Some less common sensor types can provide a solution to presence or motion sensing needs.
By Andrew Waugh, Product Manager for Sensor and Safety Products, AutomationDirect
While photo eye, limit switch and dc proximity sensors are the most commonly used types in machine automation applications, there are many other sensors available to help solve application issues. Here’s a look at some of the less common sensor types, along with selection criteria and common uses.
Color sensors are one of the few options available—other than much more complex and expensive vision systems—for color object detection, color quality control and color print accuracy applications. These sensors typically use a white light source to evaluate ROYGBIV colors. The mnemonic ROYGBIV defines the order and colors of the rainbow—red, orange, yellow, green, blue, indigo and violet.
These color sensors, usually with small spot diameters, reliably detect colored objects at varying scanning distances in automated high-speed applications, based on the previously defined (taught) reference colors. Integrated LED technology is often used to create the white light, with automatic adjustment of intensity.
Settings, such as taught samples, output functions, and switching output tolerances are frequently adjustable at the sensor through buttons and an OLED display. Some sensors provide the ability to select settings and configuration parameters with an IO-Link interface, expanding capability.
Typical color sensors are available for simultaneous detection of 3 to 12-colors, Figure 1. Each color or switch output is taught using an “okay” sample, which is the color of the object to detect, and a “not okay” sample, usually the background. Semiconductor color chip technology can divide a selected color into six or more spectral ranges, with separately adjustable tolerance or switching thresholds for each.
Color sensors can be configured with single-lens optics or fiber optic cable extensions to provide flexible mounting options at a variety of sensing distances. Single-lens optics use a reflective (diffuse) method to detect color, bouncing the light off the object. When used with glass fiber-optic cables, a sensor can be operated in a reflective mode, as well as a through-beam mode to look through an object. Some sensors can distinguish among up to twelve colors, each with a related and teachable switch output.
Common applications include verifying color in a painting process, measuring color in a printing operation, part sorting based on color, and detection of color registration markings on labels and packages. While color sensors are robust and reliable, problems can occur.
Glossy surfaces can cause sensor errors, which can often be addressed by mounting the sensor at a 10° to 30° angle from perpendicular to the surface or object being sensed. Too little reflected light can also be a problem with small or irregular shaped objects. And although there is a range of working distance, sensors must be mounted within this distance, and not too close or too far away from the object.
Presence through contrast
Contrast sensors are photoelectric sensors that evaluate contrast by sensing the red, green or blue (RGB) light reflected by the object when using a suitable light source. The sensor detects the difference in the light wavelength when reflected by an object, a mark or a background.
Contrast sensors typically have a sensing distance of 6 to 40 mm, and use either a white light, or an RGB light with adjustable color. White light sensors evaluate the reflected light based on a teach-in function used to set the target and background colors.
In more advanced applications, RGB lights can automatically select the optimal color light source for an application based on the colors of the target and background, usually determined using a teach function. The sensor then uses a threshold halfway between the target and background values to trigger an on and off sensor state.
These contrast sensors are used in a variety of industrial applications including paper and printing machinery, packaging lines, and beverage and bottling lines. They typically have a fast switching frequency of 5-50 kHz, making them suitable for use in these and other high-speed applications. Some sensors are built with a stainless-steel housing to provide IP69K washdown protection for use in pharmaceutical, food and beverage applications.
Another common application for a contrast sensor is detecting print marks such as lines or boxes of solid color in the margins of printed material in a web application, Figure 2. Due to the detection speed and repeatability of the sensor, they can be used to confirm or adjust position in web cutting or printing equipment. These sensors can also be used to detect presence of shrink-wrap seals, tamper-proof seals, labels, inserts and barcodes.
Ac proximity switches
While dc proximity switches are far more popular than their ac counterparts, the technology used in the ac version is similar to the dc version, and the operation is nearly identical. On new designs, if dc voltage is not available, the use of ac proximity switches is a reliable choice.
Proximity sensors detect the presence of metallic objects, typically at a range of less than 1.5 in. The sensing distance varies depending on the diameter of the sensor, with detection of ferrous metals allowing the greatest distances to the sensor. Other metals, such aluminum, reduce the detection range.
Many low-cost, reliable ac proximity sensors are available in a variety of shapes and sizes for a wide range of applications. Popular sizes include M8 (8 mm), M12 (12 mm), M18 (18 mm), M30 (30 mm) metal barrel, all for shielded and unshielded use. These ac sensors operate on a wide range of voltage from 20 to 253 Vac, and typically use a 2-wire electrical hookup.
These sensors are commonly used in many applications, similar to dc proximity switches. In the past, many applications used ac control circuits that today are now commonly dc control circuits. Fortunately, there are still options for replacement ac proximity sensors.
Area sensors properly applied
An area sensor can be thought of as a multi-beam array of photoeyes. It is used to detect presence within the controlled area of the sensor array, typically a square or rectangular plane. These sensors provide a simple method to detect different sized and shaped objects as they drop or pass through the two-dimensional target area, Figure 3.
An area sensor emitter produces near-infrared light at non-dangerous levels. The object is detected by reducing or obscuring the intensity of the reflected light beams hitting the receiver. The sensor’s photoelectric arrays use a linear series of lenses, usually with a 5-mm or 10-mm pitch. The sensor pitch, height and teach method affect the minimum detectable object size, as well as the maximum response time. The smallest detectable objects range from 1.5 mm to 10 mm.
These multi-beam and through-beam sensors can cover wide areas and are available in basic and advanced versions. The height of the area can range for approximately 70 mm to almost 1000 mm, with up to a 6-meter sensing distance between the emitter and receiver.
Discrete outputs of these devices are typically two states, light or dark, off or on. Some also have an analog output of 4-20 mA or 0-10 Vdc proportional to the unblocked detection height. When installed on a machine, some beams may be blocked, so a blanking function allows the active optic window to be adjusted to the specific application.
Applications include detecting a part dropped into another part, or into a reject bin for part counting and error proofing. Poka-yoke mistake proofing is another suitable application, and, by using several area sensors, a matrix of manual pick or place positions can be sensed using row/column logic. Sensors with an analog output can also be used as a coarse height or area measurement device.
Area sensors must not be considered safety devices. While similar in appearance and function to a safety light curtain, they are not suitable for safety applications, such as protecting an operator for a machine hazard.
Optical rotary encoders
A rotary encoder senses rotary position using an internal coded disc and a sensing head. Encoders are used to sense position, rotation and speed on machines, tooling and actuators.
An incremental encoder can connect to a high-speed PLC input or counter module for accurate position monitoring and control. Some absolute encoders can be wired to standard PLC dc input cards where the Gray code represents position, so high speed input modules are not required. Accessories such as mounting brackets and flexible couplings in aluminum and glass-reinforced resin simplify installation.
A variety of encoders are available in light-, medium- and heavy-duty configurations with various body diameters, shaft diameters and environmental ratings. Open collector, totem-pole or line driver outputs can be specified. Different pulses per revolution (PPR) can also be specified depending on application requirements.
There are some rules to follow when specifying PPR to simplify design and integration. The PPR frequency must not exceed that of the input device. The frequency is the encoder revolutions per second times its PPR. Some encoders have quadrature outputs, which increase the readable frequency by a factor of four.
Selecting a PPR that is an even multiple of the engineering units used is good practice. For example, if one revolution is one inch of travel, then consider choosing an encoder with a 1000 PPR. This simplifies calibration as each pulse is 1/1000th of an inch.
When it comes to sensors, keep in mind that there are more than photo eyes, limit switches and dc proximity sensors. Adding less common sensors makes sense in many applications—and can dramatically improve operation.