Choosing the Right Sensor for Measuring Distance

Distance-measuring devices help with positioning, material flow control, and level detection. However, there are several options to consider when it comes to choosing the correct sensor technology to measure distance. Here I’ll cover the three most commonly used types in the industrial automation world today, including photoelectric, ultrasonic, and inductive.

Photoelectric sensors

Photoelectric sensors use a light source, such as a laser or light-emitting diode, to reflect the light off an object’s surface to calculate the distance between the face of the sensor and the object itself. The two basic principles for how the sensor calculates the distances are the time of flight (TOF) and triangulation.

    • Time of flight photoelectric distance measurement sensors derive the distance measurement based on the time it takes the light to travel from the sensor to the object and return. These sensors are used to measure over long distances, generally in the range between 500 millimeters and up to 5 meters, with a resolution between 1 to 5 millimeters, depending on the sensor specifications. Keep in mind that this sensor technology is also used in range-finding equipment with a much greater sensing range than traditional industrial automation sensors.

    • In the triangulation measurement sensor, the sensor housing, light source, and light reflection form a triangle. The distance measurement is based on the light reflection angle within its sensing range with high accuracy and resolution. These sensors have a much smaller distance measurement range that is limited to between 20 and 300 millimeters, depending on the sensor specifications.

The pros of using photoelectric distance measurement sensors are the range, accuracy, repeatability, options, and cost. The main con for using photoelectric sensors for distance measurement is that they are affected by dust and water, so it is not recommended to use them in a dirty environment. The object’s material, surface reflection, and color also affect its performance.

Photoelectric distance measurement sensors are used in part contouring, roll diameter measurement, the position of assemblies, thickness detection, and bin-level detection applications.

Ultrasonic sensors

Ultrasonic distance sensors work on a similar principle as photoelectric distance sensors but instead of emitting light, they emit sound waves that are too high for humans to hear, and they use the time of flight of reflecting sound wave to calculate the distance between the object and the sensor face. They are insensitive to the object’s material, color, and surface finish. They don’t require the object or target to be made of metal like inductive position sensors (see below). They can also detect transparent objects, such as clear bottles or different colored objects, that photoelectric sensors would have trouble with since not enough light would be reflected back to reliably determine the distance of an object. The ultrasonic sensors have a limited sensing range of approximately 8 meters.

A few things to keep in mind that negatively affect the ultrasonic sensor is when the object or target is made of sound-absorbing material, such as foam or fabric, where the object absorbs enough soundwave emitted from the sensor making the output unreliable. Also, the sensing field gets progressively larger the further away it gets from the sensing face, thus making the measurement inaccurate if there are multiple objects in the sensing field of the sensor or if the object has a contoured surface. However, there are sound-focusing attachments that are available to limit the sensing field at longer distances making the measurements more accurate.

Inductive sensors

Inductive distance measurement sensors work on the same principle as inductive proximity sensors, where a metal object penetrating the electromagnetic field will change its characteristics based on the object size, material, and distance away from the sensing face. The change of the electromagnetic field detected by the sensor is converted into a proportional output signal or distance measurement. They have a quick response time, high repeatability, and linearity, and they operate well in harsh environments as they are not affected by dust or water. The downside to using inductive distance sensors is that the object or target must be made of metal. They also have a relatively short measurement range that is limited to approximately 50 millimeters.

Several variables exist to consider when choosing the correct sensor technology for your application solution, such as color, material, finish, size, measurement range, and environment. Any one of these can have a negative effect on the performance or success of your solution, so you must take all of them into account.

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.

Do Photoelectric Sensors Really Measure Up?

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I recently spent the week at the IMTS show in Chicago. If you have never been to this show I highly recommend you go in 2012. The show occupied over 1.1 million square feet and there were over 1700 manufacturers and or vendors. If you do attend this show I think, you really need take at least two days to go through it. No matter how comfortable you think your shoes are I guarantee that your feet will hurt after going through the four buildings of exhibits.

While I was working at our booth, I had several customers ask me about using photoelectric sensors in measuring applications. The questions ranged from the basic how they work, what are some of the application concerns to the infamous what is the accuracy of the sensor.

Here are some of the highlights to summarize our conversations:

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