Contact vs. Non-Contact Measurement and Linear Displacement Sensors
Linear displacement sensors are used to measure the distance between two points or two plane surfaces. They use various technologies, but there are two basic types: contact and non-contact. As their names suggest, contact sensors make physical contact with the object that is being measured and non-contact sensors do not. This is an obvious but important difference, but there is much more to consider.
Non-contact measurement is faster than contact measurement, especially for applications with high sampling rates. Because contact-type devices must touch and then traverse the object, measurement is slower. Non-contact systems can also measure more points at one time and without putting pressure on the object. They are also less prone to sensor wear and won’t dampen the motion of a target.
Although non-contact measurement has its advantages, contact-based measurement is a good choice for applications with low levels of cleanliness. Contact devices are also recommended for measuring exterior features that are not visible to non-contact devices. There are differences between contact sensors, however, and there also various non-contact measurement technologies.
Contact Displacement Sensors
There are many types of contact displacement sensors, including linear variable differential transformer (LVDT), string pot gauge, glass scale gauge, and sliding scale. All of these contact sensors provide a continuous output of the distance being measured, such as a voltage that is proportional to the distance, or a digital indication in engineering units such as inches or meters.
Figure 1 below shows a string pot, a cable-actuated position sensor that uses a spring-loaded spool to detect and measure linear position. Figure 2, a simple proximity switch, shows how a push rod can be used to indicate when an object being measured has reached a fixed distance. Proximity sensors can be linear devices but are typically just a switched output that indicates when a fixed distance is reached.
Figure 1: String Pot Linear Distance Sensor
Figure 2: Simple Proximity Switch
Non-Contact Displacement Sensors
Non-contact displacement sensors use technologies that include eddy current, capacitance, laser triangulation, confocal chromatic, and fiber optic. MTI Instruments, a leading providing of precision measurement solutions, provides highly-accurate capacitance sensors, fiber optic sensors, and laser systems that measure variables such as position, thickness, and distance with micrometer resolution.
The following sections provide a brief description of each technology along with images of MTI products and links where you can find more information.
Capacitance sensors are used for high-resolution measurements that require a high level of accuracy. They are not recommended for use in dirty environments, and are not affected by magnetic fields, temperature, humidity, nuclear radiation, or pressure. For thickness measurements that require nanometer accuracy, capacitance sensors provide excellent linearity. They are the best choice for semiconductor wafers.
Fiber Optic Sensors
Fiber optic sensors can accommodate a wide range of targets and shapes. The probes are immune to electromagnetic interference (EMI) and are suitable for surfaces made of metallic, composite, plastic, glass, or ceramic materials. Fiber optic sensors provide a sensitive, linear output with a large measurement range and standoff distance. They are the best choice for measuring high-frequency vibrations.
Laser Triangulation Sensors
Laser triangulation sensors are ideal for measuring moving targets in high-speed applications because they have a high frequency response. With their visible laser positioning, they also support ease of positioning and alignment. In addition laser sensors have larger standoffs than fiber optic or capacitive sensors meaning the sensor can be located further away from the target. Both 1D laser systems and 2D-3D laser systems are available.
Measuring Linear Displacement
There are two basic types of linear displacement sensors: contact and non-contact. Within each category of sensor, various technologies are used. Linear sensors provide a continuous measurement over a range of distance or displacement as opposed to proximity sensors which only indicate when a particular displacement has been reached. Proximity sensors usually have a go /no-go switched output where linear displacement sensors have an analog or digital output that is proportional to the distance/displacement.
MTI Instruments, a leading provider of precision measurement solutions, can help you to select the best non-contact sensor technology for your application. To get started, contact us.