What is a displacement sensor?

What are Displacement Sensors (Displacement Gauges) and Dimension Measurement Systems?

What is a displacement sensor?

A displacement sensor (displacement gauge) is primarily used to measure the range of where an object has to travel and in relation to a reference position. Displacement sensors have multiple uses. Its primary use is for dimension measurement to figure out an object’s width, height, and thickness. Accuracy, application and usage environment is required when selecting the best equipment.

What is a measurement sensor?

A measurement sensor is a device that is used to measure an object to ascertain its dimensions such as by converting changes of light into electrical signals when the object intersects a laser beam.

New dimensional measurement technology has provided many benefits to the industry, such as enabling new manufacturing processes, increasing measurement throughput, and providing detailed information for process improvement during the manufacturing process. It’s vital that before you purchase and use new measurement technology, that you understand the full capabilities of the instrument in question. Understanding new measurement systems is much tougher then it appears. It’s so tough in fact that, at least in the U.S. industry, understanding the technology has proven to be a substantial barrier to the adoption of advanced measurement systems.

There are two primary categories that measurements with displacement measurement sensors fall under: contact measurement conducted while in direct contact with the object and non-contact measurement utilizing magnetic fields and sound waves. Below are the features for each method:

Item	Non-contact type				Contact type
Item	Optical type	Eddy current type	Ultrasonic type	Laser focus method	Contact type
Detection targets	Most targets	Metal	Most targets	Most targets	Solid
Measurement distance	Normal	Short	Long	Short	Short
Accuracy	High	High	Low	High	High
Response speed	Fast	Fast	Slow	Normal	Slow
Dust, water, oil, etc.	Poor	Strong	Normal	Normal	Strong
Measurement surface	Small	Normal	Large	Small	Small

Applications

Displacement sensors have many applications that serve a variety of purposes. There are a few examples of these applications below.

Outer diameter – measuring the diameter of a cylinder mass produced in a factory to determine whether or not the components are acceptable.

3D shape – not only checking to see if an item has warped, but also measuring the object in every direction on a flat surface.

Stroke and positioning – measuring the stroke amount of an object like a camera module. This is to check to see if the object has arrived in the correct position.

Height and step – ascertaining if the height of an object is completely uniform, and checking to see if there are any unusual steps that were taken.

Warpage and flatness – checking to see if an item, such as a steel beam, is warped in some way shape or form. You’re also checking how flat an object is, such as a metal plate.

Thickness – measuring to ensure that an object has the desired thickness. For example, if a block of wood is the desired thickness and falls within industry standards.

Triangulation Measurement Method

These sensors use a triangulation method to measure an object. Some sensors employ what is known as CMOS (CCD) and others employ PSD. We’ll take a look at both to gain a better understanding of what they are.

PSD Method

The light from a light source, such as a candle or flash light, is condensed by the lens and guided onto the object. The device where the light source is being redirected is called a one-dimensional position sensing device (PSD). The PSD receives the light through a receiving lens. While this is happening if the object being measured moves or shifts in any way, the image formation positions on the PSD will change and the balance may be thrown off. These sensors are rapidly becoming obsolete and being replaced by much more accurate and less expensive CMOS sensors.

CMOS (CCD) Method

A sensor that uses a CMOS (CCD) as the element receiving light can provide a much more accurate displacement measurement without being changed by the color of an object as well as the texture of the object. Furthermore, the sensor can detect the amount of light on individual pixels in the CMOS (CCD) and can convert them from a distance.

The position of the laser spot on the CMOS detector array is proportional to the distance between the sensor and the target. Typically, 1D sensors have a larger measuring range than capacitance probes with a little less resolution 2-200 mm ( 0.08 – 8 inch) range and down to 0.25 µm resolution (10 micro inch). MTII’s 1D sensors have analog and digital outputs, do not use a separate controller, and sample the target presence at up to 40k samples/second. This results in a frequency response of about 20 kHz.

Lasers are suited for applications where larger target standoff is needed and they also work on non conductive targets.

2D/3D Line Laser Displacement Sensors:

These sensors consist of a line laser emitter and a CMOS camera assembly. The sensor detects the whole profile of the target (linear displacement of the laser stripe) under the sensor at up to a 6 kHz rate, essentially using the same laser triangulation principle of the 1D laser sensors. If the sensor moves across a target surface, and tracks that motion with an encoder connected to the 2D sensor, it then becomes a 3D measuring system that can map the surface profile of the target (3D linear displacement).

The 2D/3D sensor is more sophisticated than 1D, requiring external software to process the height profile/displacement of what it is measuring.

The 2D/3D Protrak sensor has a GigE interface that can be used to capture 3D images of the target. In conjunction with standard camera vision processing software it can be used for OCR, barcode reading, and automatic inspection. These sensors are ideal for welding seam inspection, industrial robot process guidance, and automatic inspection.