During project planning, design engineers should conduct a thorough review of the environmental, mechanical, electrical, and magnetic parameters of the industrial application. Engineers should analyze the entire magnetic circuit of the industrial application—including sensor products and magnetic actuators. Taking all critical factors into account, engineers may recommend the use of powerful, custom-designed sensor products to meet the unique requirements of the industrial automation system.
This article will discuss how to select highly accurate and reliable sensor products for industrial automation applications. After defining and describing the advantages of several analog and digital sensors, it will provide examples of industrial applications utilizing these devices.
Analog rotational and linear Hall effect sensors offer stability and accuracy.
Analog sensors allow end users to obtain instantaneous feedback on the position of magnets in industrial automation systems. Previously, analog Hall effect sensors measured the magnetic flux density of magnets and were highly susceptible to changes in application temperature. Recent advances in analog Hall effect technology have enabled Hall effect chips to measure the angle of the flux field, rather than its amplitude, thus significantly reducing their sensitivity to temperature variations. Hall effect sensors can now provide highly stable analog outputs over a wide temperature range.
The following Hall effect sensors are ideal for custom analog sensing applications:
Rotating Hall effect sensors use no moving parts. This semiconductor-based sensor combines a Hall effect sensing element with circuitry to provide an analog output signal corresponding to changes in a rotating magnetic field. Rotating Hall effect sensors offer two output options: analog or pulse width modulation (PWM). The device is programmable, allowing engineers to correlate a specific output voltage or PWM with a precise degree of rotation.
Multiple programming points can rotate up to 360 degrees. Each programming point represents a voltage or PWM output value corresponding to a given magnetic field angle. This produces a high-precision, high-resolution ratio measurement output signal relative to the degree of rotation.
advantage:
● Unlike mechanical rotation or resistive film rotation devices, it will not experience resistance changes or mechanical wear.
● Provides excellent stability at normal operating temperatures up to +105°C
● Provides 0-360 degree rotation accuracy with 0.5-4.5 VDC output or PWM duty cycle of 10-90%.
application:
● Replace the resistive film and potentiometer mechanical components, as they may experience wear and oxidation, leading to signal loss from the control unit.
● Motion control applications include measuring the angle of a baffle valve in a fluid and precisely controlling the flow rate.
● Detect the dial position in the control circuit
Linear Hall effect sensor
A linear Hall effect sensor measures the linear motion of a magnetic field, rather than its rotation. The sensor can be programmed for a set output voltage that is proportional to a given distance traveled. (Its output options are the same as those of a rotating Hall effect sensor.) Using a single Hall effect chip, the sensor measures the linear motion of the magnetic actuator and the relative flux angle, with a maximum travel of 30 mm. This results in a proportional output signal relative to the precise motion of the sensor.
advantage:
● Sensors and actuators can be placed in the final installation area of the application (including all magnetic influences of the surrounding environment).
● This mounting position allows engineers to optimize the application's output signal because any shunt, magnetic field mechanical tolerances, or superposition tolerances will be included in the programmed output signal to correspond to the direction of magnetic flux as the magnet rotates.
application:
●Used as a liquid level sensor to monitor liquid level
● Sensors detect the position of the moving float, which is equipped with magnets.
● Provides highly precise positional tolerances for automated pick-and-place systems.
Reeds and the Hall effect provide highly reliable digital sensing.
Industrial automation users who want to verify that an object (such as an elevator door) is in the correct position prefer digital outputs. The following digital sensors offer superior reliability for custom magnetic sensing applications:
Reed switch
The contacts of the reed switch are made of precious metal and sealed inside a glass tube. This electrical switch can operate without any power supply.
advantage:
● Unaffected by moisture and other environmental hazards, it can achieve reliable operation for millions of cycles.
application:
● Microprocessor-controlled logic-level electrical loads
● Digital on/off applications, such as door closing or position detection systems in the industrial/motion control market (because they can switch AC or DC loads); see Figure 2 for an example of a freight or passenger elevator door using a reed switch for door closing detection.
Digital Hall effect sensor
These sensors combine Hall effect sensing elements with circuitry to provide digital on/off output signals corresponding to changes in the magnetic field, without using any moving parts. The active circuitry of a Hall effect device always draws a small amount of current.
advantage:
● Provides high reliability
● Programmable to activate within a given magnetic field tolerance to meet precise sensing requirements.
application:
● Limited to industrial and motion control applications with low DC voltage and current values.
● Widely used in high-speed sensing applications, such as speed sensing on conveyors; an example of a Hall effect speed sensor for detecting a rotating 16-pole ring magnet, see Figure 4.
in conclusion
Numerous environmental hazards exist in industrial automation and motion control environments. Designing a magnetic circuit integrating a powerful magnetic sensor is crucial for providing accurate and reliable industrial automation solutions. For certain industrial applications, a custom magnetic sensor may be required to achieve long-term reliability. Reed switches and Hall effect sensors are well-suited for digital applications requiring exceptional reliability, such as elevator door closing detection systems. Rotary and linear Hall effect sensors enhance the stability and accuracy of custom analog applications, such as fluid flow and automated pick-and-place systems.
