Product Detection and Ranging Using Ultrasonic Sensors

Agricultural machinery, logistics operations, material handling systems, and industrial processing equipment are a few of the applications demanding increasingly accurate item detection and distance measurements. Often the objects can vary in color, be shiny, and/or have transparent surfaces, making accurate detection and range measurements challenging. Adverse conditions such as dust, dirt, fog, and operating temperatures up to +70°C, can present more obstacles to accurate item detection and distance measurements. The area over which the sensors need to operate can vary from a few millimeters (mm) to several meters, both in width of the detection area and the distance to the object. The availability of analog and digital outputs plus the programmability of the measurement is often needed to support flexible system implementations. Ultrasonic sensors can be used to address all of these application needs.

This article examines the operating characteristics of ultrasonic sensors and how to use these sensors in item identification and distance measurement applications. The article will describe adjustable operating modes, including; switching point (DtO) mode, switching window (Wnd) mode, and object between sensor and background (ObSB) mode. It will then present the options to synchronize up to 50 sensors versus multiplexing up to 50 sensors, compare adjustable measurement filters and sound beam shaping, and close with examples of ultrasonic sensors from SICK that offer varying ranges, measurement speeds, and combinations of analog and digital outputs.

An ultrasonic sensor emits a high-frequency acoustic pulse (typically from about 80 kHz up to several hundred kHz) reflected by the object being detected (Figure 1). The round-trip time needed for the pulse of acoustic energy to travel from the sensor to the object and back is measured by the sensor. Internal electronics calculate the distance to the object by multiplying the speed of sound times the total travel time divided by 2. The distance calculation includes temperature compensation (the speed of sound varies with the temperature) to ensure accurate measurements. In addition, the measurement provides color-independent object detection and range measurement, plus immunity to contaminants such as dirt, dust, humidity, and fog.

Product Detection and Ranging Using Ultrasonic SensorsFigure 1: Ultrasonic sensor distance measurement; Distance = temperature compensated speed of sound x total acoustic time of flight (t2) / 2. (Image source: SICK)

Ultrasonic sensors can detect colored, shiny, or transparent surfaces that can be challenging for optical sensor technologies. They can be used to detect solid, powdered, or liquid materials, making them useful across a wide range of applications. Materials that are less sound absorbent can be detected at longer ranges using a given sensor. Under optimal conditions, with an easily-detected object, an ultrasonic sensor can be used up to its ‘limiting range’ (Figure 2). The sensitivity and detection range of a sensor is dependent on the acoustic reflectivity, size, and alignment of the object to be detected. In some cases, acoustic sensors can detect very small objects such as metal wires. Ultrasonic sensors are highly versatile and are used in diverse applications, including:

  • Filling level monitoring
  • Detection of different types of objects
  • Collision prevention
  • Identification of empty containers
  • Rip detection
  • Diameter control
  • Object positioning
  • Dimensional measurements

Product Detection and Ranging Using Ultrasonic SensorsFigure 2: Ultrasonic sensor scanning range increases for larger objects, such as the Aligned plate (top) versus a Pipe (bottom). The less sound the object being measured absorbs, the greater the possible detection range, up to the limiting range. (Image source: SICK)

Operating modes

There are several operating modes for individual ultrasonic sensors and groups of sensors. The individual sensor can be used in three basic modes:

  • The simplest mode is distance to object (DtO) (sometimes referred to as ‘simple switching point’ or ‘sensor on object’). In DtO, the detection and distance to the object are transmitted as soon as the object is within range.
  • Window (Wnd) mode is an extension of DtO where the detection of an object only occurs when the object is within a defined distance (the ‘window’).
  • Object between sensor and background (ObSB) mode teaches the sensor a background to reference, and the sensor then detects any object that obscures the background. For example, ObSB can detect flat objects (such as envelopes) on a conveyor belt.

Filtering and adjusting sensor operation

In addition to selecting the needed operating mode, ultrasonic sensors can be multiplexed, synchronized, filtered, and the sound cone itself can be adjusted (Figure 4). Filtering can be used to adjust the sensitivity of the measurement. The greater the level of filtering, the better the ability to detect small or irregularly shaped objects. Lower levels of filtering can improve the ability to detect fast-moving objects.

The switching frequency measures how often an ultrasonic sensor turns on and off per second. Switching frequencies can vary, with typical values from 1 to 25 Hz. Hysteresis measures the difference between the switching point and the reset point and is typically adjustable to the requirements of a specific application.

Some applications require the use of multiple sensors, for example, to cover a wider detection area than can be handled with a single sensor. Crosstalk between the sensors can lead to spurious data and needs to be controlled. For example, if one sensor picks up the signal from another sensor, it can result in the “detection” of an object that does not exist. The use of adjustable sound cones together with multiplexing and synchronization can control crosstalk in multiple sensor applications.

Product Detection and Ranging Using Ultrasonic SensorsFigure 3: Ultrasonic sensor features include multiplexing or synchronization of multiple sensors (top left), adjustable sound cones (right), and average value filtering (center bottom). (Image source: Digi-Key)

Sensor synchronization eliminates crosstalk by switching all the sensors simultaneously, eliminating the potential for errors from varying sensor timing. It also reduces the potential for interference between sensors. Synchronization can enable sensors to be mounted closer together. The use of multiple sensors can increase the size of the detection zone.

