Telemetry may be defined as measurement at a distance. In this article our focus will be on DC telemetry system and its types such as voltage and current telemetry system. Let’s have a brief idea about electrical telemetry.
Electrical Telemetric System
The electrical telemetring system consists of a transmitter, which converts the input measured into an electrical signal, which is transmitted through a telemetering channel and is received at the other end by a receiver located at a distance in a remote location.
This signal is then converted into a suitable usable form by the receiver and then can be indicated, recorded by the final stage device, which is calibrated in terms of the measurand.
The electrical telemetry systems can be broadly classified as
(i) dc systems, and (ii) ac systems.
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DC Telemetring System
The dc telemetring system is categorized as
(i) Voltage Telemetring system
(ii) Current Telemetring system
(iii) Position Telemetring system
In a dc telemetring system the signal is transmitted through a channel, which utilizes direct transmission through cables in order to convey the information.
Voltage Telemetry System
- A voltage telemetring system transmits the measured variable as a function of an ac or dc voltage.
- A simple voltage telemetring system is shown in figure.
- As seen from the diagram, a slide wire potentiometer is connected to a battery.
- The variable sliding contact of the potentiometer is moved by the pressure sensitive bourdon tube, which expands as the pressure is applied.
- The telemetring channel consists of a pair of wires, which are connected to a voltage-measuring device such as a null balance dc indicator or recorder.
- As the measured pressure changes, the bourdon actuates the sliding contact, which in turn changes the voltage. The dc null balance potentiometer measures the voltage and positions the pointer on a scale calibrated to measure pressure.
- The advantage of using a null balance dc potentiometer is, it reduces the current carried by the telemetring channel to a minimum.
- The primary elements used by most of the systems produce a voltage signal. The primary elements used by most of the systems produce a voltage signal. The primary elements can be thermocouples, tachometers, etc.
- The application of voltage systems in the industry is limited to distances of upto about 300 metres.
- Most of the receivers often use self-balancing potentiometers for such systems.
- A voltage telemetring system requires high quality circuits than the current type. The signal-to-noise ratio must be comparatively high.
- Since the power level is small for the voltage telemetring system, the transmission channel must be well protected from noise and interferences, which can be of the same order of the signal.
Disadvantages of Voltage Telemetry System
- These systems are affected by line resistance, leakage, interfering sources nearly, noise and require higher-quality circuits than current systems, especially for low voltages.
- The voltage telemetering system is limited for transmission upto 300 metres distances.
- A voltage telemetering system is very much suited for adding several input voltages in series provided the measurement is linear. However, this system needs a relatively more expensive receiving terminal.
- This system is normally not suitable to the use of many receivers at the same time.
Current Telemetry System
The basic version of a current telemetring system is shown in Figure.
- This is similar to the voltage telemetring system except that the slide wire potentiometer is now connected in series with the battery.
- The sliding contact of the potentiometer is connected to the pressure sensitive bourdon tube, which moves as the bourdon tube expands.
- In this case, the telemetring channel consists of a pair of wires, which are connected to a current measuring device.
- As the pressure changes, the bourdon tube expands, which in turn changes the position of the sliding contact on the (potentiometer) slide wire.
- This changes the current flow in the circuit. This current is measured by a milliammeter, whose scale has been calibrated to read pressure directly.
The most commonly used current telemetring systems are the
- motion, and (ii) force balance types which are improved version of the basic type.
Motion Balance System
In a motion balance system, the potentiometer or the slide wire is replaced by a position detector, such as LVDT as shown in Figure.
- In this system, the bourdon tube is connected to the core of the LVDT. When pressure is applied to the bourdon tube, the bourdon tube expands causing a displacement.
- This moves the core of the LVDT, thereby producing an output voltage, which is then amplified and rectified. This voltage produces a dc current of the order of 4 nA to 20 mA in the telemetry channel and is measured by the milliammeter.
- The scale of the milliammeter is directly calibrated in terms of pressure that is being measured.
Force Balance Systems
A force balance system is as shown in Figure, a part of the current output is fed back to oppose the motion (movement) of the input variable.
- The bourdon tube operates the system which rotates the feedback force coil.
- This in turns changes the flux linkage between the primary and the secondary coils.
- This change in flux linkages varies the amplitude of the amplifier. The output signal is then fed back to the feedback force coil, which in turn produces a force opposing the bourdon input.
- A force balance system increases the accuracy, as smaller motions are required which results in better linearity.
Advantages of Current Telemetry System
- The current systems can develop higher voltages than most voltage systems and, consequently, it can be made more immune to the effect of thermal and inductance voltages in the interconnecting leads as well as line resistance.
- Simple D.C. milliammeters can be used with special calibration for line resistance.
- Several receivers can be operated simultaneously.
- The received signals can be added or subtracted directly.
- Changes in line resistance are compensated by basic feedback method.
- The response of the system to an input change is almost instantaneous.
Disadvantages of Current Telemetry System
- The energy level is adequately high to minimise the effects of extraneous voltages.
- This system is also not suitable for long distance since the current output is varied by means of an adjustable resistance in the line.