Telemetry in Instrumentation Systems

Telemetry can be defined as the science of gathering information at some remote location and transmitting the data to a convenient location to be examined and recorded.

Importance of Telemetry

Within instrumentation there is usually a requirement for telemetry in order to transmit data or information between two geographical locations. The transmission may be needed to enable centralized supervisory data logging, signal processing or control to be exercised in large-scale systems which utilize distributed data logging or control subsystems. In a chemical plant or power station, these subsystems may be spread over a wide area.

Telemetry may also be required for systems which are remote or inaccessible, such as a spacecraft, a satellite or an unmanned buoy in the middle of the ocean. It can be used to transmit information from the rotating sections of an electrical machine without the need for slip rings. By employing telemetry-sensitive signal processing and recording, equipment can be physically remote from hazardous and aggressive environments and can be operated in more closely monitored and controlled conditions.

Techniques utilized in Telemetry

Telemetry can be accomplished by employing different techniques: optical, mechanical, hydraulic, electric, and so forth. The mechanical methods, either pneumatic or hydraulic have acceptable results for short distances and are used in environments that have a high level of electromagnetic interference and those circumstances where, for security reasons, it is not possible to use electrical signals such as explosive environs. Modern technologies like optical fiber systems allow the measurement of broad bandwidth and have high immunity to noise and interference. Other newest telemetry systems are based on ultrasound, capacitive or magnetic coupling and infrared radiation however these telemetry methods aren’t regularly used. The main advantage of electric over mechanical telemetry techniques is that electrically based telemetry does not have practical limits regarding the distance between the measurement and the analysis locations, and can be easily adapted and upgraded in already existing infrastructures.

Increasingly, telemetry in instrumentation is being undertaken using electrical, radio frequency, microwave or optical fiber methods. The communication channels employed include transmission lines utilizing two or more conductors which may be a twisted pair, a coaxial cable, or a telephone line physically connecting two sites; radio frequency (RF) or microwave links which allow the communication of data by modulation of an RF or microwave carrier; and optical links in which the data is transmitted as a modulation of light down a fiber-optical cable. All these techniques utilize some portion of electromagnetic spectrum.

Electric telemetry techniques can be classified depending on the transmission channel that they use as wire telemetry and wireless (or radio) telemetry. Wire telemetry is the simplest technological solution. Some of the limitations of wire telemetry are the low bandwidth and low transmission speed that it can support but it can be used where the transmission wires can utilize the already existing infrastructure, for instance, in many electric power lines that can also be utilized as wire telemetry carriers. Wireless telemetry is more complex than wire telemetry, as it requires a final radio frequency (RF) stage. In spite of its complexity, wireless telemetry is widely used because it can transmit information over long distances; hence, it is used in those applications in which measurement location is not normally accessible. It can also transmit at higher speeds and has sufficient capacity to transmit several channels of information if needed.

A General Telemetry System

The figure below illustrates a general telemetry system (not all blocks are shown):

With reference to the figure above, the telemetry system block diagram consists of:

• Transducers – to convert physical variables to be measured into electrical signals that can be easily processed.
• Conditioning circuits – to amplify the low-level signal from the transducer, limit its bandwidth and adapt impedance levels.
• Signal processing circuit – this may be integrated with the previous circuits.
• Subcarrier oscillator whose signal will be modulated by the output of the different transducers once processed and adapted.
• Codifier circuit, which can be a digital encoder, an analog modulator or a digital modulator, that adapts the signal to the characteristics of the transmission channel, which is a wire or an antenna.
• Radio transmitter, in wireless telemetry, modulated by the composite signal.
• Impedance line adapter, in case of wire transmission, to adapt the characteristic impedance of the line to the output impedance of the circuits connected to the adapter
• Transmitting antenna for wireless telemetry.

At the receiving end, we have similar modules. For wireless telemetry, these modules are:

• Receiving antenna – designed for maximum efficiency in the RF band used.
• Radio receiver with a demodulation scheme compatible with the modulation scheme.
• Demodulation circuits for each of the transmitted channels.

For wire telemetry, the antenna and the radio receiver are replaced by a general front end to amplify the signal and adapt line impedance to the input impedance of the circuits that follow.

The transmission in telemetry systems in particular wireless ones is done by sending a signal whose analog variations in amplitude or frequency are a known function of the variations of the signals from the transducers. In modern systems, digital telemetry systems send data digitally as a finite set of symbols, each one representing one of the possible values of the composite signals at the time it was sampled. The effective communication distance in a wireless system is limited by the power radiated by the transmitting antenna, the sensitivity of the receiver and the bandwidth of the RF signal. As the bandwidth increases, the contribution of noise to the total signal also increases, and as a result more transmitted power is required to maintain the same signal-to-noise ratio (SNR).

Measurements systems may need to acquire either different types of same type of data at different locations in the process that is being monitored. These different information signals can be transmitted using the same common carrier by multiplexing the data signals. Multiplexing allows different signals to share the same channel. Multiplexing techniques normally utilized include frequency division multiplexing (FDM) or time division multiplexing (TDM). In FDM, different subcarrier frequencies are modulated by the different measurement channels, which cause the information spectrum to shift from base band to the subcarrier frequency. Thenceforth the subcarrier frequencies modulate the RF carrier signal, which allows the transmission of all desired measurement channels simultaneously. In TDM, the whole channel is assigned entirely to each measurement channel, although only during a fraction of time. TDM methods use digital modulation to sample the different measurement channels at different times. Subsequently, these samples are used sequentially to modulate the RF carrier.

Related articles:

• Pneumatic Signal Transmission System
• Sources of Coherent Interference/Noise in Instrumentation Systems
• Basics of Radio Frequency (RF) and Wireless Communication Systems
• Features of Analog Signal Transmission
• RF Power and Voltage Measurement