Littelfuse – How to choose the ideal sensor for industrial automation applications

By Gwenn Gmeinder, Sensors Business Development Manager, Littelfuse

Motion-control and automation systems for industrial use have to cope with the hazards of dirt and dust commonly found in factory settings. In addition, conveyors and pick-and-place systems can be subject to vibration and shocks which threaten the stability of the control circuit’s sensors. The sensing technology used for position measurement and object detection in industrial equipment must therefore provide exceptional durability to maintain the required accuracy over a long operating lifetime.

Both analogue and digital sensor types can meet this need, as the examples in this Design Note show.

Analogue Hall-effect sensors for stability and precision
In the past, an analogue Hall-effect sensor would measure the flux density of its paired magnet, which meant that its operation was greatly influenced by the ambient temperature. Improvements to analogue Hall-effect technology enable the sensor chip to measure the angle of the flux field instead of its amplitude, making it much less sensitive to temperature changes. As a result, today’s Hall-effect sensors provide a stable analogue output across a wide temperature range.

Both rotary and linear Hall-effect sensors are suitable for analogue sensing applications. In Hall-effect sensors from Littelfuse, multiple programming points are available up to the full 360° of rotation. Each programming point represents a voltage or PWM output value which corresponds to a given angle of the magnetic field. This results in a high-accuracy, high-resolution ratiometric output signal relative to the degree of rotation.

Benefits of Rotary Hall-effect SensorsApplications
• Do not experience changing resistance values or mechanical wear as a mechanical or resistive film rotary device does
• Provide high stability over operating temperatures up to 105°C
• 0.5V to 4.5V output or 10% to 90% PWM duty cycle
• Replacing resistive film devices and potentiometers, which can suffer from wear and oxidation
• Motion control
• Detecting dial position in control circuits

A linear Hall-effect sensor measures the linear movement of a magnetic field rather than the rotation.

Littelfuse linear sensors are programmable for a set output voltage which is ratiometric for a given travel distance. The sensor measures the linear movement and relative flux angle of a magnetic actuator over ≤30mm of travel with a single Hall-effect chip. This results in a ratiometric output signal relative to the movement of the sensor.

Benefits of Linear Hall-effect SensorsApplications
• Sensor and actuator can be placed in their final mounting area in the application to enable ‘in situ’ optimisation• Level sensors
• Automatic pick-and-place systems

Digital reed and Hall-effect sensors for high reliability
Digital position sensors provide a more convenient output configuration for some applications, and Littelfuse digital sensors provide outstanding reliability in magnetic sensing applications.

A reed switch’s contacts are hermetically sealed in a glass tube, as shown in Figure 1.

Fig. 1: A reed switch is hermetically sealed

Fig. 1: A reed switch is hermetically sealed

The contacts are made of precious metal. This electrical switch does not require any power to operate.

• Immune to moisture and other environmental hazards. Enables reliable operation for millions of cycles
• Can switch AC or DC loads
• Microprocessor-controlled, logic-level electrical loads
• Digital on/off applications such as door closure detection systems (see Figure 2)
Fig. 2: Reed sensor detects the opening and closure of an elevator door

Fig. 2: Reed sensor detects the opening and closure of an elevator door

Littelfuse digital magnetic sensors combine a Hall-effect sensing element with circuitry to provide a digital on/off output signal which corresponds to a change in the magnetic field. The sensing system itself thus has no moving parts, as shown in Figure 3.

The Hall-effect device’s active circuitry continuously draws a small amount of current.

Benefits of Digital Hall-effect SensorsApplications
• High reliability
• Can be programmed to activate at a given magnetic field tolerance for precise sensing requirements
• Industrial and motion-control applications with low DC voltage and current values
• High-speed sensing applications such as rotary speed sensing on conveyors (see Figure 4)
Fig. 3: Hall-effect sensor assembly in a rotary motion-sensing application

Fig. 3: Hall-effect sensor assembly in a rotary motion-sensing application