Why optical displacement sensors replace tactile sensorsMarch 14, 2023
Displacement sensors are widely used in many industries to measure the distance between an object’s position and a reference location. In addition to measuring the range an object travels, displacement sensors can also be used to gauge an object’s dimensions, such as width, length and height. Two broad categories of displacement sensors exist: tactile sensors that gauge displacement by directly contacting the target, and non-contact sensors that perform measurements via electromagnetic waves or optical beams without touching the target. The proper selection of a displacement sensor depends on an evaluation of the application, including required accuracy and the environment in which the sensor will be used.
Tactile displacement sensors such as the linear variable differential transformer (LVDT) convert the motions of objects to which they are mechanically attached into electrical signals. An LVDT houses a movable core within a hollow tube-like assembly containing a primary winding in the middle of two secondary windings. The primary winding is energized with an alternating current (AC) and as the core moves within the center of the tube, a differential AC voltage is generated between the secondary windings. This differential voltage is converted into the LVDT’s electrical output signal, often a direct current (DC) voltage or current.
Optical displacement sensors
Two types of optical displacement sensors are laser triangulation, and confocal-chromatic; both shown in Figure 1. Laser triangulation sensors measure distance to an object via angle calculation. The reflected light from a target illuminated with a projected laser spot enters a receiving element at a particular angle, depending on the distance. The distance to the target is calculated by comparing the location of the light on the receiver element, and the distance from the laser projector to the receiver element.
Confocal-chromatic sensors gauge distance by sensing differences in color. The sensor emits a beam of white light through an arrangement of lenses that break down the light into monochromatic wavelengths via chromatic aberration (also known as imaging error or longitudinal color aberration) of the lenses. The light reflected by the object is gathered and analyzed by a spectrometer within the sensor. The color of the reflected light corresponds to the distance between the target and the sensor.
Advantages and limitations
Tactile displacement sensors and optical displacement sensors each achieve the same result, measuring distance between an object’s position and a reference location, but each type of displacement sensor has advantages and limitations.
Tactile LVDT sensors are based on a proven inductive measuring principle, have a robust sensor design for industrial applications and exhibit good temperature stability. Tactile sensors excel in industrial measurement tasks with their excellent protection against humidity and dirt, good resistance against vibration and shock, and high signal quality.
Tactile sensors do, however, have some disadvantages, including lower measurement speed, requiring contact with the target and mechanical wear. Typical measurement speeds of LVDT sensors and gauges vary from about 30 Hz to 300 Hz, slower than standard optical displacement sensors which realize 50 kHz or even higher. Also, the target must be physically connected to the plunger, limiting maximum measurement speed. Furthermore, the touch points of the plunger can be visible on the target, a particular limitation for sensitive targets like thin glass sheets. Mechanical wear of an LVDT gauge with integrated spring can also add up over time, necessitating replacement of the gauge after its specified lifetime cycles have been expended.
Optical displacement sensors, on the other hand, are a modern technology exhibiting high speed and high accuracy, non-contact measurement without touching the target, and a tiny measurement spot. Fast measuring rates make optical displacement sensors ideal for dynamic measurement tasks. Certain confocal-chromatic sensors can dynamically adapt the target surface’s illumination by regulating the exposure of the CCD line to compensate for changes in the reflectivity of the target. This allows very accurate measurements even at high rates and with changing surface colors. The small measurement spot size — less than 3 µm in some confocal-chromatic sensors — enables even the finest structures and details to be easily detected. Optical displacement sensors are also easily integrated in industrial measurement systems, with compatibility with multiple interfaces such as field bus, analog, RS422, Ethernet, EtherCAT, Profinet and EtherNet/IP.
A drawback of optical displacement sensors is poor performance in dusty, high temperature environments. Dust obscures the line of sight between the sensor and the target, inhibiting accuracy. In addition, at high temperatures, typically over 50° C, the optical and mechanical components of a laser sensor can expand enough to reduce the sensor’s measurement accuracy. This effect can be reduced with a cooled housing.
Tactile sensors and optical sensors are two categories of devices that perform displacement and position measurements. Each type of displacement sensor has advantages and limitations, requiring an evaluation of each application’s operating requirements. Ultimately, though, optical sensors offer higher accuracy, higher measurement speed and contactless measurement.
Learn more about Micro-Epsilon's confocal-chromatic or laser sensors for use in measurement and monitoring tasks in industrial applications like factory automation, semiconductor production, robotics and plastics production.