of input signal that is required to make a change or start from zero. This is the minimum value below which no output change can be detected when the input is gradually increased from zero. In digital system, the out put is displayed in incremental digits. Thus, in digital instruments the threshold is the minimum input signal which is necessary to produce at least one significant digit of output to indicate on the display.
8.2.2 Resolution
When an instrument is showing a particular output reading, there is a lower limit on the magnitude of the change in the input measured quantity that produces an observable change in the instrument output. That means, when the input is slowly increased from some arbitrary input value, which is non-zero, the output does not change at all until certain increment is exceeded. This increment is called resolution or discrimination of the instrument. Thus, the resolution refers to the smallest change of input for which there will be a change output.
In the analog instruments, the resolution is also determined by the observer’s ability to judge the position of pointer on the scale. One of the major factors influencing the resolution of an instrument is how finely its output scale is divided into subdivisions. Using a car speedometer as an example again, this has subdivisions of typically 20 km/h. This means that when the needle is between the scale markings, we cannot estimate speed more accurately than to the nearest 5 km/h.
The difference between threshold and the resolution of the measuring instrument could be understood this way. Threshold defines the smallest measureable input, while the resolution defines the smallest measureable input change. Both of these values may be expressed in terms of an actual value or as a fraction / percentage of the full scale value.
8.2.2.1 Numericals
1. Determine the resolution of a voltmeter which has a range readout scale with 100 divisions and a full-scale reading of 100 V. If one tenth of a scale division can be read certainty, determine the resolution of the voltmeter.
Solution:
100 scale division = 100V
One scale division = 100 /100 = 1V
Resolution = 1 x 1/10
= 0.1 V
2. A transducer measures a range of 0-200 N force with a resolution of 0.20 percent of full scale. What is the smallest change in the force which can be measured by this transducer?
Solution:
Range of force = 0-200 N
Resolution = 0.20 % of full scale
Smallest change in force which can be measured = 200 x 0.20 / 10
= 0.4 N
8.3 Noise
Noise is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects. In general noise is an error or undesired random disturbance of a useful information signal, introduced before or after the detector and decoder. The noise is a summation of unwanted or disturbing energy from natural and sometimes man-made sources. The quality of a signal is often expressed quantitatively as the signal-to-noise ratio.
Noise may be defined as any signal that does not convey any useful information. Extraneous disturbances generated in the measuring system itself or coming from outside, frequently constitute a background against which a signal may be read.
There are many sources of noise. Noise may originate at the primary sensing device, in a communication channel or other intermediate links. The noise may also be produced by indicating elements of the system.
The common sources of noise are given below:
i). Stray electrical and magnetic fields present in the neighbourhood of the instruments produce extraneous signals which tend to distort the original signal. The effects of these stray fields can be minimized by adequate shielding or relocation of the components of the instruments.
ii). Mechanical shocks and vibrations are another source of trouble. Their effect can be eliminated by proper mounting devices.
8.4 Loading Effect
An ideal measuring instrument should not change or distort the original signal. The sensing element should not draw the process or current from the circuit, thereby resulting in the true measurement of parameters being measured. Unfortunately, in the real world, all instruments draw current and invariably draw energy from the system. This is referred to as the ‘loading effect’. This causes parameters being measured to change in value.
The loading effects may occur due to both electrical and mechanical elements. The loading effects of an Instrument are the alternations that are caused in the circuit conditions such as voltage, current etc. when the instrument is introduced in the circuit for the purpose of measurement. In simple terms, loading effects of an instrument ends up distorting the signal they are supposed to measure in the form of attenuation or phase shift. The instrument therefore reads the altered value of the quantity and thus an erroneous measurement is resulted. These loading effects can be better explained by the following examples.
Let a voltage has to be measured across a resistance R in the circuit. For this purpose, a voltmeter V is connected across the resistor R in parallel. We know that a voltmeter has a very high resistance value. But since this value is finite, a fraction of the total current passing through R will pass through the Voltmeter V. This will lead to power dissipation in the Voltmeter. The voltmeter extracts this power out of the circuit and thus end up varying the values of the circuit parameters on being introduced. Another example of loading effects is found in the measurement of current by an Ammeter. An ammeter is a very low resistance device that is connected in series in a circuit for the measurement of current. When current passes through it, because of the low finite resistance of the ammeter, there is a small voltage drop across the ammeter which results in power dissipation. This power is again borrowed from the circuit and therefore affects the circuit parameters.
So to minimize this ‘loading effect’, the best you can do is to use a measuring instrument that has very high impedance so that the current it draws is minimal.
But practically an infinite or zero impedance in an instrument is not possible. So a condition of minimum loading effects is tried to be reached by making the values of the impedances of instruments very high or very low according to their use. That is why Voltmeters always have very high while Ammeters always have very low impedance values.