Using Magnetic Sensors to Improve the Reliability of Haptic Human Machine Interfaces

【Introduction】For a long time, rotary dials have been widely used in various human-machine interface (HMI) applications. Buttons, knobs, and dials are included in the touch interface of a variety of everyday devices, from automobiles to white goods to mobile phone touchscreens. These functions not only convert human input into electrical signals, but also provide an important function – haptic feedback. This feature is especially useful for safety-related functions in time-sensitive situations, such as when the user is moving the interior of the cabin in a darkened cockpit, or when adjusting the flow of a medical pump (see Figure 1), the buttons should be easily located by touch , without additional visual identification.

Using Magnetic Sensors to Improve the Reliability of Haptic Human Machine Interfaces

Figure 1: Console and steering wheel controls inside the car

However, new designs are constantly being introduced, and newer solutions are emerging to accommodate new form factors, provide more and more functionality, and extend the life of the HMI. Traditional buttons and rotary dials became a limiting factor at this point. Because of their large size, rigid coupling from the button to the solder joints on the PCB, and the layout of the contacts results in a specific, limited lifetime.

For example, many white goods have a seemingly simple one-button interface on the panel. The entire machine, including the knobs, is subject to vibration, temperature fluctuations and humidity during hours of operation. This harsh environment can adversely affect the life of the knob, and the shaft of the button also provides a channel for moisture to penetrate other control electronics. Mitigating these problems can be costly.

The use of modern magnetic sensors for HMI applications can greatly reduce overall design cost while increasing reliability and flexibility in mechanical design. This article will provide an in-depth introduction to the basic ideas and design considerations for this optimized HMI application.

Utilize a simple and cost-effective sensing principle

Magnetic angle sensors with integrated Hall effect are able to detect the rotation of the magnetic field. For example, a sensor with a rotating part connected to a small indicating magnet can realize a fully functional contactless knob. Simply align the magnetic angle sensor with the rotation axis of the magnet, and the actual rotational position of the knob can be sensed by the rotation of the magnet. The sensor operates without a mechanical connection between the electronics and the haptic rotor, such as a wiper potentiometer. The use of magnetic sensors allows for a simple and highly reliable design with an extremely long service life.

Figure 2 shows a knob implemented with the MA800, a magnetic sensor from MPS for improving HMI applications. It uses MPS’ proprietary SpinAxis™ technology, enabling a small size, cost-effective angle sensor.

Using Magnetic Sensors to Improve the Reliability of Haptic Human Machine Interfaces

Figure 2: Knob cross section with magnetic induction

The entire magnetic sensor pair occupies a small footprint, leaving enough room to customize the tactile or visual appearance of the knob with bearings, detents or light guides. As shown in Figure 2, its footprint is less than 5mmx5mm, which is only a small part of the potentiometer product size. Potentiometers are typically 10mm measured along one side.

Design for reliability and meet manufacturing tolerances

Magnetic sensors are extremely reliable due to their non-contact principle. This is in stark contrast to traditional slip ring potentiometers. Slip ring potentiometers have been criticized for their contact properties, and their typical lifespan is around 250,000 cycles. However, magnetic sensors also suffer from some limitations and distortions in some aspects, which need to be avoided.

The first potential source of distortion is lateral displacement of the sensor relative to the axis of rotation. These displacements can be caused by imprecise production processes or by wear and tear of equipment during use. It causes the non-linearity of the magnetic field, which is picked up by the sensor.

For example, with the setup in Figure 2, a mismatch below 0.5° means a lateral tolerance of ±0.2mm, well within typical manufacturing tolerances. If for some reason a larger tolerance is required, a larger magnet diameter or ring magnet can be used to achieve a robust design. For example, increasing the magnet diameter from 5mm to 8mm can increase the lateral tolerance to ±0.4mm.

Magnet size is critical for handling air gap variations between the magnet and the sensor. The point is that this varying deviation cannot cause the magnetic flux density at the sensor location to exceed its required conditions. As shown in Figure 2, the air gap can vary between 0.0mm and 3.1mm without exceeding the MA800 specification. By making a trade-off between air gap and magnet size, it provides flexibility in mechanical design to meet manufacturing tolerances. For example, wider target magnets allow for larger air gaps.

