Instrumentation Sensors Book

15.2.4Noise and Correction Time

Differencing amplifiers with high common mode rejection ratios are used to amplify low-level signals in high-noise environments, to obtain a high signal-to-noise ratio. These amplifiers were discussed in Section 4.3.1. The time elapsed from the detection of an error signal to the correction of the error is discussed in Chapter 16.

15.3Conditioning Considerations for Specific Types of Devices

The method of signal conditioning can vary depending on the destination of the signal. For instance, a local signal for a visual display will not require the accuracy of a signal used for process control.

15.3.1Direct Reading Sensors

Visual displays are not normally temperature-compensated or linearized. They often use mechanical linkages, which are subject to wear over time, resulting in a final accuracy from 5% to 10% of the reading, with little or no conditioning. However, with very nonlinear sensors, the scale of the indicator will be nonlinear, to give a more accurate indication. These displays are primarily used to give an indication that the system is either working within reasonable limits, or is within broadly set limits (e.g., tire pressure, air conditioning systems, and so forth).

A few sensors have outputs that are suitable for direct reading at the point of measurement, but cannot be used for control or transmission. Such devices include: sight glasses for level indication; liquid in glass, for temperature, rotameter, for flow; hydrometer, for density or specific gravity; and possibly, a liquid-filled U-tube manometer, for differential or gauge pressure measurements.

Visual indicators should be clear and the scale well-defined. Rotameters need to be selected for flow rates and fluid density, and their output values should be corrected for temperature variations from lookup tables. Care needs to be taken to ensure that thermometer bulbs are correctly placed in the fluid for temperature measurement, and do not touch the container walls, since this can effect the temperature reading. When measuring liquid levels, and liquid and gas pressures, the instrument should have conditioning baffles to minimize pressure and level fluctuations, which can introduce uncertainties into the readings.

The Bourdon tube, capsule, and bellows convert pressure into mechanical motion, which is well-suited for conversion to direct visual indication, as discussed in Section 7.3. These devices are cost-effective and in wide use, but are not tempera- ture-compensated, and the cheaper instruments do not have zero or span adjustment. More expensive devices may have screw adjustments and a limited temperature range.

15.3.2Capacitive Sensors

Capacitive sensing devices can use single-ended sensing or differential sensing. Sin- gle-ended sensing capacitance is measured between two capacitor plates, as shown in Chapter 8, Figure 8.7. Differential sensing can be used when there is a capacitor plate on either side of, and in close proximity to, a central plate or diaphragm, as

shown in Chapter 7, Figure 7.5(a). In differential sensing, the two capacitors (A and B) can be used to form two arms of an ac bridge, or switch capacitor techniques can be used. For single-ended sensing, a fixed reference capacitor (B) can be used with a variable capacitor (A). Capacitive sensing can use ac analog or digital measuring techniques.

Figure 15.4 shows an ac bridge that can be used with capacitive sensing. Initially, the bridge is balanced for zero offset with potentiometer R3, and the output from the bridge is amplified and buffered. The signal will be converted to a dc signal and further amplified for transmission.

Switch capacitor sensing techniques can use open loop or closed loop sensing techniques. Figure 15.5 shows an open loop switch circuit for sensing capacitance

changes. The top capacitor is switched from VREF to 0.5VREF, and the bottom plate is switched from 0.5VREF to ground. Any difference in the capacitance of the upper and

lower plates will appear as a charge on the input to the first amplifier. This amplifier is used as a charge amplifier and impedance matching circuit. The output of the first amplifier goes to a sample and hold circuit, where the charges are held in a capacitor and then become a voltage, which is amplified by the second amplifier to give a dc output voltage that is proportional to the capacitance difference. The second amplifier also modulates the 0.5VREF voltage and feeds it back to the switches, so that the voltage across each capacitor is proportional to the distance between the capacitor plates. This prevents electrostatic forces due to the driving voltages from producing a deflection force on the diaphragm [1]. This can be a problem for micromachined devices where the capacitor spacing is less than 3m. This type of technique gives good linearity (better than 1%). In applications such as capacitive level sensors, the temperature of the liquid also must be measured, so that corrections can be made for the changes in the dielectric constant of the liquid due to temperature changes.

15.3.3Magnetic Sensors

The resistance of MRE devices change in a fluctuating magnetic field, and MRE devices are also temperature-sensitive. Figure 15.6 shows the circuit used to condition the signal from an MRE into a digital signal in on/off applications. The MRE sensor contains four elements to form a bridge circuit. The four elements are

Figure 15.4 (a) Capacitive diaphragm pressure sensor, and (b) ac bridge for use with a capacitive sensor.

hysteresis, but is large, expensive, sensitive to stray magnetic fields, and has poor linearity, as shown in Section 11.2.1. LVDTs should be screened from magnetic fields, and should be used in closed-loop configurations, as shown in Chapter 7, Figure 7.6. The closed-loop configuration has several advantages over linear conversion. The feedback counteracts mechanical movement, and therefore linearizes the conversion, minimizes hysteresis effects, and reduces strain in the mechanical driving device.

15.3.7Semiconductor Devices

Wide ranges of measurements are made using semiconductor devices. These devices are used to measure pressure, acceleration (MEMS), temperature, light intensity, strain, force, and so forth. These integrated electronics in these devices give high sensitivity and conditioning, do not suffer fatigue, do not need recalibration, and can handle large overloads, but have a limited temperature operating range from −40° to +150°C. Information on device characteristics and usage can be obtained from manufacturers’ data sheets and application notes.

