What are the types of signal transmission?

13.2.8 Baseband transmission

Baseband transmission is described as the raw electrical signal transmission typically surrounding some zero value. A signal is considered “baseband” when it includes frequencies at or near 0 Hz up to the highest frequency in the signal. For instance, a baseband digital transmission signal emanates a series of 0s and 1s from a PC to some other device. For an analog device, such as a telephone, human voice impresses an alternating current over a conductor of a phone line to transmit sound to the receiver. Baseband signals can be distance limited; therefore, some type of modulation scheme is necessary in order to condition the signal for long-distance transmission. The physical device used for modulating and demodulating a carrier signal is typically colocated on to the same device called a modem (or modulator/demodulator).

12.4.2 Positioners

Positioners are not now considered to be the universal solution to many problems. In fact, in some control circuits the effect of fitting a positioner can be detrimental. Whilst it is an advantage to use positioners in slow systems, it can be a disadvantage in the case of control loops with short reset times. The best solution is to correctly size the actuator and spring and ensure that the valve functions correctly without the need for a positioner.

The use of a positioner however should be considered for:

split-range systems, where a controller controls more than one valve; although it is now preferable to split the controller output electronically, see Section 12.4.5, and control each valve in a range by a full range signal from the control room

valves where the actuator working pressure is greater than the control signal pressure, higher actuator pressures are used to provide sufficient force to ensure correct valve movement, a frequently used range is 0.4 to 2.0 barg, 6 to 29 psig

actuators operating at higher pressures to increase actuator “stiffness”

where it is necessary to achieve the best possible control with a minimum of overshoot and the fastest possible reaction in systems with long pneumatic signal lines between the valve and regulator. In this case it would be better to use electric analogue or digital signal transmission

where the control loop reacts slowly to changes in valve position, and very accurate positioning is therefore desired

A positioner should also be considered for relatively slow systems, such as mixing/separation, level and temperature control when the volume and mass of the fluids handled are large compared to the process action.

The valve positioner is a feedback mechanism. It allows valves to be positioned precisely in accordance with the output signal of the controller, in the presence of major disturbances caused by unbalanced forces acting on the valve stem, changes in actuator temperature, etcetera. Even a relatively small actuator, that otherwise would have to be bench-set to an unacceptable degree, can be stroked precisely when a valve positioner is used to signal the valve position. Positioners will accept a pneumatic control signal between 0.21 and 1.03 barg. The air supply pressure should be at least 0.4 bar above the maximum outlet pressure and inlet pressures up to 6 barg are fairly standard. The positioner outlet pressure to the actuator will adjustable between 0% and nominally 100% of the supply pressure. Positioner hysteresis should be not greater than 0.5%.

Figure 12.32 indicates, diagrammatically, the arrangement of a standard bellows positioner fitted to a diaphragm actuator.

Generally, the positioner housing is attached to the actuator yoke, as shown in Figure 12.33.

The positioner senses the movement of the valve plug by means of a lever, which is connected to the stem between the plug and the actuator, see Figure 12.34. The signal from the controller, 0.21 to 1.03 barg, is transmitted to the positioner bellows via the instrument connection. The bellows expands as the input signal pressure increases, while the internal tension spring tries to restrain movement. Compressed air from the air supply line supplies the relay where it meets both the relay valve trim and a small restrictor jet. The air then passes to a chamber which is totally enclosed by a diaphragm, except for an escape nozzle to atmosphere. This nozzle remains open if the beam and flapper assembly are positioned at the mid-point of its travel.

Whether the beam and flapper assembly are moved to the right or to the left depends upon the bellows signal input setting and the valve position. If for example, the beam and flapper assembly is moved to the left by the position cam or the control signal bellows, the nozzle will then be constricted and more air will flow through the restrictor than through the nozzle, causing pressure to build up in the diaphragm chamber. The relay valve trim presses to the left, closing the exhaust channel and opening the supply pressure line to the top of the actuator diaphragm.

