Contactless technology used in Merritt's steering column combination switch

The column combination switch displayed by Merritt Automotive Electronics at the Shanghai exhibition used contactless technology, which aroused the interest of many people.      

Contactless switch is one of the most confusing names in automotive mechatronics. When people first hear this term, they will ask: "How can a switch be contactless? The working principle of a switch is to make contact." In order to understand what is non-contact in mechatronics, you must first understand the meaning of contact switch.

Figure 1 (top) Simplified example of a contact switch

The slider moves as indicated by the arrows and, depending on its position, shorts or disconnects a contact area on the printed circuit board (PCB). The electrical contact that exists between the two blocks is the coded position of the switch, so this type of switch is often called a contact switch. This switch is the most common switch technology today. It is cheap, simple and reliable. But is it really reliable? It hardly has a long service life. When the contacts wear out, the PCB surface corrodes, or the spring loses its elasticity. Of course, these are damages that occur after long-term use. Contact switches can of course also be made more robust, but this requires more costs and may also make the PCB board size larger, while the housing of the switch tends to become smaller and smaller.

This is the origin of contactless technology, which enables the signaling of the position of a knob, lever or button without the use of electrical contacts. There are many types of switches of this type, each with its advantages and disadvantages. Two types are mainly presented here: optical and electromagnetic contactless switches.

Optical switches rely on light signals to determine their position. In simple form, they are made of a light source (LED), a photosensor and a moving screen or reflector (see Figure 2). When this screen is between the light source and the photosensor, it blocks the light source. There are already ready-made optical barrier detectors on the market as components of optical switches. However, sometimes it is not possible to use a barrier, in which case a reflector may be a better solution. The light source and photosensor are placed next to each other on a PCB board, and using a reflector on this, the presence of a state change can be detected.

Figure 2. Examples of contactless optical switches in the closed (a) and open (b) states. A barrier moves between the light source and the light-sensitive element, blocking the light.

Optical switches have some distinct advantages over contact switches. They last longer, are less susceptible to damage, and are relatively inexpensive, even compared to contact switches. They wear out more slowly and don't suffer from corrosion like PCB contacts. In terms of price, some optical switches may be similar to contact switches. This may be counterintuitive, but contact switches usually require multiple precision-made components, which increases the overall price.

Of course, optical switches also have some significant disadvantages compared to contact switches. LEDS light sources require more power than the slider of a contact switch. Optical switches may not be suitable when a wake-up function is required. It is well known that optical switches are susceptible to the impact of dirt and dust covering their components over time, which forces the product's housing to be designed to be dust-proof and undergo long-term verification, both of which increase the cost of the final product.

Another solution for near-contactless switches is magnetic switches. This switch consists of a permanent magnet as a magnetic field source and a magnetic field sensor to detect the position of the magnet. (See Figure 3 for an example)

Figure 3. Example of a magnetic contactless switch. A cylindrical magnet moves over a magnetic field sensor.

Note that the magnet can move in both directions.

IC vendors have developed a variety of sensors for contactless switches over the years. There are two possible modes of operation for magnetic contactless switches: In the first case, the sensor reacts to a certain threshold of magnetic energy, and a simple on-off switch is constructed with this single sensor. The threshold sensor output is usually an open collector output, allowing the output to be connected directly to the ECU through the wiring harness. The second type of magnetic switch uses a sensor to report a continuous value of the magnetic flux in one or more axes. This switch usually requires a microcontroller to process the sensor data and determine the magnet position, which is then transmitted over a digital bus. This approach increases the complexity of the switch, but allows it to correct for temperature and implement more complex magnet movements (such as XY joysticks).

Magnetic switches have advantages over contact and optical switches. The first advantage is obvious: magnets are not affected by dust and environmental pollution. Magnetic switches can be used in dirty environments, such as immersed in liquids. Another less obvious advantage is energy consumption. Magnetic sensors usually use less current than the LEDs of optical switches. This makes magnetic switches the preferred device when wake-up is required. Another advantage of magnetic switches comes from its mechanical simplicity. It does not require precise positioning of the slider on the PCB, nor does it require an optical barrier that moves precisely through the detector gap. The installation of the magnet does not require high precision and can be calibrated during the calibration process.

Of course, magnetic switches also have certain disadvantages, one of which is the possibility of interference with their operation by external magnetic fields. This type of switch requires a thorough EMC/ESC design verification and may require additional circuitry/shielding to keep it operational in the presence of external magnetic fields. In addition, any magnetic material nearby, whether it is metal, plastic or even grease, may interfere with its normal operation. The last disadvantage is price. Magnetic sensors are more expensive than optical barriers or contact switches and may require a microcontroller.

In general, contactless switches have obvious advantages over contact switches, but they may not be suitable for all application scenarios. As a supplier of automotive mechatronics, Merit Automotive Electronics can provide customers with both contactless and contact switches to meet any possible needs of customers.

Author of this article: 

Andrzej Wetula, Senior Software Engineer at Merit