Types of contact vibration measurement
- Potentiometer type
- LVDT type
- Piezoelectric sensor
- Piezo resistance sensor
- Resistance sensor
- Inductive sensors
A potentiometric transmitter is a one-dimensional position sensor. It is based on a potentiometer, an adjustable potential divider. A voltage is applied across a resistance path. A wiper passes along this resistance path, thus dividing the resistance into two parts, as shown in the figure (resistors R1 and R2).
Linear variable differential transformer
A linear variable differential transformer (LVDT) is a type of transformer that is based on induction. It can be used to measure relative displacement. As shown in Figure 2, this sensor works along one axis and the direction of movement can be determined. LVDT basically consists of three coils and a core. The main coil is connected to a power source for excitation. Two other coils are placed on each side of the main coil and are placed in opposite series.
At the center of this assembly is a core coil that influences the magnetic flux from the primary to secondary windings. Depending on the movement of the core, which is attached to the vibrating body, the direction and distance can be deduced from the output signal. The carrier frequency ranges from 50 Hz to 25 kHz, which is typically defined as 10 times the frequency of the core movement. Using this setup, it is possible to measure displacements of more than 50 cm and an accuracy of up to 0.1 µm. The temperature range is between -270°C and 600°C
Figure 2: LVDT – Cutaway view with primary winding A and secondary windings B
The principle of electrodynamics
The principle of electrodynamics is used in a relative velocity sensor. It is based on the phenomenon of induction. In order to apply this principle, a light coil and a permanent magnet are used. The magnet is fixed on the vibrating object. The magnet either moves without contact or is guided inside the coil.
Due to the movement of the magnet, a voltage is induced in the coil. This voltage can be measured and is directly proportional to the speed of the vibrations. The separation of the wires is the only limit to the maximum voltage. For example, there are sensors with a working frequency between 1 Hz and 2 kHz
Absolute velocity can be measured with a seismometer. Seismography consists of a seismic mass and a spring inside a chamber. Due to the immobility of the mass, in case of vibration, there is a relative movement between the seismic mass and the housing. A coil, which is fixed to the housing, can be used and creates an inductance. Due to the motion of the mass, a voltage is induced in the coil. This voltage velocity can be measured because it is proportional. In order to avoid resonant peaks, a relief offset is often installed in such a seismograph.
Figure 3: Seismograph
In today’s seismometers, the mass is motionless relative to the housing. Therefore, there is no voltage amplitude due to seismic mass movement. However, force, which is required to maintain mass balance, is measured through voltage. Modern seismographs are able to record lower frequencies from 3-10 Hz to 100 Hz. It is possible to detect movements in the range of about 1 nanometer and several centimeters. The principle of seismic mass can also be used to measure direction and acceleration.
The piezoelectric sensor works based on the seismic principle and the piezoelectric effect. Here, quartz ceramic and piezo ceramic replace the spring used in seismography. The piezo material is fixed on one side to the vibrating body and on the other side to the seismic mass. Vibration forces lead to pressure and compression of piezo materials. The piezoelectric effect describes the occurrence of an electric charge due to a change in the length of a polarized material.
This charge is proportional to the force of action and can be used. Since force is the product of mass and acceleration, it can be easily calculated. Piezo materials are very rigid, so this may be necessary. This can be achieved by adding barriers or immersing the parts in oil. The weight of piezoelectric sensors varies from less than 1 gram to several grams. The linear frequency range of piezoelectric sensors varies from below 0.1 Hz to 104 Hz. Therefore, piezoelectric sensors can measure accelerations below 1 gram and up to several thousand grams.
Figure 4: Piezoelectric sensor
Piezo resistance sensor
The piezo resistance sensor uses four semiconductor strain gauges. These strain gauges are mounted on the vibrating body with a seismic mass using a bridge circuit. Vibrations lead to deformation of strain gauges. While moving in one direction, two pressure gauges are stretched and the other two are compressed, which leads to a change in voltage. An advantage compared to the piezoelectric effect is the possibility of measuring constant accelerations. It is possible to measure acceleration up to 1000 g. Piezoelectric sensors are more suitable for high frequencies while semiconductor sensors are preferred at low frequencies.
Piezo resistance sensor
The piezo resistance sensor uses four semiconductor strain gauges. These strain gauges are mounted on the vibrating body with a seismic mass using a bridge circuit. Vibrations lead to deformation of strain gauges. While moving in one direction, two pressure gauges are stretched and the other two are compressed, which leads to a change in voltage.
An advantage compared to the piezoelectric effect is the possibility of measuring constant accelerations. It is possible to measure acceleration up to 1000 g. Piezoelectric sensors are more suitable for high frequencies while semiconductor sensors are preferred at low frequencies.
The functional principle of the resistance sensor is the same as the piezo resistance sensor. The only difference is that strain gauges are not made of materials that have a piezo effect. This leads to similar properties. But the measurable signal is less.
The inductive sensor for measuring acceleration is based on the principle that the reaction force of the seismic mass can be converted into a path. Now, the distance covered can be calculated by measuring the induced voltage, and hence the amount and direction of the vibration can be determined. However, this path-dependent measurement requires a much larger sensor than comparable acceleration sensors.
Specifications of vibration sensors