A vibration sensor is a device that measures vibration on machines such as motors and pumps. By analyzing these vibrations, maintenance engineers can tell if the equipment is operating normally. If unusual vibrations are occurring, this could indicate a fault, which may require preventative maintenance to avoid a failure. There are many different types of vibration sensor, each suited to particular applications.
Where are vibration sensors used?
Vibration sensors are most often used on rotating equipment such as motors, pumps, compressors, gear boxes and gas turbines.
A wide range of industries, including automotive and aerospace, food and beverage, pulp and paper, oil and gas, mining and metalworking rely on rotating equipment to enable applications including conveyor systems, ovens and kilns, wind turbines, dryers and rolling mills.
A growing number of industries are using vibration monitoring of rotating machinery to ensure the health of their production and process plant. It is normal for systems such as motors and pumps to vibrate, but any unusual behavior could indicate faults caused by issues such as worn bearings or misaligned drive shafts
For example, fans and compressors are critical for the proper operation of HVAC systems. Monitoring vibration in these systems ensures that air flows properly and keeps the room or machinery at the required temperature.In the chemical industry, many substances are corrosive to pipework and other vessels and must be kept moving. This requires pumps and motors that are reliable and offer uninterrupted working. Monitoring vibrations ensures that problems can be spotted early, and action taken to maintain their availability, avoiding damage to plant vessels and possibly expensive and risky cleaning processes.
The food and beverage industry is very reliant on rotating machinery for equipment such as grinders and mixers as well as pumps for liquids and fans for heating and cooling applications. Maintaining reliability is vital to avoid interruptions to production. By monitoring vibrations and assessing any unusual patterns, maintenance engineers can predict when a motor or pump needs to be replaced and can fit this work around production schedules. This avoids unplanned downtime that could lead to wasted products and materials and a failure to supply customers.
How does a vibration sensor work?
Vibration sensors convert motion into electrical energy using a transducer. There are several physical phenomena that can be used to achieve this conversion. One of the most common methods is the piezoelectric effect, where mechanical pressure on a crystalline material causes a corresponding electrical output that can be anazysed and used to portray the values of the original vibrations.
Other methods include Micro-Electromechanical System or MEMS devices, which is a chip-based technology. Capacitive MEMS devices measure vibration by changing their capacitance in response to an acceleration. Another technique uses eddy currents induced in the measured object by a coil. The change in distance between the coil and the target object produces a proportional output signal.
Microphone sensors are also used. These convert the noise generated by vibrations into electrical signals.
What is a shock sensor?
A shock sensor is similar to a vibration sensor, but it responds to sudden shock loads, rather than measuring continuous vibration. Some shock sensors are based on the piezoelectric principle, generating an output in response to a bending stress.
An alternative method is magneto dynamic shock sensors. These use a magnet mounted on a spring to induce a current in a coil when a shock moves the magnet.
Shock sensors produce little output when experiencing a slow acceleration, such as being moved from one site to another. However, when subject to a rapid shock, such as being dropped, they will produce an output corresponding to the speed and size of the force experienced. They can also be used to indicate the direction of the shock force by producing a plus or minus output.
Applications of shock sensors include preventing read and write errors in hard disk drives that could be caused by a fall. They can also be used to measure tire rotation speed, waking up tire pressure monitoring systems to preserve battery life.
Types of vibration sensor
A common type of vibration sensor is the strain gauge. These can work either through the piezo resistive effect or the piezoelectric effect.
Piezoresistive strain gauges take the form of a foil. This contains an electrically conductive grid, which undergoes a deformation as the component under test, such as a motor, experiences vibrations. These deformations change the electric resistance of the grid and by reading the time taken for an electric current to pass through it, the vibration of the object can be assessed.
A typical arrangement is a piezo film sensor, which consists of a rectangular section of piezo electric film that is adhered directly to the component’s surface.
The other main type of strain gauge is a piezoelectric sensor, in which a mechanical force applied to certain materials produces a charge across the material. This force can be measured as a voltage proportional to the pressure.
The charge across the sensor will leak away over time due to factors such as imperfect insulation and the internal resistance of the sensor. This makes them unsuitable for measuring static pressure. However, they are sensitive to dynamic changes in pressure across a wide range of frequencies and pressures, making them ideal for use as vibration sensors.
Strain gauges can often be mounted in free space. When the assembly is deflected by direct contact, the device acts as a flexible "switch" that generates an output. If the assembly is supported by its contacts and left to vibrate in free space, the device will behave as an accelerometer or vibration sensor.
By far the most common vibration sensors in use are accelerometers. They work by measuring changes in the velocity of the component. These changes in velocity are indicated by vibrations, which cause the accelerometer to produce an electrical signal. These signals can be processed to produce useable vibration data.
An accelerometer works using an electromechanical sensor, designed to measure either static or dynamic acceleration. Static acceleration is the constant force acting on a body, such as gravity or friction. Dynamic acceleration forces are non-uniform, with the best examples being vibration or shock.
Piezoelectric accelerometers are the most widely used type, producing a strong, clear signal at most frequencies, although piezoresistive accelerometers are also becoming increasingly common. These are adept at detecting high and low frequencies, though are significantly more expensive than the piezoelectric type.
