In order to control the process performance, we need a control system, which consists of a sensor, a controller and a final control element. Obviously, the sensor is a very important part of the control system. It monitors the process and serves as a signal source for the control system. In our previous discussion, we always assumed, there was some suitable measuring device available, but not all measuring devices can be used in automatic control. The basic requirements for a sensor used in a control loop are the abilities :
- to indicate the values of measured variables
- to transmit the signals to the controller
The signals could be transmitted through either a electric circuit or a pneumatic pipeline, therefore, in order to transmit the signal, the sensors must have the ability to convert the measured variable values into either electric signals or pneumatic signals.
In this section of the course, we will very briefly discuss some kinds of sensors used in process control. However it is not intended to examine their working mechanism in any detail. Common variables in Chemical Engineering covered in the following discussion are pressure, temperature, flowrate, and liquid level. Analytical instruments for chemical composition measurement are usually specially designed for the specific purpose and hence are not included.
Many kinds of pressure transducers are widely used in industry for pressure measurement. Although devices like the manometer and Bourdon tube etc are quite common, they are not suitable for control purposes due to the difficulties for signal transmission.
In most pressure transducers, there is a diaphragm to contact the fluids and protect the measuring setup isolated from the measured fluids, most of which may be corrosive. Due to the existence of this diaphragm, most pressure transducers can be used as pressure differential transducers, as long as the second (lower) fluid is introduced into the other side of the diaphragm. Actually, when measuring the pressure, it is measuring the pressure difference between the measured pressure and the pressure of atmosphere guage pressure.
Strain meter (resistance):
In this sensor, under the diaphragm there is a pressure sensitive electric resistance. It is a spring type of resistance wire. Under pressure, some of the spring is pressed together causing a short circut and reducing the resistance. The resistance will be inversely proportional to the strain on it, or the pressure. This changing resistance may be measured by being included into a electric circuit. The electric signal, current or voltage, is easily transmitted.
Piezo - electric sensor:
In this sensor, under the diaphragm is a quartz crystal. The working principle is based on the characteristics of quartz crystal. If the crystal is cut in a special way and placed between two plates, then the electromotivated force (e.m.f) set up between the plates will be a measure of the pressure applied to the crystal. This property of crystal is called piezo-electric effect. By measuring this e.m.f. setup, the applied pressure can be indicated and transmitted. This technique is mainly used for higher pressure measurements.
Traditional transducer (air pressure transmitted):
The principle of this system is that the pressure on a diaphragm is arranged to control the flow of air into, or out of, a chamber on the opposite side of the diaphragm, until a balance is obtained. The balancing pressure is an indication of the measured pressure. In this case, the measured signal is transmitted in a pneumatic circuit through the air pipeline.
Pressure operated sensor:
As there exists a unique quantitative relationship between the liquid level (head) and the static pressure at the bottom of the tank, the latter is widely used as an indication of the liquid level. Thus this is another case of pressure measurement and pressure transducers discussed above could be used for level measurement.
Float operated sensor:
In this kind of system, there is a float on the surface of liquid. The change in liquid level will cause movements of this float. By monitoring this movement a signal of the level is generated and transmitted.
Capacity bridge sensor:
This equipment consists of an electrode, an electronic unit and an indicator (or transmitter). The electrode is in the form of a long metal rod which reaches from the top to the bottom of the vessel. The electrode is bare when the liquid is electrically non conducting, but is sheathed in polymer like polyethlene etc when the liquid is conducting. The electronic unit is merely a power supply and a highly stabilized capacitance measuring bridge. One arm of the bridge is formed by the capacitance between the level sensing electrode and the earth (the vessel wall). A change in the capacitance owing to the rise and fall of the material around the electrode produces an out-of-balance current flow from the bridge which is measured and transmitted.
This kind of sensor is based on the difference in the reflecting and transparent properties of liquid and the gas above it. It takes the form of a light source and a receiver to the reflected light. When there is no liquid around, the receiver can detect the reflected light, and this light signal is converted to a electric signal which can be transmitted. When the sensor is surrounded by liquid, the receiver can’t get the same amount of light reflected. The change in the light received is then converted into the change in the electric signal which is indicative to the amount of liquid. Thus to monitor level, you need a number of these kinds of sensors in series, spread over the whole height of the tank. This signal can also be transmitted.
Differential Pressure Method:
This is still the most commonly used method. Whatever the construction of the meter, the principle involved is same. The net cross-section area of the stream is reduced, causing an increase in the velocity, and hence an increase in kinetic energy. This increase in kinetic energy is obtained at the expense of pressure energy, so that the pressure of fluid is reduced. By measuring this pressure reduction, or the pressure differential, the velocity of the fluid can be calculated. Examples of this kind are orifice plates and Venturi tube. The nature of this measurement is to measure the pressure differential and then to use pressure transducers.
Rotating vane meter:
Liquid passing through the meter is directed on to the rings of the vane, and rotates it at a rate which depends on the velocity of the liquid. This rotation can be arranged to drive some electric transducers to give out electric signal, like frequency.
Resistance thermometer and thermistor:
The electrical resistance of metals depends on temperature. By measuring the changing resistance, the temperature can be determined. The change in resistance can easily be converted to a electrical signal transmittable. Commonly used thermometers are made of Platinum or Nickel because they have a stable and preferable resistance-temperature coefficients.
A thermistor is made of semiconductor, a mixture of metal oxide. Unlike metals, the semiconductors have a negative resistance coefficient. This is the main difference between a thermometer and a thermistor.
If an electric circuit consists of all metallic conductors and all parts of the circuit are at the same temperature, there will be no electric force in the circuit, and hence there is no current. However, if the junctions between two metals are at different temperature, then there will be an e.m.f. and a current will flow. This e.m.f is called the thermoelectric e.m.f. , and the junction between the two metals is a thermocouple. The e.m.f will depend on the temperature difference between the two junctions. Therefore, when one junction (cold end) is kept at zero degrees, the e.m.f will indicate the temperature of the heated junction (measured temperature).
It has been shown the electric circuit for transmitting this signal does not alter the signal itself, which is indicated by the law of intermediate metals which states: In a thermoelectric circuit composed of two metals A and B, with junctions at temperature T1 and T2, the e.m.f is not altered if one or both the junctions are opened and one or more metals are interposed between A and B, provided that all the junctions by which the single junction at temperature T1 may be replaced are kept at T1, and all those by which the junction at temperature T2 may be replaced are kept at T2.
Diodes have an important parameter called pass-required voltage. Below this voltage, there is no current through the diode. Above this voltage, the diode allows current to pass through.
The pass required voltage of silicon diodes depend uniquely on temperature, and thus this voltage signal can be used to indicate temperature. The main advantage of silicon semiconductor thermometers is that this pass-required voltage has a temperature coefficient which is essentially the same for all silicon devices of -2mV/ degC, and this linear change feature is a great advantage for control porposes.