Some photoelectric applications require not only knowing if the object is present or not but exactly where the object is while providing a continuous or dynamic value representative of the objects location. For instance, if a robot is stacking a product is the stack at the correct height or how many additional pieces can be placed on the stack, how large is the coil or roll diameter of a product, and how high is the level or how much further can the product move before it is in position. Distance sensors can provide this dynamic information and in some case provide a digital output as well for alarms.
These sensors are normally based on diffuse sensing technology. However, in some cases retro-reflective technology is used for extremely long sensing distances. As with diffuse sensors there is only one device to mount and wire. However, due to the technology required for the higher resolutions, lensing, electronics and outputs these devices are typically much more expensive than a discrete diffuse sensor.
Similar to a diffuse sensor the distance sensor emits a pulsed light that strikes an object and a certain amount of light is reflected back to the sensor’s receiver. The sensor then generates an analog output signal that is proportional to the distance to the target. The technology that is utilized within the sensor to determine the distance is either Time of Flight or Triangulation.
Time of Flight sensors are more immune to target color and texture than light intensity based system because of the time component. These devices measure greater distances than the triangulation method however there is a sacrifice in resolution.
Triangulation sensors emit a pulsed light towards the target object. The light is then reflected back to the receiver. When the light reaches the sensor it will strike the photosensing diode at some angle. The distance between the sensor and the target determines the angle in which the light strikes the receiver. The closer the target is the sensor the greater the angle.
Triangulation based sensors being dependent on the amount of reflected light are more susceptible to target characteristics such as color and texture. These sensors are characterized by short to mid-range sensing distance however they provide higher resolutions than TOF sensors.
Output signals are either 0…10 volts, 1…10 volts or 4…20mA each of which has their pros and cons. Voltage outputs, 0 – 10 or 1- 10 volts, are easier to test and there is typically a broader offering of interface devices. However voltage outputs are more susceptible to noise from motors, solenoids or other coils and voltage drops of the wire. In addition generally voltage output cable runs should be less than 50 feet. Also since 0 volts is an acceptable output value broken wires, device failures, or power failures can go undetected.
Current outputs, 4 – 20 mA, provide the best noise immunity, are not affected by voltage drop and the cables lengths can exceed 50 feet. Since the sensor will be providing 4mA at zero distance its lowest possible signal, if the sensor should fail, the cable damaged or a power failure the interface device can detect the absence of the signal and notify an operator. Current outputs are more difficult to test and in some cases are affected by temperature variations.
For more information about photoelectric sensors, request your copy of Balluff’s Photoelectric Handbook.
Diffuse sensors depend on the amount of light reflected back to the receiver to actuate the output. Therefore, Light-on (normally open) operate refers to the switching of the output when the amount of light striking the receiver is sufficient, object is present. Likewise, Dark-on (normally closed) operate would refer to the target being absent or no light being reflected back to the receiver.
Retroreflective and through-beam sensors are similar in the fact they depend on the target interrupting the light beam being reflected back to the receiver. When an object interrupts the light beam, preventing the light from reaching the receiver, the output will energize which is referred to as Dark-on (normally open) operate switching mode or normally open. Light-on (normally closed) operate switching mode or normally closed output in a reflex sensor is true when the object is not blocking the light beam.
