Photoelectric Basics – Distance Measuring

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.

RetroreflectiveThese 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.

PrintTime 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.

PrintTriangulation 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.

Photoelectric Output Operate Modes and Output Types

Photoelectric sensors are used in a wide variety of applications that you encounter every day. They are offered in numerous housing styles that provide long distance non-contact detection of many different types of objects or targets. Being used in such a variety of applications, there are several outputs offered to make integration to control systems easy and depending on the sensing mode when the output is activated in the presence of the target.

DiffuseDiffuse 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.

RetroreflectiveRetroreflective 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.

signalsOutputs from photoelectric sensors are typically either digital or analog. Digital outputs are on or off and are usually three wire PNP (sourcing output) or NPN (sinking outputs). The exception to this is a relay output that provides a dry or isolated contact requiring voltage being applied to one pole.

Analog outputs provide a dynamic or continuous output that varies either a voltage (0-10 volt) or current (4-20mA) throughout the sensing range. Voltage outputs are easier to integrate into control systems and typically have more interface options. The downside to a voltage output is it should not be ran more than 50 feet. Current outputs can be ran very long lengths without worry of electrical noise. As additional advantage of the analog output is that it has built in diagnostics, at its minimum there will always be some current at the input unless the device completely fails or the wire is damaged.

Some specialty photoelectric sensors will provide a serial or network communication output for communications to higher level devices. Depending on the network, IO Link, for instance, additional diagnostics can be provided or even parameterization of the sensors. io-link
Interested in learning more about photoelectrics basics? You can also request a copy of the new Photoelectric Handbook.

The Other Retro-Reflective Sensors

Most of the time when we think of Retro-Reflective sensors the first thing that comes to mind is a photoelectric sensor. Photoelectric offerings use a reflector to reflect light from the internally mounted emitter and receiver. Retro photoelectric sensors come in many form factors with light source options such as infrared, red light and laser types.

Ultrasonic sensors are commonly forgotten when reflective sensors are needed in a particular application. Ultrasonic sensors when set up in “Window Mode” are similar to a photoelectric sensor however the ultrasonic sensor can use an existing background as the reflective surface such as a metal plate or a solid background. The sensor simply returns a signal as soon as an object fully covers the reflector. This mode is ideal for detecting difficult targets that photoelectric sensor can have trouble with such as poorly reflective materials

ultrasonicThe example shows an Ultrasonic sensor set up in window mode. The sensor is sending a sound wave to the background (reflector) so a target can be detected when entering the detection area between the sensor and the reflector background.

For more information Ultrasonic sensors, click here.