This section discusses the various types of pressure sensors that are used in automotive applications. In some instances, a simple pressure switch is used. In systems that require monitoring of the exact pressure electromechanical pressure sensors, piezoresistive, or piezoelectric sensor are used. These sensors convert the applied pressure to an electrical signal. A wide variety of materials and technologies has been used in these devices. These sensors are used to measure the atmospheric air pressure, manifold pressure, pressure of a gas (such as R134a), exhaust pressures, fluid pressures, and so forth. The types of sensors that can be used include; potentiometric, strain gauges using Wheatstone bridges or capacitance discharge, piezoelectric transducers, and pressure differential sensors.
FIGURE. Simple pressure switch uses contracts to complete electrical circuit.
Pressure switches will usually use a diaphragm that works against a calibrated spring or other form of tension. When pressure is applied to the diaphragm that is of a sufficient value to overcome the spring tension a switch is closed. Current that is supplied to the switch now has a completed path to ground. In a very simple warning light circuit, the closed pressure switch completed the circuit for the bulb and alerts the driver to an unacceptable condition. For example a simple oil pressure warning lamp circuit will use a pressure switch.
FIGURE. Computer monitors pressure switch circuit.
Computer monitored pressure switch circuits use the change in voltage as an indication of pressure. When the pressure change (either from low to high or high to low) changes the state of the switch, the voltage change is interpreted by the computer. Pressure switches are used to monitor the presence of pressure that is above or below a setpoint; they do not indicate the exact amount of pressure being applied.
Potentiometric Pressure Sensor
FIGURE. Potentiometric pressure sensors use a Bourdon tube, capsule, or bellows to drive a wiper arm on a resistive element.
One of the basic types of pressure sensor is the potentiometric pressure sensor. The potentiometric pressure sensors use a Bourdon tube, a capsule, or bellows to move a wiper arm on a resistive element. Using the principle of variable resistance, the movement of the wiper across the resistive element will record a different voltage reading to the computer. Although this type of sensor can be used as a computer input, a computer is not always involved. Some early analog instrument panels used this sensor unit with an air core gauge to display engine oil pressure.
A strain gauge sensor determines the amount of applied pressure by measuring the strain a material experiences when subjected to the pressure. In there simplest form, a strain gauge sensor is a piezoresistive device. A piezoresistive sensor behaves like a variable resistor because its resistance value changes as the pressure applied to the sensing material changes. This type of sensor changes resistance values as a function of pressure changes. A voltage regulator supplies a constant voltage to the sensor. Since the amount of voltage that the sensor drops will change with the change of resistance, the control module can determine the amount of pressure on the sensing material by measuring the voltage drop across the sensor. Piezoresistive sensors are commonly used as a gauge sending unit.
FIGURE. Oil pressure sensor used in gauge indicator circuit.
A Wheatstone bridge is commonly used to measure changes in pressure or strain. Although commonly used for engine control systems, body and chassis control systems that use them include tire pressure monitoring and supplemental restraint systems.
FIGURE. Wheatstone bridge.
A Wheatstone bridge is nothing more than two simple series circuits connected in parallel across a power supply. Usually three of the resistors are kept at exactly the same value and the fourth is the sensing resistor. When all four resistors have the same value, the bridge is balanced and the voltage sensor will indicate a value of 0 volts. The output from the amplifier acts as a voltmeter. Remember, since a voltmeter measures electrical pressure between two points, it will display this value. For example, if the reference voltage is 5 volts and the resistors have the same value, then the voltage drop over each resistor is 2.5 volts. Since the voltmeter is measuring the potential on the line between Rs and Rt and R2 and R3, it will read 0 volts because both of these lines have 2.5 volts on them. If there is a change in the resistance value of the sense resistor, a change will occur in the circuit’s balance. The sensing circuit will receive a voltage reading that is proportional to the amount of resistance change.
FIGURE. Piezoelectric sensors convert stress into an electric potential and vice versa. Sensors based on this technology are used to measure varying pressures.
Piezoelectric devices are used to measure pressures by the generation of voltage. Piezoelectric sensors are constructed from alumina ceramics, metallized quartz, single crystals, or ultrasonic transducer materials that make up a bi-directional transducer capable of converting stress into an electric potential. The piezoelectric materials consist of polarized ions within the crystal. As pressure is applied on the piezoelectric material some mechanical deformation occurs in the polarized crystal, which produces a proportional output charge due to the displacement in the ions. Uses of this type of sensor in the automotive industry include piezoelectric accelerometers, piezoelectric force sensors, and piezoelectric pressure sensors.
FIGURE. Piezoelectric sensor circuit.
The sensor is a voltage generator and has a resistor connected in series with it. The resistor protects the sensor from excessive current flow in case the circuit becomes shorted. Hie voltage generator is a thin ceramic disc attached to a metal diaphragm. When engine knock occurs, the vibration of the noise puts pressure on the diaphragm. This puts pressure on the piezoelectric crystals in the ceramic disc. The disc generates a voltage that is proportional to the amount of pressure. The voltage generated ranges from zero to one or more volts. Each time the engine knocks, the sensor generates a voltage spike.
Capacitance Discharge Sensors
FIGURE. Capacitance discharge sensor.
Another variation of the piezosensor uses capacitance discharge. Instead of using a silicon diaphragm, the capacitance discharge sensor uses a variable capacitor. In the capacitor capsule-type sensor, two flexible alumina plates are separated by an insulating washer. A film electrode is deposited on the inside surface of each plate and a connecting lead is extended for external connections. The result of this construction is a parallel plate capacitor with a vacuum between the plates. This capsule is placed inside a sealed housing that is connected to the sensed pressure. If constructed to measure vacuum, as the pressure increases (goes toward atmospheric), the alumina plates deflect inward, resulting in a decrease in the distance between the electrodes.
As the distance between the electrodes changes, so does the capacity of the capacitor. A measure of capacitance constitutes a measurement of pressure that is detected by a bridge circuit. The output from the bridge circuit can be either an analog DC voltage or applied to a chip that produces a frequency modulated digital signal.