The increased pressure pushes the actuator downwards, thereby moving the cam lever, causing the cam to rotate. This allows the beam and flapper assembly to move away from the nozzle, thus creating a balanced situation where the nozzle releases an equal amount of air, supplied by the restrictor. The relay valve trim then adopts the balanced position, closing both the supply of new air to the top of the actuator diaphragm and also the exhaust channel from the actuator top to atmosphere.

The valve is now balanced and remains in the position dictated by the signal input. If the signal pressure to the bellows is reduced, then the beam and flapper assembly will move away from the nozzle causing the pressure in the diaphragm chamber to fall. The exhaust channel opens and air is released. The pressure in the top of the actuator reduces i.e. the diaphragm pressure is less, whereby the actuator spring causes the valve stem to travel upwards. The cam follows the movement, causing the nozzle opening to correspond with the restriction opening. The relay is in balance and the valve adopts the position dictated by the input signal. No air is released from the top of the actuator. The valve is now once again ready to receive a new control signal via the bellows input. The cam can be “characterised”, that is the profile modified, to adjust the valve flow characteristics.

The positioner is usually fitted with gauges, see Figures 12.35 and 12.36. The gauges indicate:

supply air pressure

signal input pressure from the controller

signal pressure to the diaphragm

The positioner can be fitted with by-pass valves which allow the output signal of the controller to be transmitted directly to the actuator diaphragm.

NOTE: It is dangerous to use positioners fitted with by-pass valves, in conjunction with control valves operating in split-range arrangements, or where the diaphragm pressure is higher than the controller signal pressure. The use of by-pass valves in such systems is therefore not recommended and they should be removed, if any are fitted, before the valves are commissioned.

Figure 12.37 shows a schematic arrangement of a positioner piped to a piston cylinder. The instrument air pressure signal from the controller, 0.21 to 1.03 barg, 3 to 15 psig, acts on the positioner bellows. The bellows tend to expand as the signal pressure increases and the bell crank pivots about the fulcrum.

This has the effect of closing relay nozzle A and opening relay nozzle B. This causes:

the pressure to increase in relay A which compels the air pressure above the piston to increase; top cylinder pressure

the pressure to decrease in relay B which causes a corresponding reduction in air pressure below the piston; bottom cylinder pressure

This “out-of-balance” causes the piston to move downwards in the cylinder until the valve plug is correctly positioned in relation to the input air signal transmitted from the controller. The piston movement is transmitted back to the bell crank via the “range spring”, the bottom of which is attached to the upper end of the piston rod. When the axial force produced by the range spring balances the thrust of the control signal bellows, the pressures in both relays A and B are equalised and the piston comes to rest.

Piston actuators can be supplied to either open or close the valve as the control signal pressure increases. This is achieved by reversing the position of the bellows as indicated in Figure 12.37.

Positioners for piston actuators can be built “in-line” on the end of the cylinder as shown diagrammatically in Figure 12.38.

Positioners can be fitted to pneumatically operated rotary valves. The positioner is mounted on the valve spindle itself as shown in Figure 12.39. This arrangement is much simpler than earlier schemes which required a specially designed type of actuator. The basic difference between the two variants is that the cylinder actuator requires a double relay, and air must be supplied both above and below the piston. Figure 12.40 clearly demonstrates the principle of operation.

If the pressure variations in the output of the controller are small, then the spool valve supplying air to the cylinder will remain static. The change in the pressure will be transmitted directly to the rolling diaphragm. If the change in output is large, then the spool valve will either admit, or exhaust, air as required and cause rapid movement of the diaphragm. Feedback of the valve position is provided by the characterised cam attached to the valve spindle, and the feedback spring. The reducing signal input causes the signal module to release air through the exhaust port and causes the opposite sequence of events throughout the system. The diaphragm actuator adopts a new position corresponding to the size of the signal from the controller.

