Introduction
Pressure sensor's function is based on physical phenomenon. It can be used at least two different situation:
water, air or between two points of measurement.
Types of pressures and basic physics of pressure sensing
Static pressure is a definition for physical pressure.
For example you would be skiing or walking somewhere and this sensor type will register your steps.
P (pressure) = F (force) / A (area), m^2)).
where P is the pressure (Pa), F is the force (N) and A is the area (m2). [1]
Force unit can be determined by F= m*a, (mass*acceleration).
Volume pressure, known as the density can be used when determining the volume of the container.
p (rho) = m (mass) / V (volume).
Archimedes's principle states can be determined by for example a person goes bath and the waterspills over bathtub. The amount of spilled
water has the same volume than a person.
Physical formula is: P= p*h*g
P = p (rho) * h (height in metres) *g (gravitation force (9,81 m/s^2))
Pascal's law states that an increase in pressure at any point in a liquid causes a corresponding increase at every other point in the
liquid. [2.]
For example if pressing a cylinder on the other side and because of the water is solid, second cylinder will be pulled out upward.
This method is a common in for example car brakes.
Different type pressure sensors
There are three different types of pressure measurements.
1. Absolute Pressure Measurement
The pressure is absolute when the vacuum is perfect, it means that there is no matter present and the air pressure is zero. Absolute pressure sensors are rarely used because it is hard to get a perfect vacuum state. These types of sensors are used in barometric or altitude related measurements.
2. Differential Pressure Measurement
In differential pressure measurement two different positions are compered to each other. For example measuring pressure in different heights. The unit of differential pressure sensors is psid and it is used when a high amount of pressure is measured. These type of sensors are used in monitoring filtering, the sensor feels the change in the pressure and the system can be leveled.
3. Gauge Pressure Measurement
Gauge pressure measurement is a subtype of differential pressure measurement. The idea is to compare pressure at any point to the current atmospheric pressure. The Gauge pressure measurement is not consistent because the atmospheric pressure varies with altitude so it cannot be used in critical measurements. Still gauge pressure sensors are used in tire pressure and blood pressure measurements. [1.]
Where to use pressure sensors?
Pressure sensor is a widely used sensor type. Like the sensor`s name tells, every device with the sensor senses pressure.
The sensor is used for converting analog signal into electrical signal. Therefore the needs of the sensor are related to electronics which needs to convert analog into electrical signal.
There are so many different targets for pressure sensors but below are the main categories:
- Touch Screen Devices: With pressure sensors you can determine how slight pressure is applied to the touch screen. Usually there is 4 different pressure sensors at the corners. By them it is given precise location of the pressure point.
- Automotive industry: Pressure sensors are widely used in cars. You may have thought how the oil and cooling pressure is calculated before blinking the oil or cooling led on the HUD. It is all about pressure sensors. At the cars there are also air bag systems which needs to quickly recognize if there is non-normal pressure coming from the edge of the cars and work if need to.
- Bio Medical: Pressure sensors are needed to optimize patient`s health and his requirements. (f.e. Blood pressure monitors)
- Aviation: Particular level of air pressure need to be in passengers lobby to provide normal breathing.
- Marine Industry: Especially on submarines it is very precise how much oxygen the crew has; pressure sensors regulate the amount. [1.]
Piezoelectric transducers
We chose as our Piezoelectric sensor a PiezoStar® Pressure Sensor Type 6125C. (3)
Transfer function
Transfer function for piezoelectric transducer can be calculated as:
[4.]
Sensors sensitivity
Sensitivity-33 pC/bar
Acceleration sensitivity:
- axial: <0,003 bar/g
- radial:<0,0005 bar/g
Sensitivity change:
- 250 C +- 100 1 %
- RT...350 C +- 2 %
There are different kind of sensors on the market with different amount of bars. New piezo materials (PiezoStar crystal) inprove the sensors performance. [3.]
Other Characteristics
Operating temperature range -20...350 C
Measurin range 0...300 bar
Natural frequence >70 kHz [3.]
Conducting for piezoelectric transducers
Piezoelectric sensor component includes a microchip which sends information and he data from piezoelectric sensors can be collected by using voltmeter. The voltage is equivalent to load voltage which loads the capacitance of charging full.
We are also able to analyze the specifications what happens signal conditioning by using charge amplifier. A signal-conditioning circuit must have low input impedance but piezoelectric transducer have high input impedance which is bad thing. A typical input impedance for piezoelectric transducer is 3500 kΩ but it varies in different piezoelectric models.
The pressure sensor input variants about the pressure, temperature and material. The signal may need amplification so there has to be an amplifier added.
The input bias currents of this type of amplifier is usually below 100 pA, it should be fine as long as the feedback resistor value is below 1 GΩ. Specified excitation voltage for all standard ICP sensors and amplifiers is generally within the range of 18 to 30 volts.
The signal is converted to digital signal with an A/D converter. It is placed usually in some kind of an embedded system outside of the sensor. The sensor only reacts to pressure changes.
The system has some output noise which is approximately some hundreds of nV/sqrt(Hz). Noises are coming from voltage and current. [5;6]
Piezoelectric sensors are usually very stable so they don't need calibration after purchasing but in extreme conditions characteristics may change over time. In this case the sensors are usually sen to the manufacturer. [10; 11]
Sources
- Pressure definition:http://www.school-for-champions.com/science/pressure.htm#.U1ilsRDFJ8E
- Archimedes' Law: http://www.auburn.edu/~nzk0001/pub/phys1600/10_archimedes.pdf
- Determining the density: http://physics.about.com/od/fluidmechanics/f/density.htm
- Preeti Jain, Pressure Sensors, http://www.engineersgarage.com/articles/pressure-sensors-types-working
- Robert E. Bicking, Fundamentals of Pressure Sensor Technology 1998, http://www.sensorsmag.com/sensors/pressure/fundamentals-pressure-sensor-technology-846
- PiezoStar® Pressure Sensor Datasheet by Kistler
- Piezoelectric Transducers and Applications, editor: Antonio Arnau Vives
- Eduardo Bartolome, Signal conditioning for piezoelectric sensors, http://www.ti.com/lit/an/slyt369/slyt369.pdf
- CONVENTIONAL CHARGE OUTPUT SENSORS, http://www.pcb.com/TechSupport/tech_signal
- Calibration services http://www.pcb.com/SensorCalibrationServices/CalibrationServices
- Accuracy and Calibration http://www.mmf.de/accuracy_and_calibration.htm
Group members: Kiia Tammi, Mikko Heiskanen and Aleksi Väisänen