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.

Back to the Basics: Measuring

In the last post about the Basics of Automation, we discussed how objects can be detected, collected and positioned with the help of sensors. Now, let’s take a closer look at how non-contact measurement—both linear and rotary—works to measure distance, travel, angle, and pressure.

Measuring travel, distance, position, angle and pressure are common tasks in automation. The measuring principles used are as varied as the different tasks.

Sensor Technologies

  • Magnetostrictive enables simultaneous measurement of multiple positions and can be used in challenging environments.
  • Magnet coded enables the highest accuracy and real-time measurement.
  • Inductive is used for integration in extremely tight spaces and is suitable for short distances.
  • Photoelectric features flexible range and is unaffected by the color or surface properties of the target object.

Different Sensors for Different Applications

Distance measurement

Janni1Disc brakes are used at various locations
in wind power plants. With their durability and precise measurement, inductive distance sensors monitor these brake discs continuously and provide a timely warning if the brake linings need to be changed.

In winding and unwinding equipment, a photoelectric sensor continuously measures the increasing or decreasing roll diameter. This means the rolls can be changed with minimal stoppages.

Linear position measurement

Janni4Workpieces are precisely positioned on the slide of a linear axis. This allows minimal loss of production time while ensuring quality. Magnetic encoders installed along the linear axis report the actual slide position to the controller (PLC) continuously and in real time — even when the slide is moving at a speed of up to 10 m/s.

In a machine tool the clamping state of a spindle must be continuously monitored during machining. This improves results on the workpiece and increases the reliability of the overall system. Inductive positioning systems provide continuous feedback to the controller: whether the spindle is unclamped, clamped with a tool or clamped without a tool.

Rotational position measurement

Janni5Workpieces such as a metal plate are printed, engraved or cut on a cut/print machine. This demands special accuracy in positioning it on the machine. Magnetic encoders on both rotating axes of the machine measure the position of the workpiece and ensure an even feed rate.

In a parabolic trough system,
sunlight is concentrated on parabolic troughs using parabolic mirrors allowing the heat energy to be stored. To achieve the optimal energy efficiency, the position of the parabolic mirror must be guided to match the sun’s path. Inclination sensors report the actual position of the parabolic mirror to the controller, which then adjusts as needed.

Pressure and Level Measurement

Janni7Consistently high surface quality of the machined workpiece must be ensured in a machine tool. This requires continuous monitoring of the coolant feed system pressure. Pressure sensors can reliably monitor the pressure and shut down the machine within a few milliseconds when the defined pressure range is violated.

Janni8In many tanks and vats, the fill height of the liquid must be continually measured. This is accomplished using ultrasonic sensors, which note levels regardless of color, transparency or surface composition of the medium. These sensors detect objects made of virtually any material (even sound-absorbing) including liquids, granulates and powders.

Stay tuned for future posts that will cover the essentials of automation. To learn more about the Basics of Automation in the meantime, visit www.balluff.com.

A Sound Solution for Object Detection and Measurement

Ultrasonic sensors can detect objects that many traditional sensing technologies cannot because of their ability to see targets regardless of color, transparency and surface texture. Even in dusty, humid, or hazy environments, they may be the only sensor that is able to provide the desired results. Ultrasonic sensors are often used in liquid level measurement applications and detecting clear films.

Ultrasonic sensors emit a burst of short, high frequency sound waves that propagates in a cone shape towards the target. When the sound waves strike the target, they are bounced back to the sensor. The sensor then calculates the distance based on the time span from when the sound was emitted until the sound was received.

Continue reading “A Sound Solution for Object Detection and Measurement”