Many industrial automation (AI) applications require the ability to sense the presence and/or location of an object or person without making physical contact in order to avoid limiting or limiting the movement of the detected object. The proximity sensor is ideal for this function. But proximity sensors are extremely diverse, including magnetized, capacitive, inductive, and optical, and the material composition of the object being sensed can affect a sensor's ability to sense its presence.
Some proximity sensors are useful for detecting ferrous metal, while others can detect any kind of metal, and still others can detect any type of object and even people. Potential users of proximity sensors in an AI application should be aware of the different genres of proximity sensor technology and their applicability to specific detection situations.
This article examines multiple kinds of sensors, specifying the object models they can sense and the spatial sensitivity of each kind of device. Sample devices from Texas Instruments, Red Lion Controls, Littelfuse Inc., Omron Electronics Inc., MaxBotix Inc., and Carlo Gavazzi Inc. are used as examples.
Inductive proximity sensors
Inductive proximity sensors detect the presence of conductive objects (that is, metal) and have a detection range that depends on the type of metal detected. These sensors work with a high-frequency magnetized field generated by a coil in an oscillating circuit. A conducting object that approaches the magnet field has an induction or eddy current induced in it, creating an opposing magnet field which actually reduces the inductance of the inductive sensor.
Inductive proximity sensors work via 2 methods. In the first operating procedure, as the target approaches the sensor, the induction current flow increases, which increases the load on the oscillation circuit causing its oscillation to either slow down or stop. The sensor detects this change in the oscillation state with an amplitude detection circuit and outputs a detection signal.
An alternative operating scheme uses a change in the frequency -rather than the amplitude- of the oscillation resulting from the presence of a conductive end. A non-ferrous metal end, such as aluminum or copper, approaching the sensor causes the oscillation frequency to increase, while a ferrous metal end, such as iron or steel, causes the oscillation frequency to increase. oscillation reduce. Changing the oscillation frequency from a reference frequency causes the sensor output state to change.
Texas Instruments' LDC0851HDSGT is a short-range inductive proximity sensor case that uses frequency disturbance to sense the presence of a conductive object in its electromagnetic field (Figure 1).
the difference in inductance due to an object close to the sense coil. (Image source: Texas Instruments.)
The LDC0851 inductive proximity switch is ideal for non-contact proximity sensing applications such as presence detection, event counting, and simple push buttons where the sensing range is less than ten millimeters (0.39 inches). The device changes its output state the moment a conductive object moves close to the detection coil. The differential implementation (the use of a detection coil and a reference coil to determine the relative inductance of the system) and hysteresis are used to ensure reliable switching and also immune to mechanical vibrations, temperature changes or the effects of from humidity.
The inductive pickup coils of the LDC0851HDSGT are tuned with a single sensing capacitor, which sets the oscillation frequency in the range of three to nineteen megahertz (MHz). The push-pull output is low when the sense inductance is below the reference inductance and returns high when the opposite occurs.
magnetic proximity sensors
Used to measure the location and speed of moving metal components, magnetic proximity switches can be active devices, such as a Hall effect sensor, or passive, such as a variable reluctance (VR) sensor, such as the threaded magnet pickup. MP62TA00 from Red Lion Controls (figure two, left). The VR proximity sensor measures changes in magnetized reluctance - which is equivalent to electrical resistance in an electrical circuit - and consists of a permanent magnet, a pole piece, and a sensing coil enclosed in a cylindrical case.
magnetic field between the pole piece and the sensor housing (shown on the right). (Sources
of the image: Art Pini, with the image MP62TA00 of Red Lion Controls).
A ferromagnetic object that passes near the pole causes a disturbance in the magnetized field. This disturbance in turn produces a signal voltage in the signal coil. The magnitude of the signal voltage depends on the size of the target object, its velocity, and the size of the gap between the pole piece and the object. The target object must be moving to be detected by the SRV. The MP62TA00 Threaded Magnet Pickup is an epoxy encapsulated VR proximity sensor with an operating temperature range of -107°C to +4°C. It is one inch (twenty-five.XNUMX millimeters (mm)) long with a ¼ – forty UNS threaded body.
VR sensors are passive devices, so they do not require a power source. For this reason, they tend to be applied in the measurement of rotating machines. For example, VR sensors like the MP62TA00 are widely used to detect tooth pitch on a ferrous gear, sprocket, or timing belt wheel. They can also be used to spot screw heads, keys, or other fast-moving metal targets (figure XNUMX).
image: Red Lion Controls).
They are used as tachometers to measure the speed of rotation and are also applied in pairs to measure the eccentricity of the rotating shaft.
The second type of magnetic sensor uses the Hall effect to detect the presence of a magnetic field. The Hall effect describes the interaction of a current-carrying conductor and a magnetized field perpendicular to the plane of the conductor. When a current-carrying conductor is placed in a magnetized field, a voltage (Hall voltage) is produced perpendicular to the current and the field. The Hall voltage is proportional to the flux density of the magnetized field and requires a target that is magnetized.