Note that in this paper, all magnetic analyses utilize existing published magnetic simulation tools. The simulation tool allows designers to quickly examine specific magnetic configurations and conduct in-depth investigations into the effects of tolerances or misalignments.

button

In addition to the rotary function, some HMI applications require button functions. The MA800 can detect button events when the magnetic field strength changes due to changes in the air gap. Figure 3 shows a typical application of a button and knob combination.

Using Magnetic Sensors to Improve the Reliability of Haptic Human Machine Interfaces

Figure 3: Knob cross section with button function

Figure 4 shows the magnetic field gradient in relation to the distance between the magnet and the sensor, and the difference between single and double thresholds (more on this below). An axial displacement of 1 mm, a nominal air gap of 1.5 mm and a pressed position with an air gap of 0.5 mm produces a field strength difference of 60 mT. The MA800 has an adjustable magnetic threshold that provides different output signals to the system controller to safely flag such changes.

Using Magnetic Sensors to Improve the Reliability of Haptic Human Machine Interfaces

Figure 4: Button function implementation of MA800

This non-contact process utilizes non-destructive testing principles. The key is to ensure that changes in the lifetime of this mechanism do not reduce the reliability of the sensing. Figure 4 overlays the detection threshold of the MA800 and the associated hysteresis interval at the top of the field strength curve in a typical configuration. The design point of the nominal magnetic setup is to ensure that both endpoint thresholds are sufficiently distant from the magnetic switching threshold to ensure that foreign particles that may accumulate over time do not obstruct the travel of the mechanical part to the point where the switching threshold is not exceeded.

When the difference in magnetic field traveling along the axis is large enough to safely pass multiple switching thresholds, different thresholds are recommended to detect high and low field strengths. Figure 4 shows a schematic diagram of this dual threshold configuration. Dual thresholds add a safety feature to the entire application, which can detect mechanical failures and prevent buttons from getting stuck in the middle.

When choosing a non-contact sensing mechanism (thus avoiding knob and button contact issues), the pressing mechanism becomes the biggest limiting factor. Figure 3 also illustrates the principle that an axially positioned click magnet attracts a steel ring on the shaft. The magnetic force between these two creates a threshold for axial travel that is easily adjustable through material constants and coatings. Also, it has no wear and tear over its entire life cycle.

Choose a compact, low-power solution

MPS also offers the MA782 for low power battery powered applications. Its magnetic design principles are the same as the MA800. The sensor’s refresh rate is configurable and the average sensor current consumption can be reduced to less than 10µA.

In addition, the MA782 provides a dedicated signal to indicate when motion exceeds a certain angular threshold. With this monitor, the angle sensor can be used as a wake-up trigger for the entire system, which can keep the microcontroller (MCU) and Display in sleep mode, saving a lot of battery power.

Coupled with the MA782’s UTQFN-14 (2mmx2mm) package, it achieves both ultra-low power sensing and ultra-small size. This combined advantage can play an important role in emerging applications such as home wireless thermostats or hinge control in folding cell phones (see Figure 5).

Using Magnetic Sensors to Improve the Reliability of Haptic Human Machine Interfaces

Figure 5: Folding phone hinge using MA782 as sensor and monitor

With their small size and low power consumption, these sensors are ideal for applications where there is no room for the sensor (and PCB) at the end of a rotating shaft or hinge to meet their stringent design requirements. Devices such as the MA782 sense magnetic fields away from the axis of rotation through special compensation and accurately restore the linear relationship between mechanical angle and sensor output.

in conclusion

In order to implement HMI dial and button functions using magnetic sensors, various issues need to be considered when designing the solution, such as decoupling between the mechanics of the haptic elements and the electronics, while leaving enough space for the surrounding mechanics. This article provides simple design guidelines for designing cost-effective contactless HMI solutions with unmatched lifetime and low power consumption.

Source: MPS

The Links:   PM200CSA060 BSM25GP120