Positioning sensors identify positions by detecting the target&#;s distance, height, width, and other information. Different sensors can be selected according to the workpiece shape, conditions, and application.
Contact-type displacement sensors that operate on the typical principle of linear variable differential transformers (LVDT) detect and measure the changes in the height of the target by capturing the mechanical linear movement of the probe as displacement. For non-contact displacement sensors, laser displacement sensors are the most common among optical types due to the linearity of the light. Laser displacement sensors can be further classified into reflective and thrubeam models. Reflective models detect the position and height based on the distance from the target. Thrubeam laser displacement sensors can continuously detect not only the position of objects passing vertically through the band of signal light, but also their outer and inner diameters.
Sensors used for positioning workpieces can be selectively applied according to the workpiece shape and detection conditions. Detection methods are broadly categorized into two groups: contact and non-contact. Contact displacement sensors (touch sensors) that bring a probe into direct contact with a target are typical contact-type positioning sensor. Laser displacement sensors are a typical example of non-contact positioning sensors, using the linearity of laser light for detection. The following section explains the types and working principles of these typical positioning sensors.
Linear variable differential transformer (LVDT) method:
An LVDT displacement sensor detects mechanical linear movement as displacement and converts it to an electrical signal.
This type of sensor has a core at its center with coils located around it. A probe is attached to the tip of the core to make up a spindle. The spindle is pushed against the target with a spring mechanism, sliding along with the changes in the target&#;s height. A magnetic field is generated when current passes through the coil, moving the core inside. The resistance of the coil and the output signal level change according to the amount of core movement. The movement of the core shows changes in the shape of the target, which means displacement can be measured from the changes in the output signal level. These changes can be used for height measurement and positioning.
Scale (pulse count) method:
Contact displacement sensors using the scale method are also called pulse-count sensors.
The vertical movement of the spindle caused by it coming into contact with the target is converted by an internal scale into signals to detect the position. Pulse counting comes in two types: magnetic pulse counting, which counts pulses using an S(N)-pole magnetic field; and optical pulse counting, which counts pulses using a light receiving element with slits on the scale as a light source.
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The detection accuracy of the scale method depends on the accuracy of the built-in scale.
KEYENCE developed the Scale Shot System II, the world&#;s first system that uses a CMOS sensor for high-speed scanning of an absolute value glass scale that incorporates slits with complex patterns that vary depending on the position. This allows the sensor to measure height (position) with precision, obtain the absolute position, and detect position information, which means it does not need zero point adjustment. These properties also offer other advantages, such as protection against tracking errors and good temperature characteristics.
However, contact-type positioning sensors are not suitable for use with soft workpieces that can be deformed by the probe and workpieces that are prone to scratching.
Reflective laser displacement sensors:
One sensor head emits laser light on the target, and the reflected light is received by the built-in light receiving element (e.g. CMOS). This type of sensor can capture the distance to the target based on the changes in the position of the reflected light received by the CMOS through the receiver lens and detect the position and height. This principle is called the triangulation method because the laser&#;s path is a triangle.
Laser displacement sensors that use a CMOS as the light receiving element can provide high-accuracy position detection.
Thrubeam laser displacement sensors:
This type of sensor creates a band of laser signal light between the laser transmitter and the receiver. When a target passes through the signal light, part of the signal light is blocked. The changes in the amount and position of laser light coming through are captured by the light receiving element (e.g. CCD) to detect the target without coming into contact with it.
The width of the band of the laser varies, which allows use in a variety of applications, such as target position and edge detection as well as clearance and inner/outer diameter measurement, regardless of the workpiece shape or purpose.
Laser displacement sensors can not only position targets without touching them, but also support a wide range of applications by placing multiple sensor heads according to the workpiece shape or purpose. Multiple reflective models can be placed targeting flat workpieces from different directions to measure their flatness based on height differences. Thrubeam models can detect the outer and inner diameters of objects.
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