Contrast sensors are photoelectronic sensors that work according to the principle of a reflex sensor: Emitted light is reflected by the object, reabsorbed and evaluated.
White light is used to detect printing and contrast markings in different colors. Laser light is used to distinguish different objects based on contrast differences.
Contrast sensors with white light operate with very high switching frequencies and a rectangular light spot. All colors can be optimally detected thanks to the white light.
Laser light is used at greater distances of up to 150 mm. The focused small light spot is ideal for very small and fine objects.
White light is produced in the RGB color model by additive color mixing. This is achieved by mixing red, green and blue light.
The RGB photodiode consists of three areas corresponding to the RGB color space. Depending on the reflected color, the light is evaluated by the three channels according to the reflection properties.
The monochrome photodiode consists of a single continuous surface. It evaluates the received brightness and sorts it into a grayscale.
Jitter refers to the time variation in the detection of an object. This is the variance of the response time. Low jitter ensures high positioning accuracy at high machine speeds.
Contrast sensors with white light achieve maximum switching frequencies of 50 kHz. Contrast sensors with laser light achieve switching frequencies of up to 20 kHz.
The temperature range is from min. –25 °C to max. 60 °C. The temperature drift of the sensors is low. As a result, they have consistently good measurement performance under fluctuating temperature conditions.
White light contrast sensors are suitable for applications with near-range contrast markings up to 40 mm. Contrast sensors with laser light can be used for contrast differentiation up to 150 mm.
For shiny objects, the contrast sensor is mounted at an angle of approx. 10 degrees.
For materials with a printed pattern, a black contrast marking is recommended if the pattern of the background material does not contain black. This means that only very dark marks are detected and patterns of different colors are hidden. The following applies invariably: The higher the contrast between the mark and the background, the better.
For transparent films, it is important to consider what material is behind the film itself. If the material is light, we can use either a dark or a light marking on the transparent film. If the material behind the film is dark or there is nothing behind it, the application with a light marking will work better than with a dark one.
Shiny surfaces reflect a lot of light back to the sensor, similar to a mirror. In this case, a dark mark would be best suited. In applications with shiny objects, the inclined design of the sensor is also crucial to increase the accuracy of the color information and to increase vibration resistance.
Contrast sensors have a defined working range in which they work. It must be ensured that the objects to be detected are within the specified working area. Variations in distance affect the received light intensities and may reduce detection performance.
For markings on a constant, monochrome background, it is recommended to use print mark mode. Contrast mode is recommended for black marks on a patterned background. If only marks with specific colors are to be detected, this can be achieved with the color mode. Jump detection can be used in both print mark and contrast mode for applications where different materials are used and the mark is not always the darkest or lightest of the possible patterns.
Excessive vibration, ambient light or electrical interference can affect the performance of contrast sensors.
Improper mounting may affect the accuracy of a contrast sensor. In this case, the direction of movement to the side of the sensor and the inclination as well as the specified working distance must be observed. Crinkly or shiny materials may also affect accuracy.
Contrast sensors with white light have a rectangular light spot. This makes them particularly suitable for detecting rectangular marks in dynamic processes. However, other objects can also be detected as long as the light spot can be mapped on the object.
Illumination can affect the light reflection of objects. To optimize the performance of a contrast sensor, the lighting situation should therefore be constant.
