8 Signal transmission

 There is a necessity in many measurement systems to transmit measurement signals over quite large distances from the point of measurement to the place where the signals are recorded and/or used in a process control system. This creates several problems for which a solution must be found. Of the many difficulties associated with long distance signal transmission, contamination of the measurement signal by noise is the most serious. Many sources of noise exist in industrial environments, such as radiated electromagnetic fields from electrical machinery and power cables, induced fields through wiring loops, and spikes (large transient voltages) on the a.c. power supply. Signals can be transmitted electrically, pneumatically, optically, or by radiotelemetry, in either analogue or digital format. Optical data transmission can be further divided into fibre-optic transmission and optical wireless transmission, according to whether a fibre-optic cable or just a plain air path is used as the transmission medium. These various options are explored in the following sections.

 

8.1 Electrical transmission

The simplest method of electrical transmission is to transmit the measurement signal as a varying analogue voltage. However, this can cause the measurement signal to become corrupted by noise. If noise causes a problem, the signal can either be transmitted in the form of a varying current, or else it can be superimposed on an a.c. carrier system.

 

8.1.1 Transmission as varying voltages

As most signals already exist in an electrical form as varying analogue voltages, the simplest mode of transmission is to maintain the signals in the same form. However, electrical transmission suffers problems of signal attenuation, and also exposes signals to corruption through induced noise. Therefore, special measures have to be taken to overcome these problems.

Because the output signal levels from many types of measurement transducer are very low, signal amplification prior to transmission is essential if a reasonable signalto-noise ratio is to be obtained after transmission. Amplification at the input to the transmission system is also required to compensate for the attenuation of the signal that results from the resistance of the signal wires. The means of amplifying signals have already been discussed in section 5.1.

It is also usually necessary to provide shielding for the signal wires. Shielding consists of surrounding the signal wires in a cable with a metal shield that is connected to earth. This provides a high degree of noise protection, especially against capacitiveinduced noise due to the proximity of signal wires to high-current power conductors. A fuller discussion on noise sources and the procedures followed to prevent the corruption of measurement voltage signals can be found in Chapter 5.

 

8.1.2 Current loop transmission

The signal-attenuation effect of conductor resistances can be minimized if varying voltage signals are transmitted as varying current signals. This technique, which also provides high immunity to induced noise, is known as current loop transmission and uses currents in the range between 4 mA and 20 mA* to represent the voltage level of the analogue signal. It requires a voltage-to-current converter of the form shown in Figure 8.1, which is commonly known as a 4–20 mA current loop interface. Two voltage-controlled current sources are used, one providing a constant 4 mA output that is used as the power supply current and the other providing a variable 0–16 mA output that is scaled and proportional to the input voltage level. The net output current therefore varies between 4 mA and 20 mA, corresponding to analogue signal levels between zero and the maximum value. The use of a positive, non-zero current level to represent a zero value of the transmitted signal enables transmission faults to be readily identified. If the transmitted current is zero, this automatically indicates the presence of a transmission fault, since the minimum value of current that represents a proper signal is 4 mA.

Current-to-voltage conversion is usually required at the termination of the transmission line to change the transmitted currents back to voltages. An operational amplifier, connected as shown in Figure 8.2, is suitable for this purpose. The output voltage V is simply related to the input current I by V = IR.

 

8.1.3 Transmission using an a.c. carrier

Another solution to the problem of noise corruption in low level d.c. voltage signals is to transfer the signal onto an a.c. carrier system before transmission and extract it from the carrier at the end of the transmission line. Both amplitude modulation (AM) and frequency modulation (FM) can be used for this.

AM consists of translating the varying voltage signal into variations in the amplitude of a carrier sine wave at a frequency of several kHz. An a.c. bridge circuit is commonly used for this, as part of the system for transducing the outputs of sensors that have a varying resistance (R), capacitance (C) or inductance (L) form of output. Referring back to equations (7.14), and (7.15) in Chapter 7, for a sinusoidal bridge excitation voltage of V_s = V sin (ωt) , the output can be represented by V0 = FV sin (ωt) . V₀ is a sinusoidal voltage at the same frequency as the bridge excitation frequency and its amplitude FV represents the magnitude of the sensor input (R, C or L) to the bridge. For example, in the case of equation (6.15):

                        FV ={ L_u/(L₁ + L_u) - R₃/(R₂ + R₃)}  V

After shifting the d.c. signal onto a high-frequency a.c. carrier, a high-pass filter can be applied to the AM signal. This successfully rejects noise in the form of low-frequency drift voltages and mains interference. At the end of the transmission line, demodulation is carried out to extract the measurement signal from the carrier.

FM achieves even better noise rejection than AM and involves translating variations in an analogue voltage signal into frequency variations in a high-frequency carrier signal. A suitable voltage-to-frequency conversion circuit is shown in Figure 8.3, in which the analogue voltage signal input is integrated and applied to the input of a

comparator that is preset to a certain threshold voltage level. When this threshold level is reached, the comparator generates an output pulse that resets the integrator and is also applied to a monostable. This causes the frequency f of the output pulse train to be proportional to the amplitude of the input analogue voltage.

At the end of the transmission line, the FM signal is usually converted back to an analogue voltage by a frequency-to-voltage converter. A suitable conversion circuit is shown in Figure 8.4, in which the input pulse train is applied to an integrator that charges up for a specified time. The charge on the integrator decays through a leakage resistor, and a balance voltage is established between the input charge on the integrator and the decaying charge at the output. This output balance voltage is proportional to the input pulse train at frequency f.