Pneumatic actuators may be arranged to provide “split range” control, which enables full valve travel to be obtained by only using part of the 0.21 to 1.03 barg control signal pressure range. This allows several valves to be controlled by the same controller. Valve positioners may be used to achieve split range operation of control valves. A typical application is the steam range shown in Figure 12.41. When the system is in balance, the amount of steam entering the range at A is sufficient to maintain a constant pressure on the range. If the steam demand at B exceeds the supply, the pressure on the range will tend to reduce. This is sensed by a reverse-acting pressure controller, PIC, in such a way that its output signal will start to increase. As a result more steam will be admitted to the back pressure turbine through valve 1. If there are no other constraints this can continue until the limit of its flow capacity is reached. If this is not sufficient to maintain pressure on the range, extra steam is admitted directly from the boiler through valve 2.

Conversely, if supply exceeds demand, the flow to the back pressure turbine is throttled and, if that is not sufficient to restore the range pressure to its set point, valve 3 is opened to pass the excess steam to a condenser. When the demand decreases the sequence is reversed. Valve 2 begins to close in stages. When this is closed, valve 1 begins to close until the system is once more in balance with a signal to the valves of about 0.5 barg, 8 psig. If the demand is further reduced, the signal output begins to fall below 0.5 barg, valve 3 begins to open in stages and is fully open at 0.2 barg, whereupon excess steam is released from the system. If the demand increases, valve 3 begins to close again until the system is balanced at the pre-set system pressure, where the signal output is at 0.5 barg.

One of the many advantages of such a system is that the complete circuit can be supervised by just one controller which can be adjusted up or down as required to control all valves in the system. In a practical arrangement of this kind there must be adequate relief capacity to cope with instrument failure.

In the classical arrangement, the output signal of the direct-acting pressure controller of 0.21 to 1.03 barg (or 4 to 20 mA if the controller is electronic), is divided into three bands by carefully adjusting the calibration of the positioners on valves 1, 2 and 3:

valve 3: 0.2 barg (open) to 0.5 barg (closed)

valve 1: 0.5 barg (closed) to 0.7 barg (open)

valve 2: 0.7 barg (closed) to 1.0 barg (open)

This arrangement is quite effective, but the calibration is difficult to achieve under operating conditions in the field, and tends to drift under the influence of vibration and extremes of temperature usually experienced on industrial plant out in the open.

The more satisfactory of arranging split range control of, say, a steam range, is to divide the controller output signal electronically by three electronic selector relays in the control room, where long term stability of the settings can be guaranteed to about 0.1%. Each of the three valves is controlled with the full range output, 0.21 to 1.03 barg of the appropriate signal selector relay.

The positioners described so far are all pneumatic/mechanical devices. The pneumatic control signal is compared to the valve stem position and adjustments are made if necessary. The valve/actuator/positioner must be located as close as possible to the controller to reduce piping pressure drop losses. In some complicated process installations and in utility distributions systems, the controller may be a considerable distance from the valve. Distribution systems controlled from a central control room may transmit control signals via radio or fibre optic link. An interface is required between electrical signals and the pneumatic operating system. An electro-pneumatic positioner fulfils this requirement.

The electro-pneumatic positioner is fitted to the actuator yoke and a link is connected to the valve stem in the normal way. The control signal is either 4 to 20 mA or 2 to 10 V. The control signal is compared to the valve position via a cam, which can be “characterised” if necessary. The positioner reduces the air supply pressure to a modulated 0.21 to 1.03 barg control supply which is directed to the diaphragm or piston. These positioners can be single or double acting and set up to be reverse-acting. Split range operation can be accommodated by internal adjustments. Typical electro-pneumatic positioner operating parameters are shown in Table 12.7.

Table 12.7. Typical electro-pneumatic positioner operating parameters

ParameterValueControl signal4 to 20 mA at 8.5 V dc minimumSupply air pressure1.4 to 6.9 bargOutput air pressure0 to 100%Sensitivity0.1% spanLinearity1.5% spanHysteresis0.5% span

An alternative electro-pneumatic positioner control for diaphragm actuators can be achieved by using solenoid valves. The driving control signal can be pneumatic or electric; when pneumatic is used it is converted immediately to an electric current signal. The 4 to 20 mA positioner signal is compared electronically with the 4 to 20 mA control signal. If the actuator must compress the spring further, a solenoid valve in the air supply is opened until the signals coincide. If the actuator has travelled too far, a solenoid valve in a vent line is opened and allows the spring to return the actuator until the signals coincide. The positioner Smart electronics are can be configured using the HART protocol over the current loop.

