1. How do ultrasonic ToF proximity sensors work?
Ultrasonic time of flight proximity sensors measure the position of a target object up to 5 meters away by emitting high frequency ultrasound and determining the time taken for the ultrasound to reflect from the target and return. The distance to the target can be calculated from the measured time because the speed of sound is a known quantity. The sensor consists of two central components; the ultrasonic emitter and the ultrasonic detector, which are responsible for emitting and detecting the ultrasound. The remaining components of the sensor are the electronics for driving the emitter (which requires a high frequency AC driving voltage) and conditioning the output signal.
The emitter and detector are typically piezoelectric transducers. These are crystals that vibrate in response to an AC voltage (thereby emitting ultrasound) or emit a voltage in response to a vibration (thereby detecting ultrasound). The emitted ultrasound has a frequency of 100-400 kHz and therefore a wavelength between 3.4 mm and 0.85 mm. The wavelength places a practical limit on the measurement resolution and so sensors operating at high frequencies achieve higher spatial resolution. However, high frequency sensors also have shorter measurement ranges because the attenuation of ultrasound is greater at high frequencies.
Ultrasonic proximity sensors output either analog signals, e.g. 4-20 mA, or Boolean signals of logic 0 or logic 1 depending on whether a threshold distance has been crossed. Sensors with Boolean outputs include built-in hysteresis. The practical effect of the hysteresis is shown below. As the target moves to within 200 mm of the sensor, the sensor is triggered high. However, the sensor will only be triggered low once the target moves to at least 205 mm away. This prevents an oscillation of the sensors output state around a particular value.
2. General characteristics of ultrasonic ToF proximity sensors
Ultrasonic ToF proximity sensors are medium to high cost sensors capable of measuring targets at distances of up to 5 meters (though most are limited to 2 meters). They are available with either analog or Boolean output signals. Their accuracy is reasonably high (particularly if they are temperature compensated) and they can achieve spatial resolutions of up to 0.2 mm. Ultrasonic ToF proximity sensors are non-contact and solid state meaning that they are easy to assemble, do not experience mechanical wear and have an extremely long lifetime. They typically operate at sampling rates of 2-8 Hz, significantly lower than most other proximity sensors. As well as having excellent environmental protection (IP67), their readings are largely unaffected by contamination with dirt, debris or liquid.
3. Input and output signals
Ultrasonic ToF proximity sensors operate on a 10-30 Vdc supply voltage from which the sensors internal electronics generates short pulses of high frequency AC voltage to drive the ultrasonic emitter. The sensor measures the time between emission and detection of the ultrasound pulse, from which it calculates the distance to the target.
Analog output sensors output a 0-10 V or 4-20 mA output signal representing the distance of the target from the sensor face. Boolean output sensors output either 0 V (for a logic 0) or the supply voltage (for a logic 1), depending on whether a threshold distance has been crossed. If the sensor’s output is normally open (NO) then movement of the target to within sensing range will cause the sensors output to switch from logic 0 to logic 1. If the sensor’s output is normally closed (NC) then movement of the target to within sensing range will cause the sensors output to switch from logic 1 to logic 0. Boolean output sensors have either a NO output, a NC output (for 3 wire sensors) or both (for 4 wire sensors).
4. Applications of ultrasonic ToF proximity sensors
Ultrasonic time of flight proximity sensors are widely used across many applications. They are particularly popular in collision detection, for which they are used in motor vehicles and industrial robots. Within industrial robotics, they can be mounted to robots to detect impending collision or statically mounted to detect the presence of a person or foreign object within the robot’s work area. Furthermore, ultrasonic ToF proximity sensors are widely used for counting parts on conveyor belts. They are a particularly popular choice for applications where the resolution of optical ToF sensors is insufficient.
5. Typical specification
|Measurement range||0.2-5 m|
|Cost||Medium to high|
|Sample rate||2-8 Hz|
|Velocity||<5 m/s to maintain accuracy|
|Ambient temperature||-20 to 70°C|
|Supply voltage||10-30 Vdc|
|Output signal||4-20 mA / 0-10 V or Boolean|
|Vibration resistance||10 g|
|Shock resistance||30 g|
|Passive / active||Active|
|Contact / non-contact||Non-contact|
6. Purchasing tips
- Range vs Resolution: Operating at a low frequency improves measurement range but reduces spatial resolution. Conversely, a high frequency improves resolution but reduces range. There is therefore an inherent trade off between range and resolution.
- Adjustable range: Many proximity sensors include an adjustable range. For analog output sensors this enables scaling of the output to a specific part of the measurement range. For Boolean output sensors this enables adjustment of the threshold distance at which the output switches states.
- PNP or NPN: The transistor used to switch output states can be of either PNP or NPN type (also known as current sourcing and current sinking types respectively). When interfacing the sensor to a PLC, it is vital that the sensor is of the opposite type to the PLC port e.g. a current sourcing sensor will only work with a current sinking PLC port.
- Boolean output sensors: They have either a NO output, a NC output (for 3 wire sensors) or both (for 4 wire sensors).
7. Advantages of ultrasonic ToF sensors
Ultrasonic ToF proximity sensors:
- Can detect almost all material, irrespective of their optical, of electromagnetic properties
- Are highly immune to external electromagnetic and acoustic interference. However, ultrasound at a similar frequency to that used by the sensor, can cause interference.
- Have excellent environmental resistance (typically IP67) and are not affected by dirt, debris and liquids.
8. Disadvantages of ultrasonic ToF sensors
Ultrasonic ToF proximity sensors:
- Cannot detect targets very close to the sensor face. The blind spot is normally 10-20% of the maximum measurement distance.
- Are relatively expensive when compared to electromagnetic proximity sensors.
- Their spatial resolution, although reasonable, is much lower than that of optical triangulation proximity sensors.
- The accuracy of position measurement decreases with increasing target velocity because the targets position changes by the time the reflected ultrasound signal is detected. At a target velocity of 10 m/s, a position error of almost 3% can be expected.
- Have sampling rates of only 2-8 Hz, significantly lower than other proximity sensors and insufficient for many applications.
- Are not suitable for detecting very soft materials as these tend to absorb the ultrasound.
- Cannot be used in vacuums because without air there is no propagation of the ultrasonic wave.
9. Application tips
- Speed of sound: The speed of sound at standard temperature and pressure is 343 m/s but varies with changes in temperature and pressure. Since ultrasonic proximity sensors measure distance according to the time taken for a sound wave to reflect from a target, variations in the speed of sound cause measurement error. Whereas temperature variations are often (but not always) internally compensated for, pressure variations are not. By determining an accurate speed of sound, a correction factor can be calculated to compensate for any variations in the speed of sound.
- Reliable measurement range: Boolean ultrasonic ToF proximity sensors may not always perform robustly towards the upper extremes of their measurement range, particularly if the target is smaller than the ultrasonic beam cross-section. Many manufacturers therefore provide a typical recommended operating range within the total measurement range. For mission critical applications it is advisable to remain within the recommended operating range.
- Multiple sensors: Multiple Boolean output sensors can be connected in series or parallel to a single process controller input port to achieve logic functions. Two sensors connected in series act as an AND gate. Two sensors connected in parallel act as an OR gate. The circuits used to implement parallel and series connections can be found here