Single- and multi-axis accelerometers can detect both the magnitude and the direction of the proper acceleration they experience. Expressed as a vector quantity, this can be used to sense vibration, the orientation of the sensor, coordinate acceleration, shock and falling in a resistive medium.
Single axis accelerometers have a single crystal and thus respond to vibration in one plane. Three axis accelerometers measure the vibration in three axes, X, Y and Z, using three crystals, each positioned to respond to forces acting in a different plane. The output of the accelerometer has three signals, each representing the vibration experienced on one of the axes.
MEMS accelerometers are also available and come in two main types - piezoelectric and capacitive accelerometers. A MEMS piezoelectric accelerometer combines a rigid body and a physical mass attached to a piezoelectric crystal. When motion occurs, the mass will produce stress, stretching the piezoelectric material and generating a charge that is proportional to the acceleration. They are mostly used to measure vibration and shock.
In a MEMS capacitive accelerometer, the change of acceleration drives the movement of the proof mass, reducing the distance between two poles of the parallel plate capacitor. Acceleration is then calculated by measuring the change of capacitance. Known for its accuracy, it is the most commonly used type of accelerometer.
Compared to standard piezoelectric accelerometers, MEMS accelerometers have a number of advantages. As well as a relatively compact size compared to standard devices, MEMS accelerometers can measure down to DC, making them very useful for assessing slow moving machinery. This trait finds application in measuring vibration in wind turbines and the types of slowly rotating equipment used in industries such as metal processing and food and beverage, where rotational speeds of less than 60 rpm are common.
MEMS sensors can also recover from shock loads more quickly than standard piezoelectric accelerometers, allowing them to detect both impact events and subsequent vibration measurements reliably.
Microphone vibration sensors
Another option is microphone sensors. Vibrations create sound, some of which consists of high frequencies that are undetectable by humans. Microphone sensors can detect changes in these high frequency sounds and are a cost-effective method of providing some basic information.
With their wide frequency range, microphones can provide a clear and accurate representation of the frequencies present in a machine’s vibration pattern. However, although they can detect changes in frequencies, they cannot provide absolute values.For this reason, they are often used alongside accelerometers.
A typical product is the CM-01B Contact Microphone from TE Connectivity. This uses a sensitive yet robust PVDF piezo film, combining it with a low-noise electronic preamplifier. The design minimizes external acoustic noise while offering extremely high sensitivity to vibration. Features include a large bandwidth, high sensitivity, high impact resistance, light weight and low cost.
Other types of vibration sensor
Several other types of vibration sensor are available.
An eddy-current sensor is a type of non-contact sensor. It works by inducing eddy currents in the measured object by means of a coil. If the sensor is fixed in place and the object is experiencing vibration, that movement will register in the magnetic field. The change in distance between the coil and the target object produces a proportional output signal.
Because eddy-current sensors measure movement relative to the sensor’s position, they must be fixed in place. The fact that they don’t need to make contact with the asset makes them ideal for delicate assets or setups.
Another non-contact technique, laser displacement, uses a laser beam aimed at the asset. The beam reflects back to a detector. Changes in the object’s position will cause the beam to hit a different part of the detector, which is interpreted as a measurement of the object’s movement.
Gyroscopes are a contact technique that measures angular velocity - how quickly something is turning or rotating. They use MEMS technology to achieve an accurate measurement of how many degrees an object rotates per second. Gyroscopes aren’t usually used on their own, instead supplementing data from an accelerometer with orientation data.
One of these is the technology to be used. You may need a contact or a non-contact technology, want to measure very low or very high frequency vibrations, need to measure vibrations in more than one plane or require a very compact device. These requirements may limit the vibration detection technologies that can be used.
Sensitivity is another consideration. For example, the sensitivity of industrial accelerometers ranges between 10 and 100 mV/g. Choosing the correct sensitivity for an application involves knowing the range of vibration amplitude levels the sensor will be exposed to during measurements.
If the machine produces high amplitude vibrations (greater than 10 g rms) at the measurement point, a low sensitivity (10 mV/g) sensor is preferable. Conversely, if the vibration is less than 10 g rms, a 100 mV/g sensor should be chosen. In no case should the peak g level exceed the acceleration range of the sensor.
To select the frequency range, we need to determine the frequency requirements of the application. The high frequency range of the sensor is constrained by the sensitivity as it approaches resonance. Many sensor amplifiers also filter the high end of the frequency range to attenuate the amplitude of the resonance.
Most vibrations of industrial machinery contain frequencies below 1000 Hz, but there may be components of interest at higher frequencies. For example, with a shaft, the highest frequency of interest could be a harmonic that will be a product of the running speed and the number of bearings on the shaft.
Yet another factor is temperature, with sensors needing to survive the temperature ranges experienced in the proposed application. This is important, as the thermal expansion of the sensor’s metal casing or other metallic components could cause erroneous outputs.
There are many different sensors and finding the right vibration sensor for the application is important. Achieving an accurate measure of vibration is essential if condition-based monitoring of rotating machinery is to achieve all its potential benefits.