15.4Digital Conditioning

Many analog signals are converted to digital signals for transmission. In many cases, the output from a sensor can be converted directly into a digital signal, and optoisolators can provide ground isolation. The value of capacitors and resis- tive-type devices can be accurately sensed using digital techniques, eliminating the need for analog amplification, but conditioning of the sensor signal may still be required. However, all of the conditioning in the digital domain can be performed by the processor in the controller, using software or lookup tables obtained from a knowledge of the temperature characteristics of the sensing device, and using physical variables.

15.4.1Conditioning in Digital Circuits

Conditioning is performed for nonlinear devices by using equations or memory lookup tables [4]. If the relationship between the values of a measured variable and the output of a sensor can be expressed by an equation, then the processor can be programmed based on the equation to linearize the data received from the sensor. An example would be a transducer that outputs a current (I) related to flow rate (v) by:

where K is a constant.

The current numbers from the sensor are converted into binary signals, where the relationship still holds. In this case, a linear relationship is required between current and flow rate. This can be obtained by multiplying the I term by itself. The resulting number is proportional to v2, and the generated number and flow now have a linear relationship. Span and offsets now may require further adjustment.

There are many instances in conversion where there is not an easily definable relationship between variable and transducer output, and it may be difficult or impossible to write a best fit equation that is adequate for linearization of the variable. In this case, lookup tables are used. The tables correlate transducer output to the true value of the variable, and these values are stored in memory. The processor can retrieve the true value of the variable from the transducer and temperature reading by consulting its lookup tables. This method is extensively used, for instance, with thermocouples.

15.5Pneumatic Transmission

Pneumatic signals were used for signal transmission, and are still in use in older facilities, or in applications where electrical signals or sparks could ignite combustible materials. Pneumatic transmission of signals over long distances requires an excessively long settling time for modern processing needs, especially when compared to electrical signal transmissions. Pneumatic signal lines are also inflexible, bulky, and costly, compared to electrical signal lines, and are not microproces- sor-compatible. They will not be used in new designs, except possibly in the special circumstances mentioned above. Pneumatic transmission pressures were standardized into two ranges—3 to 15 psi (20 to 100 kPa), and 6 to 30 psi (40 to 200 kPa). The 3 to 15 psi range is now the preferred range for signal transmission. Zero is not used for the minimum of the ranges, since low pressures do not transmit well. The zero level can then be used to detect system failure.

15.5.1Signal Conversion

Pneumatic signals as well as electrical signals can be used to control actuators. Signal conversion is required between low-level signals and high-energy control signals for actuator and motor control. Electrical control signals can be either digital, analog voltage, or analog current. It is sometimes necessary to convert electrical signals to pneumatic signals for actuator control [5].

One of the many designs of a current to pressure converter is shown in Figure 15.9(a), in which the spring tends to hold the flapper closed, giving a high-pressure

Figure 15.9 Signal conversion: (a) current to pressure transducer, and (b) transducer characteristic.

output (15 psi). When current is passed through the coil, the flapper moves towards the coil, closing the air gap at the nozzle and increasing the output air pressure. The output air pressure is set to the maximum of 3 psi by the set zero adjustment when the current through the coil is 3 mA. Moving the nozzle along the flapper sets the system gain and span. There is a linear relationship between current and pressure, as can be seen from the transducer characteristic shown in Figure 15.9(b).

A linear pneumatic amplifier or booster can be used to increase the pressure from a low-level pressure signal to a high-pressure signal for actuator control.

15.6Analog Transmission

15.6.1Noise Considerations

Analog voltage or current signals are hardwired between the transmitter and the receiver. These signals can be relatively slow to settle compared to digital signals, due to the time constant of the lead capacitance, inductance, and resistance, but the signals are still very fast in terms of the speed of mechanical systems. Analog signals can lose accuracy if signal lines are long with high resistance. The signals can be susceptible to ground offset, ground loops, radio frequency (RF) and EMI noise from transmitters and motors, and so forth.

To reduce these problems, the following precautions should be taken:

•The dc supply to the transmitter is generated from the ac line voltage via an isolation transformer and voltage regulators, to minimize noise from the power supply.

•The ground connection is used only for the signal return path.

•The signal and ground return leads are a screened twisted pair, with the screen grounded at one end only.

Other necessary compensations can include: filtering to remove unwanted frequencies, such as pickup from the 60 Hz line frequency, noise, or RF pickup; dampening out undulations or turbulence to give a steady average reading; and correcting for time constants and for impedance matching networks.

15.6.2Voltage Signals

Voltage signals are normally standardized in the voltage ranges 0V to 5V, 0V to 10V, or 0V to 12V, with 0V to 5V being the most common. The requirements of the transmitter are: a low output impedance, to enable the amplifier to drive a wide variety of loads without a change in the output voltage; low temperature drift; low offset drift; and low noise. Improved voltage signal transmission can be obtained using a differential signal, as shown in Figure 15.10. In this case, the transmitter sends a differential signal via a screened twisted pair. Because any RF and EMI pickup will affect both signal lines by the same amount, any noise will cancel in the differential receiver in the controller. Ground noise and offsets do not normally affect differential signals.