The term “Telemetry” derived from two words “Tele” and “metron”. “Tele” means over a long distance or far and “metron” means to measure. Therefore, telemetry is the measurement of quantities from a long distance. Following article focuses on Electrical Telemetry.
Definition: Telemetry is a process by which data collected or measurements are made at remote or inaccessible location and transmitted to receiving end for monitoring, display and recording.
Generally, the information was sent over wires, but nowadays telemetry more commonly uses radio waves for transmission.
Types of Telemetry Systems
The electrical telemetric systems can be classified as follows.
On the basis of the Signal Transmission Medium or Link Used
1. Wire-link or wire telemetry system
2. Radio or wireless telemetry system
i. Short-range radio telemetry system
ii. Satellite radio telemetry system
3. Optical-fibre or fibre-optic telemetry system
On the basis of the Modulation Method Used
1. DC telemetry systems
i. Direct voltage telemetry system
ii. Direct current telemetry system
2. AC telemetry systems
i. Amplitude modulation (AM) telemetry system
ii. Frequency modulation (FM) telemetry system
3. Pulse telemetry systems
i. Pulse amplitude modulation (PAM) telemetry system
ii. Pulse width modulation (PWM) telemetry system
iii. Pulse phase modulation (PPM) telemetry system
iv. Pulse frequency modulation (PFM) telemetry system
v. Pulse code modulation (PCM) telemetry system
On the basis of the type of Information Signal
1. Analog telemetry systems
i. Direct voltage telemetry system
ii. Direct current telemetry system
iii. Amplitude modulation (AM) telemetry system
iv. Frequency modulation (FM) telemetry system
v. Pulse amplitude modulation (PAM) telemetry system
vi. Pulse width modulation (PWM) telemetry system
vii. Pulse phase modulation (PPM) telemetry system
viii. Pulse frequency modulation (PFM) telemetry system
2. Digital telemetry system
i. Pulse code modulation (PCM) telemetry system
Why is Telemetry Required?
Use of telemetry techniques becomes essential in some cases, where the conventional measurement techniques may not work or not worth.
Remote Location of the Measurand
Sometimes the control room is far away from the plant or quantity to be measured (measurand) in such cases telemetry plays significant role. Telemetry makes it possible to carry out measurements regardless of the distance or movement without being there.
It significantly lowers operational costs and saves the challenge going to inaccessible places.
Inaccessibility of the Measurand
Telemetry provides access to the data that is far or hard to reach places without being physically present to gather the information. Telemetry is ideal for measurement in places considered unsafe and for measuring items that can sustain easy damaged.
It enables you to be aware of the measurements that are taking place. A lot of time and money gets wasted when people opt to measure and download information manually.
Electrical Telemetry System
The block diagram of a basic telemetry system is as shown below.
The function of each of block of the telemetry system shown in the figure is explained below.
Transducer or Sensor
It converts the physical variable to be measured (measurand) into an electrical quantity. In electrical telemetric system, this quantity in most cases is either an electrical parameter such as resistance, inductance or capacitance or an electrical signal that is voltage or current.
Signal Processing Unit or Signal Conditioner-1
It converts the electrical output of the transducer (or sensor) into an electrical signal compatible with the transmitter.
The incompatibility can be such that the sensor’s output is an analog signal but the transmitter is digital one. Similarly, there can be parameter (R, L, C, X or Z) versus signal (V and I), voltage versus current incompatibility may exist. Sometimes, the magnitude of the signal may be too weak to be used by the transmitter.
In general, the purpose of transmitter is to transmit the information signal to the receiver. In case of telemetry, a signal which is a function of the value of the measurand coming from the signal conditioner-1 is transmitted using a suitable carrier signal to the receiving end. It may be used for the following functions.
(i) Amplification: If the signal to be transmitted is weak then it is amplified. In wireless transmission, to suitably amplify the power of the signal before feeding it to the transmitting antenna.
(ii) Modulation: Any one of the modulation techniques is used at the transmitter. In this process any one parameter of carrier signal is changes as per the information signal. On the basis of modulation, the telemetry system can be classified into
- Amplitude modulation (AM) telemetry system
- Frequency modulation (FM) telemetry system
(iii) Signal Conversion: Signal conversion is required for the purpose of transmission depending on the need of receiver.
For example, analog to digital conversion, or electrical signal to radio wave conversion, or electrical signal to optical beam conversion etc., depending on the nature of the carrier signal and medium of transmission.
(iv) Multiplexing: If more than one physical variable is to be transmitted simultaneously from the same location, then either frequency-division multiplexing (FDM) or time-division multiplexing (TDM) is used.
Signal Transmission Medium
It is the medium or link that connects the sending end to the receiving end. Through medium the transmitter can transmit signal to the receiver. In electrical telemetric system, the medium of transmission is wire or wireless.
Generally, wires are used wherever possible. But sometimes the electrical output of the transducer (or sensor) sensing the measurand, cannot be accessed by conventional method of connecting wires.
Therefore, the electrical output is converted to a radio wave, which is then transmitted to the user’s location. This type of telemetry is called Radio Telemetry or wireless telemetry
Its purpose of receiver is to receive the signals from the transmitter through the signal transmission medium and recover the information from the same. Mostly, receiver performs the reverse operation to that of transmitter. It may perform one or more of the following functions.
(i) Amplification: If the received signal is too weak to use for further process, then amplification is done.
(ii) Demodulation: In demodulation information signal is recovered from the modulated signal. From modulated signal the carrier signal is separated and original information signal is recovered. The demodulation process has to be complementary of the modulation performed by the transmitter.
(iii) Reverse Signal Conversion: This conversion is generally the reverse of the conversion performed by the transmitter. Thus. the receiver is required to perform digital to analog conversion, or radio wave to electrical signal conversion, or optical beam to electrical signal conversion etc. depending on the nature of the carrier signal and the signal transmission medium.
(iv) De-multiplexing: It refers to the process of separating various information signals so that they can be delivered to their respective end devices. The process in the receiver has to be essentially the reverse of the multiplexing carried out by the transmitter.
Signal processing unit or Signal Conditioner-2: It takes the output from receiver and make it suitable for the end device.
The element is so called because it appears at the end of the system. The end device may be used for variety of purposes. It may be performing one of the following functions.
(i) Analog Indication: Measured quantity may be shown by Analog indication that is through the deflection of a pointer on a scale. The device used is often a permanent magnet moving coil (PMMC) meter.
(ii) Digital Display: The value of the measurand displayed on LEDs, LCD, monitor screen etc.
(iii) Digital Storage: The values of measurand can be stored in digital format on memory. The stored data then can be used for measurement, R&D and calibration etc.
(iv) Data Processing: The digital values of the measurand may be given to a data processor, such as a microprocessor, digital signal processor or computer, for analysis etc.
(v) Closed-Loop Control: The output of signal conditioner-2 may be fed back to an automatic controller as the feedback signal.