Non-Contact Measurements

There are many instruments to measure position, distance, or vibration of an object. These can be segregated into two basic categories: contact and non contact.

Popular contact methods are: Linear Encoders, String Potentiometers, and Linear Variable Displacement Transducers (LVDTs). Some of the benefits of contact measuring systems are: long measuring range, target material insensitivity, a small spot (measuring area) size, and generally lower cost.

While contact instruments are suitable for many applications, they have a limited frequency response and can interfere with the dynamics of the object being measured. Where these factors are a concern, non contact methods have advantages. Below is a list of several types of non contact measuring technologies with some of their features.

Air Gauging: This technique uses air pressure and flow to measure dimensions or inspect parts. These devices operate on changes in pressure and flow rates to make a measurement. A clean air supply is required. It is acceptable for use on most target materials and typically used for small measuring ranges of 0.010 to 0.200 inches in production environments.

Hall Effect: This sensor varies its output voltage in response to changes in magnetic field. With a known magnetic field, distance can be determined. A magnetic target or attachment of a magnet to the target is required. These sensors are generally inexpensive and are used in consumer equipment and industrial applications. They are also commonly used in automotive timing applications.

Ultrasonic: Ultrasonic sensors operate on a principle similar to sonar by interpreting echoes of sound waves reflecting off a target. A high frequency sound wave is generated by the sensor and directed toward the target. By calculating the time interval between the sent and received signals, distance to the target is determined. Ultrasonic sensors have long measuring ranges and can be used with many target materials, including liquids. Performance is affected by shape and density of the target material. They have lower resolution than most other non contact technologies and cannot work in a vacuum. Ultrasonic sensors are frequently used to measure liquid level in tanks and in factory automation and process industries.

Photonic: Photonic sensors use glass fibers to transmit light to and from target surfaces. Displacement is determined by detecting the intensity of the reflected light. These sensors have a very small spot size and can be used to detect small targets. They can be used with most target materials and in hostile environments. They are also insensitive to interference from EMI or high voltages. Photonic sensors are generally used for small measuring ranges and can have high resolution and frequency response. But, they are sensitive to environmental contaminants and target finish variations.

Capacitance: These sensors work on the principal of capacitance changes between the sensor and target to determine distance. They can be used with all conductive target materials and are not sensitive to material changes. Capacitive sensors have a relatively small spot size and are not sensitive to material thickness, but typically require a target grounded to the measuring system. They can be constructed of very high temperature materials for measurements up to 1200°C. These sensors have a small measuring range to sensor diameter ratio and are sensitive to environmental changes and contamination.

Laser Triangulation: These sensors work by projecting a beam of light onto the target and calculating distance by determining where the reflected light falls on a detector. They can measure longer ranges than other non contact technologies; can be used with most target materials, and have a very small measuring spot size. They tend to be the most expensive type of non contact sensor. Measurement is affected by environmental contamination and surface finish variations of the target.

Inductive Eddy Current: Inductive eddy current sensors operate by generating a high frequency electro-magnetic field about the sensor coil which induces eddy currents in a target material. A conductive target is required, but a ground connection to the measuring system is not necessary. Sensor performance is affected by target material conductivity. Inductive sensors have a large spot size in comparison to other technologies. Performance is affected by temperature changes, but not by environmental contaminants or target finish characteristics. They can operate in a vacuum or in fluids. Non conductive material between the sensor and the target is not detected. The measuring distance is typically 30-50% of sensor diameter.

As with any device, both contact and non contact measuring technologies have a wide range of performance characteristics ranging from very low (on-off) to very high precision (nanometer resolution), depending on their construction. It is not only necessary to choose the correct technology, but also the correct level of performance for an application.