While synchronized sensors can be used to monitor any object or objects in a defined area, multiplexed sensors can be used to measure the dimensions of an object (Figure 4). Multiplexed sensors are switched asynchronously, performing alternating measurements. Multiplexing sensors can support very small mounting distances between sensors while maintaining the ability to make independent measurements without crosstalk. The asynchronous sensor operation increases the response time of multiplexed sensor systems. In some applications, it is possible to use synchronized sensors while still controlling crosstalk, achieving a faster system response time than multiplexing.

Product Detection and Ranging Using Ultrasonic SensorsFigure 4: Multiplexed sensors can be used to measure the dimensions of an object (left). Synchronized sensors can monitor any object or objects that might be in a defined area (right). (Image source: SICK)

UM30 ultrasonic sensors

The UM30 family of ultrasonic sensors supports all the filtering, operating modes, and adjustments outlined above. For applications that need sensing ranges from 30 millimeters (mm) to 250 mm, the model UM30-211111 offers a digital output. In contrast, the UM30-211118 provides a choice of digital or analog output, and, like all UM30 sensors, these units feature an LCD display for fast and flexible sensor adjustments (Figure 5). These ultrasonic sensors have a fast switching frequency of 25 Hertz (Hz) with an output time of 8 milliseconds (ms) and a response time of 32 ms. They have a typical ultrasonic frequency of 320 kHz.

Product Detection and Ranging Using Ultrasonic SensorsFigure 5: The model UM30-211111 offers a digital output, while the UM30-211118 offers a choice of a digital or analog output. Both feature an LCD (upper left on the sensor) for fast and flexible sensor adjustment and a sensing range from 30 mm to 250 mm. (Image: Digi-Key)

Several UM30 sensors are available with analog and/or digital outputs. Analog outputs feature an automatic selection of analog current or voltage output depending on the load. They are available with an output of 4 mA to 20 mA (load ≤ 500 Ω) or 0 to 10 Vdc (load ≥ 100,000 Ω), with a 12-bit resolution. A variety of digital outputs are offered including: 1 x PNP, ≤ 200 mA; 1 x NPN, ≤ 200 mA; 2 x PNP, ≤ 200 mA; 2 x NPN, ≤ 200 mA; or a push-pull PNP/NPN rated for ≤ 100 mA.

Applications that need longer sensing ranges from 600 mm to 8 meters can use the UM30-215113, which offers an analog output, or the UM30-215118, which offers digital or analog outputs. These sensors have a typical ultrasonic frequency of 80 kHz, with a switching frequency of 3 Hz, an output time of 60 ms, and a response time of 240 ms.

Product Detection and Ranging Using Ultrasonic SensorsFigure 6: Applications that need sensing ranges from 600 mm to 8 meters can use the UM30-215113, which offers an analog output, or the UM30-215118 that offers a choice of digital or analog outputs. (Image: Digi-Key)

UM30 sensors are available that support a wide variety of operating range/limiting range, output time/response time, and switching frequency combinations (Table 1). Shorter ranges correspond with faster output and response times and higher switching frequencies. The response times are based on a digital output or unfiltered analog output. Filtering and smoothing of an analog output, depending on the application, can increase the response time by up to 200%.

Operating range/limiting range Output time/response time Switching frequency
30 mm – 250 mm / 350 mm 8 ms / 32 ms 25 Hz
65 mm – 350 mm / 600 mm 16 ms / 64 ms 12 Hz
200 mm – 1,300 mm / 2,000 mm 23 ms / 92 ms 8 Hz
350 mm – 3,400 mm / 5,000 mm 43 ms / 180 ms 4 Hz
600 mm – 6,000 mm / 8,000 mm 60 ms / 240 ms 3 Hz

Table 1: Comparison of operating range/limiting range, output time/response time, and switching frequency tradeoffs available with UM30 ultrasonic sensors. (Table source: SICK)

IO-Link simplifies ultrasonic sensor system deployment

Using IO-Link, the detection range of UM30 sensors can be continuously adjusted. The sound cone size and shape and other detection parameters can be optimized as needed by changing application requirements. Some I-O Link benefits include:

  • Flexible sensor diagnostics that enable preventative maintenance
  • Increased machine availability through simplified sensor replacement
  • Increased system productivity with integrated communications at the Fieldbus level
  • Increased application flexibility by reconfiguring sensors online as process needs change
  • Reduced project cost through the use of unshielded cables that support interference-proof signal transmission and increased system reliability

Summary

Ultrasonic sensors can address a wide variety of object detection and distance measurement needs from 30 mm to 8 meters. SICK’s UM30 family of ultrasonic sensors supports output filtering and numerous operating modes for increased application flexibility. Up to 50 sensors can be multiplexed or synchronized. The sound cone size and shape and other detection parameters can be optimized as needed by changing application requirements. The integrated IO-Link connectivity brings increased flexibility, reliability, and efficiency and can reduce system operating costs.