Electrically actuated valves can have positioner facilities added. The stem or spindle is fitted with a potentiometer or a current position transmitter. Every stem/spindle position corresponds to a modulated electric signal between 4 to 20 mA or 2 to 10 V. The electric positioner compares the valve signal to the control signal and transmits an appropriate signal to the electric motor. Some positioners can vary the motor speed in proportion to the error between the two signals. Electric positioners can be used for split range operation in a similar manner to pneumatic positioners.

The most recent designs of positioners are non-contact devices which “watch” the valve stem or spindle. Proximity detectors sense the position of targets on the stem or spindle. Using a digital system with its own on-board microprocessor, they can analyse the valve/actuator performance — control signal, output signal, valve stem position, response time; and create an individual “signature”. If the valve/actuator performance deviates from the signature an alarm can be triggered advising pre-emptive maintenance.

(4) Multiplexing transmission modes

Multiplexing is sending multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal, and then recovering the separate signals at the receiving end. In analog transmission, signals are commonly multiplexed using frequency-division multiplexing (FDM), in which the carrier bandwidth is divided into subchannels of different frequency widths, each carrying a signal at the same time in parallel. In digital transmission, signals are commonly multiplexed using time-division multiplexing (TDM), in which the multiple signals are carried over the same channel in alternating time slots. In some optical-fiber networks, multiple signals are carried together as separate wavelengths of light in a multiplexed signal using dense wavelength division multiplexing (DWDM).

Frequency-division multiplexing (FDM) is a scheme in which numerous signals are combined for transmission on a single transmission line or channel. Each signal is assigned a different frequency (subchannel) within the main channel. When FDM is used in a transmission network, each input signal is sent and received at maximum speed at all times, but if many signals must be sent along a single long-distance line, the necessary bandwidth is large, and careful engineering is required to ensure that the system will perform properly.

Time-division multiplexing (TDM) is a method of putting multiple data streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled at the receiving end based on timing.

Dense wavelength division multiplexing (DWDM) is a technology that puts data from different sources together on an optical fiber, with each signal being carried at the same time, at its own separate wavelength. Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. Each channel carries a time division multiplexed (TDM) signal. In a system with each channel carrying 2.5 Gbps (billion bits per second), up to 200 billion bits can be delivered a second by the optical fiber. DWDM is also sometimes called wave division multiplexing (WDM).

Since each channel is demultiplexed at the end of the transmission to retrieve the original source, different data formats can be transmitted together. Specifically, Internet Protocol (IP) data, Synchronous Optical Network data (SONET), and asynchronous transfer mode (ATM) data can all travel at the same time within the optical fiber. DWDM promises to solve the “fiber exhaust” problem and is expected to be the central technology in the all-optical networks of the future.

6.2.3.6 Multiplexing Mode

Multiplexing is sending multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal and then recovering the separate signals at the receiving end. In analog transmission, signals are commonly multiplexed using frequency-division multiplexing (FDM), in which the carrier bandwidth is divided into subchannels of different frequency widths, each carrying a signal at the same time in parallel. In digital transmission, signals are commonly multiplexed using time-division multiplexing (TDM), in which the multiple signals are carried over the same channel in alternating time slots. In some optical fiber networks, multiple signals are carried together as separate wavelengths of light in a multiplexed signal using dense wavelength division multiplexing (DWDM).

Frequency-division multiplexing (FDM). FDM is a scheme in which numerous signals are combined for transmission on a single communications line or channel. Each signal is assigned a different frequency (subchannel) within the main channel.

