Many products are available to provide length or distance measurement in automated machines. From vision sensors focused through a microscope to a laser bounced off of a part or target, one of the primary criteria to be considered will be the total length to be measured and the resolution required for the task at hand. This can narrow the choice of technology and designs considerably.
LVDT
Linear variable differential transformers (LVDTs) use a magnetically permeable core moving inside three windings. The center winding excites a voltage that transfers energy to the outer windings. The secondary winding’s voltages are measured, and the core’s position is calculated by subtracting E2 from E1.
LVDTs are mechanically simple devices with the core being the one moving part. The rated measurement distance provides a very linear signal so, in some cases, the output of the LVDT can be measured directly without the need for additional electronics but typically uses signal conditioners with industry-standard analog output.
The resolution of an LVDT itself is infinite and is limited only by the electronic signal conditioner. LVDTs are absolute devices, in that the core position can be computed as soon as the electronics are powered and operational.
Due to the simple construction of the device, it should provide a long-lasting solution and physically will tolerate a small amount of axial loading without loss of linearity. It is available in measurement ranges from sub-millimeter to up to more than a meter and operates in a wide range of environmental conditions.
Draw wire
Draw wire devices use a flexible wire or cable wrapped around a spool that is turned as the wire is pulled. The measurement function is accomplished by using a rotary potentiometer or encoder connected to the spool. Several technologies can be used to provide the positioning information.
Wire position is sensed using a single-turn potentiometer, multi-turn potentiometer, incremental encoder or absolute encoder. Pull distance and measurement resolution will drive selection of measurement.
In addition to the wear in the spool bearings and potentiometer wiper, if used, the windup spring may fail and the wire could stretch or break, but overall, the draw wire is quite reliable if nothing catches on the cable. It can be very rugged and requires little routine maintenance as long as the wire or cable is kept clean of debris.
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Device output is dependent on the sensing method used, as is device resolution, which is dependent upon the analog input device, or the pulse/meter ratio if using an encoder. The response speed will largely be dependent upon the mass of the spool mechanism. The encoder or potentiometer will likely be the most fragile part of the system.
Laser measurement
Lasers used to measure distance will employ one of two basic techniques. Time of flight, or phase shift, measurements are used when measuring longer distances. This method measures the time shift between generating a pulse of light and the detection of that pulse after being reflected back. Because of the speed of light and the response time of today’s electronics, time of flight measurements are typically only accurate to within a few millimeters.
The second general technique is triangulation. A laser is pointed at a target at some degree other than 90°. The reflected light is detected by a photodetector, usually a charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) element. The point where the light is detected is used along with the angle of the laser to calculate the other lengths of a triangle. Triangulation has much greater accuracy, down to 1 micrometer (1 micron), but has a much shorter measurement range. Triangulation also has a minimum distance it can detect. A typical sensor may be rated at 40 mm to 100 mm. If the target is closer or further than the rated distance, the reflected light misses the detector device.
Laser measurement devices are non-contact, solid-state devices. Therefore, there are no moving parts to wear out. The sensors are affected by environmental conditions. Laser measurements are affected by smoke and flying debris or dust, and some have optical components that will be affected at certain temperature extremes. Vibration and shock also often affect the physical construction of the laser. The laser displacement sensors are also fragile, so handle with care.
Tremendous advancements have been made over the years in laser sensor design and software to overcome some of the problems using light to measure distance. However, lasers work best when the target they are measuring is at a constant angle to the sensor, is a constant color and does not have foreign material, such as oil or dust, on the target surface.
If it can be measured, it is very likely there is a way for a linear measurement sensor. Ultrasonic, linear encoders, radar and other devices are not covered here. All technologies here have many different manufacturers, many different variations and a great many options. The major points to consider are operating environment, resolution, response speed and whether a contact or noncontact device is required.
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