1. How do laser ToF proximity sensors work?
Optical ToF proximity sensors can measure distances between 0.1 and 100’s of meters by emitting a short laser pulse and counting the time taken for the pulse to reflect from the target and return. The distance to the target can be calculated from the measured time because the speed of light is a known quantity. The sensor consists of two central components; the laser diode and the photo detector, which are responsible for emitting and detecting the laser pulse. The remaining components of the sensor are the electronics for driving the emitter and conditioning the output signal. The electronics must operate at extremely high speed because of the phenomenally high speed of light. To give an idea of the challenge faced by the electronics, achieving a spatial resolution of 1 mm requires that the time be measured at a resolution of about 1 Pico second (that’s 1 billionth of a second!).
Optical ToF 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 laser ToF proximity sensors
Optical ToF proximity sensors are medium to high cost sensors capable of measuring targets at distances between 0.1 and 100’s of meters. They are available with either analog or Boolean output signals. Due to the extreme speed at which light travels, they suffer from low accuracy and spatial resolution. Optical 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 operate at high sampling rates of 1-250 kHz and can accurately measure the position of extremely high velocity targets. They have excellent environmental protection (IP67) and their readings are largely unaffected by contamination of the target with dirt, debris or liquid. However, they are sensitive to contamination of the sensor window with dirt and are not suitable for operation in environments subject to high intensity background light.
3. Input and output signals
Optical ToF proximity sensors operate on a 10-30 Vdc supply voltage from which the sensors internal electronics generates short electrical pulses to drive the laser diode. The sensor measures the time between emission and detection of the laser pulses, from which it calculates distance. Analog output sensors output a 0-10 V or 4-20 mA output signal that represents 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 output is normally open (NO) then movement of the target to within sensing range will cause the sensor output to switch from logic 0 to logic 1. If the output is normally closed (NC) then movement of the target to within sensing range will cause the sensor 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 laser ToF proximity sensors
Optical time of flight proximity sensors are widely used across many applications. They are particularly popular in collision detection, for which they are used in 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 space. Furthermore, optical ToF proximity sensors are widely used for counting products on conveyor belts. They are a particularly popular choice for applications where the sampling frequency and measuring range of ultrasonic ToF sensors is insufficient.
5. Typical specification
|Cost||Medium to high|
|Measurement range||0.1-300 m|
|Sample rate||1-160 kHz|
|Ambient temperature||-30 to 60°C|
|Supply voltage||10-30 Vdc|
|Output signal||: 4-20 mA /0-10 V or Boolean|
|Vibration resistance||10 g|
|Shock resistance||50 g|
|Passive / active||Active|
|Contact / non-contact||Non-contact|
6. Purchasing tips
- Adjustable range: Many optical ToF 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.
- Wavelength: Most optical ToF proximity sensors emit a red or infrared pulse. However, sensors are also available with blue laser pulses, intended for applications where there is significant red background light which would otherwise interfere with the sensor.
- Boolean output sensors have either a NO output, a NC output (for 3 wire sensors) or both (for 4 wire sensors).
7. Advantages of laser ToF sensors
Optical ToF proximity sensors:
- Can measure extremely large distances, often 100’s of meters
- Have very high sampling rates and can measure targets travelling at extremely high velocities
- Can detect most materials, with the exception of transparent materials
- Have excellent environmental resistance (typically IP67) and are not affected by dirt, debris and liquids.
8. Disadvantages of laser ToF sensors
Optical ToF proximity sensors:
- Have a low spatial resolution because of the very high speed of light.
- Cannot detect targets very close to the sensors face. The blind spot is normally 0.1-0.3 meters
- Cannot detect transparent objects and will struggle to detect highly reflective targets unless the sensor and target are accurately aligned so that the beam reflects into the detector.
- Are sensitive to intense background light. Most sensors can perform with background light intensities up to 50,000 lux. Direct sunlight can surpass 100,000 lux.
- Actual measurement range varies depending on the color and reflectiveness of the target
- Have a high power consumption, particularly for long range sensors.
9. Application tips
- Target color: Although optical ToF proximity sensors can detect objects of any color, dark colors lead to a reduction in operating range. Many manufacturers provide a recommended operating range for both white and grey targets. The ideal color is a matte white. If possible, avoid black or reflective (e.g. highly polished metal) targets.
- 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