A typical analog Internet connection through a twisted pair telephone line requires approximately three kilohertz (3 kHz) of bandwidth for accurate and reliable data transfer. Twisted-pair lines are common in households and small businesses. But major telephone cables, operating between large businesses, government agencies, and municipalities, are capable of much larger bandwidths.

Suppose a long-distance cable is available with a bandwidth allotment of three megahertz (3 MHz). This is 3000 kHz, so in theory, it is possible to place 1000 signals, each 3 kHz wide, into the long-distance channel. The circuit that does this is known as a multiplexer. It accepts the input from each individual end user, and generates a signal on a different frequency for each of the inputs. This results in a high-bandwidth, complex signal containing data from all the end users. At the other end of the longdistance cable, the individual signals are separated out by means of a circuit called a demultiplexer, and routed to the proper end users. A two-way communications circuit requires a multiplexer/demultiplexer at each end of the long-distance, high-bandwidth cable.

When FDM is used in a communications network, each input signal is sent and received at maximum speed at all times. This is its chief asset. However, if many signals must be sent along a single long-distance line, the necessary bandwidth is large, and careful engineering is required to ensure that the system will perform properly. In some systems, a different scheme, known as TDM, is used instead.

Time-division multiplexing (TDM). Time-division multiplexing (TDM) is a method of putting multiple data streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled at the receiving end based on the timing.

The circuit that combines signals at the source (transmitting) end of a communications link is known as a multiplexer. It accepts the input from each individual end user, breaks each signal into segments, and assigns the segments to the composite signal in a rotating, repeating sequence. The composite signal thus contains data from multiple senders. At the other end of the long-distance cable, the individual signals are separated out by means of a circuit called a demultiplexer, and routed to the proper end users. A two-way communications circuit requires a multiplexer/demultiplexer at each end of the long-distance, high-bandwidth cable.

If many signals must be sent along a single long-distance line, careful engineering is required to ensure that the system will perform properly. An asset of TDM is its flexibility. The scheme allows for variation in the number of signals being sent along the line, and constantly adjusts the time intervals to make optimum use of the available bandwidth. The Internet is a classic example of a communications network in which the volume of traffic can change drastically from hour-to-hour. In some systems, a different scheme, known as FDM, is preferred.

Dense wavelength division multiplexing (DWDM). Dense wavelength division multiplexing (DWDM) is a technology that puts data from different sources together on an optical fiber, with each signal carried at the same time on its own separate light wavelength. Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. Each channel carries a time division multiplexed (TDM) signal. In a system with each channel carrying 2.5 Gbps (billion bits per second), up to 200 billion bits can be delivered a second by the optical fiber. DWDM is also sometimes called wave division multiplexing (WDM).

Since each channel is demultiplexed at the end of the transmission back into the original source, different data formats being transmitted at different data rates can be transmitted together. Specifically, Internet (IP) data, Synchronous Optical Network data (SONET), and asynchronous transfer mode (ATM) data can all be traveling at the same time within the optical fiber. DWDM promises to solve the “fiber exhaust” problem and is expected to be the central technology in the all-optical networks of the future.

2.10 Signal Transmission

Measurement signals often have to be transmitted over quite large distances from the place of measurement to a display unit and/or a process control unit. Methods used for such transmission are:

Analogue voltage transmission

Analogue voltage signals can suffer corruption due to induced noise and the resistance of the connecting cables can result in attenuation of the voltage, the voltage drop across the output being reduced by that across the line resistance (Figure 2.81). Such effects can be reduced by the use of signal amplification and shielding with the connecting cables. However, because of these problems, such signals are not generally used for large distance transmission.

Current loop transmission

The attenuation which occurs with voltage transmission can be minimised if signals are transmitted as varying current signals. This form of transmission is known as current loop transmission and uses currents in the range 4 mA to 20 mA to represent the levels of the analogue signal. The level of 4 mA, rather than 0 mA, is used to indicate the zero signal level since, otherwise, it would not be possible to distinguish between a zero value signal and a break in the transmission line. Figure 2.82 shows the basic arrangement with the signal from the sensor being converted to a current signal by a voltage-to-current converter, e.g. that shown in Figure 2.76, transmitted and then converted to a voltage signal at the display.

