Elastic pressure elements or mechanical type of transducers are used for measurement of very high pressures up to about 700MN/m^2. There are three main types of pressure elements. These are:

(i.) Bourdon tube

(ii.) Bellows

(iii.) Diaphragm

Most pressure measuring devices use elastic members for sensing pressure at the primary stage. These elastic members are of many types and convert the pressure into mechanical displacement which is later converted into an electrical form using a secondary transducer.

Bourdon tube:

Pressure gauges using Bourdon tube find wide applications because of their simple design and low cost, and are most commonly used for local indications and for signal transmission to remote locations. There are three types of Bourdon elements and they are: (i.) C type (ii.) Spiral type (iii.) Helical type  

C Type:

This type of tube most commonly used for local indication but it is also used for pressure transmission and control applications. The tube which is oval in section is formed into an arc of 250˚ & hence the name is C.

One end called the tip of the tube is sealed and is called free end, is attached by a light link-work to a mechanism which operates the pointer. The other end of the tube is fixed to a socket where the pressure to be measured is applied. The internal pressure tends to change the section of the tube. The degree of linearity depends upon the quality of gauge from oval to circular, and this tends to straighten out the tube. The movement of the tip is ideally proportional to the pressure applied. The tip is connected to a spring loaded link-work and a geared sector and pinion arrangement which amplifies the displacement of tip and converts into the deflection of the pointer. The linkage is constructed for optimum linearity and minimum hysteresis as well as to compensate for wear which may develop over the time.

With neat sketches discuss significance of following aspects of signal conditionings for any one of the sensors: amplification, conversion, filtering, modulation/demodulation, and grounding.

The signals given by a transducer may be nonlinear in nature or may contain noise. Thus before sending these signals to the system it is essential to remove the noise, non linearity associated with the raw output from a sensor or a transducer. It is also needed to modify the amplitude (low/high) and form (analogue/digital) of the output signals into respective acceptable limits and form which will be suitable to the systems. Thus, activities are carried out by using signal conditioning devices and the process is termed as ‘signal conditioning’.

Amplification:

Various applications of control system such as machine tool control unit of a CNC machine tool accept voltage amplitudes in range of 0 to 10Volts. However, many sensors produce signals of the order of milli volts. This low level input signals from sensors must be amplified to use them for further control action. Operational amplifiers (op-amp) are widely used for amplification of input signals.

Conversion:

Conversion is the context of measuring instruments refers to conversion from analog to digital output or vice versa converter. Most mechanical instruments produce an output signal in the analog form. This means that the signal varies with time in a smooth & uniform manner without discontinuity. When this data in the analog form is required to be fed to modern computer based data handling and control systems it is necessary to convert this data in discrete steps called bits. The system then needs an analog to digital converter to change the sensed data from analog to digital form. Conversely in some situations where the data is received in the digital form it may be necessary to use a digital to analog converter to convert the signal into continuous uninterrupted output.

A common example for the use of A/D & D/A converter for the same application is the telephone circuitry. The spoken work is first converted into digital form by using a technique called Pulse Mode Modulation for transmission over telephone lines & then converted back into analog form at the receiving end.

Filtering:

It is the process of eliminating unwanted components of a measurand while allowing desired components to pass. Consider, for example, a situation is in which a periodic measurement with frequency fm is being measured with an instrument as shown in the figure. The sensor/transducer stage of the instrument produces a signal which is of the same form as the measured parameter but the instrument also picks up from surrounding noise signals with a higher frequency fn.

These noise signals may occur at the detector/transducer stage or the signal processing stage of the instrument.

In the amplification stage both the measured signal & the noise are amplified. The signals reaching the indicator/recorder stage thus have a distorted form in which the high frequency noise signals are superimposed on the measurand signal as shown in the figure.

This leads to a distorted final display of the measured. If now, a low pass filter is introduced between the amplification and recording stages, the high frequency noise signals will be eliminated & only the correct low frequency signals corresponding to the measured will reach the presentation stage of the instrument. The filter allows a band of frequencies known as pass band to pass through while ideally all the remaining frequencies called attenuation band are filtered out.

Modulation/Demodulation:

Quite often the signal from the sensor/transducer stage needs to be processed for smoothening, linearization, filtration or conversion from analog to digital form. For these processes the transducer signal is superimposed on a high frequency wave form called carrier in such a way that it is impossible later to recover & display the original signal.

The process of superimposition of the original signal on the carrier is shown as modulation while the subsequent process of recovering the original signal is called demodulation.

The original signal which is not mixed with the carrier is known as pure signal and the signal mixed with the carrier is called mixed signal.

Fig. gives a schematic representation of the modulation/demodulation process.

Grounding:

All low level electrical circuits need some form of grounding for one or both of the following reasons:

1. To provide an electrical reference for various sections of the device.
2. To provide a drainage path for unwanted current. As shown in fig. grounding can be of two types:

1. Earth ground or connection to earth

2. Chassis ground connection to the shielding enclosure within which the circuitry is mounted. In this case the grounding process does not involve earth at all. This is the type of grounding employed for aircraft and space craft circuitry.