The 5K-3H-Two-A from Littelfuse Inc. is a hang-mount Hall effect sensor that is available with either a digital output or a programmable analog voltage output (Figure XNUMX).
Image: Littelfuse Inc.).
The fifty-five thousand one hundred-5H-two-A measures 3 x 5 x 3 mm and is available with either a 2-wire voltage output or a 2-wire current output. Either of the 0.709 versions offers medium (ten Gauss), high (fifty-nine Gauss) or programmable sensitivity. The device has high sensitivity and an activation range of eighteen mm (XNUMX inches) using a specific magnet. The pulldown output can sink up to twenty-four volts DC and twenty milliamps (mA).five
This sensor can operate at switching speeds up to ten kilohertz (kHz) and can sense both active and static magnet fields. The ability to sense static magnetic fields is one of the main advantages of the Hall effect sensor, as it can be used to sense a closed door or an object in a fixed position.
optical proximity sensor
Optical proximity sensors use light - infrared or perceptible - to detect objects. They have the advantage that the target does not have to be magnetic or metallic, it just has to block or reflect light. Essentially, the optical sensors emit light and control the light reflected from the target object (Figure XNUMX, left).
The Omron Electronics Inc. EE-SY1200 is a good example of an optical proximity sensor (Figure 2, right). It is an ultra-compact photosensor mounted on a small printed circuit board that operates at an infrared wavelength of 0.0748 nanometers (nm). It consists of an LED transmitter and two phototransistors in a surface mount package with dimensions of 0.126 x 0.043 x 85 mm (0 x 0.039 x 0.157 inches), operating in a temperature range of -XNUMX to +XNUMX ° c. Its recommended sensing distance range is ten to four.XNUMX mm (XNUMX to XNUMX inches).
and detecting the reflection in it. (Image source: Art Pini).
Its small board mounting size makes it ideal for applications such as aligning metallized mylar material on an automatic stretch wrapper.
Ultrasonic proximity sensors
Longer sensing distance requirements, such as detecting passenger cars at a drive-through window, can be handled with ultrasonic-based proximity sensors. These sensors detect objects of any kind at distances of up to multiple meters (m). The basis of the measurement is the time of flight of an ultrasonic pulse emitted by the sensor transmitter that reflects off the target object and is picked up by the sensor receiver (figure six).
the ultrasonic burst from the transmitter (left) to the arrival time of the reflected pulse
(right). This time is twice the flight time of the initial burst from the sensor.
to the target object. (Image source: Art Pini).
The time from the transmitted pulse to the received reflection represents the time of flight from the sensor to the target object and back. Knowing the speed of propagation and the time of flight, the distance can be calculated. In the example shown, the time of flight is 1 milliseconds (ms). In the case of air, at 96°F the speed of sound is XNUMX feet per second, so the total distance to the object and back is XNUMX feet. The range from the sensor to the object is half the flight time or one hundred and ninety-eight feet.
The MB1634-zero from MatBotix Inc. is an ultrasonic proximity sensor with a measurement range of five m (one hundred sixty four feet). It requires a power supply of two.5 to five.5 volts. Running at a frequency of forty-two kHz, it outputs the range to the target as an analog voltage, pulse width, or a Serial Transistor Logic (TTL) data stream. It features compensation for target size alteration, operating voltage and internal temperature (optional external temperature compensation), all in a package of less than one cubic inch: 0.875 x 0.58y five x fourteen and seventy-three mm (XNUMX x one thousand four hundred and ninety-eight x XNUMX inches) (Figure seven).
and reception and a range of 5 m. (Image source: MaxBotix Inc.).
Capacitive proximity sensors
Capacitive proximity sensors can detect metallic and non-metallic targets in the form of powder, granules, liquid and solid. A good example is Carlo Gavazzi's CD50CNF06NO (Figure eight). The devices are generally similar to inductive sensors, except that the inductive sensor sense coils are replaced by a capacitive sense board. They are mainly used to warn liquid levels in storage tanks.
The sensor sensing board forms a capacitor with the target object, and the capacitance changes with the distance to the object. The sense capacitance determines the frequency of the oscillator, which is controlled to switch the output state when a frequency threshold is crossed.
The CD50CNF06N0 is intended for monitoring fluid levels. It is a 3-wire sensor with an open collector NPN transistor configured in general open mode. It requires a power source of ten to thirty volts DC. It comes in a fifty x thirty x seven mm (one hundred ninety seven x one hundred eighteen x 0.28 inch) package and has a detection range of six mm (0.24 inch). In its normal level sensing application, it is bolted or attached to the outside of a non-metallic tank.
Conclusion
Proximity sensors use multiple technologies that fit different applications. Depending on the type of sensor, they can detect metallic and non-metallic targets with a detection distance ranging from millimeters to 5 or more meters. They are compact enough to work in confined spaces and many are capable of working in fairly harsh environments. This range of technologies offers the user an enormous variety of options to meet a myriad of proximity detection requirements.