Digital voltage signals

Digital signals can be transmitted over transmission lines using either serial or parallel communication. With serial communication, the sequence of bits used to describe a value is sent in sequence along a single transmission line. With parallel transmission, each of the bits is sent along a separate parallel transmission line. For long-distance communication, serial communication is used.

In order to transfer data, both the sender and receiver have to agree on the meaning of the transmitted binary digital patterns. The most commonly used character set is the American Standard Code for Information Interchange (ASCII), thus using 7 bits to represent each character (Table 2.2). The format used for sending such data has to be standardised. For example, with the RS-232 form of serial transmission, a sequence of 10 bits is used, the first bit being a start of message signal, then seven bits for the data, then a parity bit to identify whether errors have occurred in the transmission, and finally a stop bit to indicate the end of the message.

Table 2.2. ASCII Code

ASCIIASCIIASCIIA100 0001N100 11100011 0000B100 0010O100 11111011 0001C100 0011P101 00002011 0010D100 0100Q101 00013011 0011E100 0101R101 00104011 0100F100 0110S101 00115011 0101G100 0111T101 01006011 0110H100 1000U101 01017011 0111I100 1001V101 01108011 1000J100 1010W101 01119011 1001K100 1011X101 1000L100 1100Y101 1001M100 1101Z101 1010

The RS-232 is a widely used serial data interface standard in which the electrical signal characteristics, such as voltage levels, the forms of plug and sockets for interconnections, and the interchange circuits are all specified. Because RS-232 is limited to distances less than about 15 m, another standard such as RS-485 tends to be used in many control systems with distances up to about 1200 m being possible.

Digital signal transmission has a great advantage when compared with analogue transmission in that signal corruption effects can be considerably reduced. With digital transmission, error coding is used to detect whether corruption has occurred. These are bits added to the sequence of bits used to represent the value and are check values which are not likely to tally with the received bits if corruption has occurred, the receiver can then request the message be sent again. For example, the sequence 1010 might be transmitted and corruption result in 1110 being received. In order to detect such errors one form of check uses a parity bit which is added at transmission. With even-parity, the bit is chosen so that the total number of 1 s in the transmission, including the parity bit, is an even number. This 1010 would be transmitted as 10100. If it is corrupted and received as 11100 then the parity bit shows there is an error.

Pneumatic transmission

Pneumatic transmission involves converting the sensor output to a pneumatic pressure in that range 20 to 100 kPa or 20 to 180 kPa. The lower limit gives the zero sensor signal and enables the zero value to be distinguished from a break in the circuit. Such pressure signals can then be transmitted though plastic or metal piping, the distances being limited to about 300 m because of the limitations of speed of response at larger distances.

Fibre-optic transmission

An optical fibre is a light conductor in the form of a long fibre along which light can be transmitted by internally being reflected of the sides of the fibre (Figure 2.83). The light sources used are LEDs or semiconductor laser diodes. Digital electrical signals are converted into light pulses which travel down the fibre before being detected by a photodiode or phototransistor and converted back into an electrical signal. Fibre optics has the advantages that they are immune to electromagnetic interference, data can be transmitted with much lower losses than with electrical cables, the fibres are smaller and less heavy than copper cables, and are more inert in hazardous areas.

Radio telemetry

Radio transmission for getting signals from measurement systems can be used when there are problems in installing electrical or fibre optics links or the transmission distances involved are large or the sensors are mounted on rotating shafts and direct wire connections between the sensors and the data display unit present problems. The transmission of analogue data usually involves frequency modulation (FM), each analogue sensor voltage modulating the carrier frequency into a varying frequency around the carrier frequency. Frequency modulation has, however, now given way to digital telemetry. With this, the output of the sensors is, after filters are used to smooth out any ‘jaggies’ that are present in what should be a smooth input, then converted into digital data. Multiple signals are often combined into one composite digital data stream for transmission. To guarantee a digital transmission is error free, blocks of code can be checked by also sending a series of bits representing their binary sum. If the transmission becomes corrupted the sum is not likely to be valid. This digital stream then modulates the radio frequency carrier which is then transmitted from the transmitter aerial to be picked up by the receiver aerial where it is then decoded to give the signals relevant to the data stream transmitted. In its simplest form amplitude shift keying (ASK) is used for the modulation of digital signals when the amplitude is modulated with the term on-off keying (OOK) for the simplest form of amplitude-shift keying modulation that represents digital data as the presence or absence of a carrier wave. The term frequency shift keying (FSK) is used when the frequency is modulated and phase shift modulation (PSK) when the phase is modulated. Figure 2.84 illustrates the radio telemetry transmission process and Figure 2.85 a simplistic sketch of the modulation methods. As an illustration of what is commercially available, there are radio telemetry instruments with frequency modulation of the radio frequency carrier and a transmission range ranging from 5 to over 50 m and which interface with strain gauge bridges and thermocouples. Such instruments are used with automobiles with sensors which can be attached to a wheel and transmit data when the wheel is rotating to the control unit in the auto.

2.10.1 Noise

The term noise is used, in this context, for the unwanted signals that may be picked up by a measurement system and interfere with the signals being measured. There are two basic types of electrical noise:

1.

Interference

This is due to the interaction between external electrical and magnetic fields and the measurement system circuits, e.g. the circuit picking up interference from nearby mains power circuits.

2.

Random noise

This is due to the random motion of electrons and other charge carriers in components and is determined by the basic physical properties of components in the system.

The three main types of interference are:

1.

Inductive coupling

A changing current in a nearby circuit produces a changing magnetic field which can induce e.m.f.s, as a result of electromagnetic induction, in conductors in the measurement system.

2.

Capacitive coupling

Nearby power cables, the earth, and conductors in the measurement system are separated from each other by a dielectric, air. There can thus be capacitance between the power cable and conductors, and between the conductors and earth. These capacitors couple the measurement system conductors to the other systems and thus signals in the other systems affecting the charges on these capacitors can result in interference in the measurement system.

3.

Multiple earths

If the measurement system has more than one connection to earth, there may be problems since there may be some difference in potential between the earth points. If this occurs, an interference current may arise in the measurement system.

Methods of reducing interference are:

1.

Twisted pairs of wires

This involves the elements of the measurement system being connected by twisted wire pairs (Figure 2.86). A changing magnetic field will induce e.m.f.s in each loop, but because of the twisting the directions of the e.m.f.s will be in one direction for one loop and in the opposite direction for the next loop and so cancel out.

Figure 2.86. Twisted pairs.

2.

Electrostatic screening

Capacitive coupling can be avoided by completely enclosing the system in an earthed metal screen. Problems may occur if there are multiple earths. Coaxial cable gives screening of connections between elements, however, the cable should only be earthed at one end if multiple earths are to be avoided.

3.

Single earth

Multiple earthing problems can be avoided if there is only a single earthing point.

4.

Differential amplifiers

A differential amplifier can be used to amplify the difference between two signals. Thus if both signals contain the same interference, then the output from the amplifier will not have amplified any interference signals.

What is the transmission of signal?

Signal transmission conveys process information from one instrument to another when controlling industrial processes. As process industries have evolved, a variety of transmission signals have been used in many different applications.

How many types of transmission signal are there in data communication?

There are two methods used to transmit data between digital devices: serial transmission and parallel transmission. Serial data transmission sends data bits one after another over a single channel. Parallel data transmission sends multiple data bits at the same time over multiple channels.

What are signals and its types?

Two main types of signals encountered in practice are analog and digital. The figure shows a digital signal that results from approximating an analog signal by its values at particular time instants. Digital signals are quantized, while analog signals are continuous.

What are three common methods of signal transmission used in networks?

The transmission methods in industrial communication networks include baseband, broadband, and carrierband. In a baseband transmission, a transmission consists of a set of signals that is applied to the transmission medium without being translated in frequency.