Contactless Magnetic Angle Sensing

Contactless magnetic angle position sensors are now giving optical encoders a run for their money. This was recently demonstrated by Monolithic Power Systems at Electronica 2018. They had on display a unique non-automotive-focused electric vehicle, mCar, with motion control and angular sensors. According to MPS, their mCar demos two main functions—motor control elements, and angular position sensors.

As Quitugua-Flores, the mechanical engineer and primary designer of the mCar at MPS explains, the steering of the car is a complete drive-by-wire concept, and there is no mechanical connection between the tires and the steering wheel. A magnetic angle sensor detects the angle of the steering wheel and converts the signal to control the tire angle necessary for the various steering modes. The magnetic angle sensor provides visible feedback via a blue LED mounted on the dashboard, with the LED lighting up when the driver turns the steering.

An electronic system takes in the magnetic angle sensor information and feeds it wirelessly to the rest of the car, thereby instructing the wheels to turn. The angular sensor, along with the board and antenna for sending the wireless signals is attached to the steering column.

The throttle and brake pedals use similar rotary magnetic angle sensors and send their signals wirelessly just as the sensor on the steering wheel does. Pivots on the brake and acceleration pedals house the angular sensors, and they measure the angle of depression of the pedals.

However, MPS has made the mCar as an R&D application, and they have not yet approached the National Highway Traffic Safety Administration (NHTSA) for compliance with their safety regulations.

According to Quitugua-Flores, another aspect of the mCar is its driver seat pivots freely. The front and rear suspension modules keep the seat suspended such that when the mCar enters a curve, the seat tilts into the turn just as it happens in a motorcycle or a plane. This keeps the driver firmly in the seat in a turn, rather than being literally pushed out of it.

An angle sensor attached to the seat detects the rotational position and sends the information to the suspension control. The shock absorbers in the mCar come with individual integrated BLDC motors that can change the length of the shock absorbers independently. Therefore, the suspension has complete control over camber or the vertical tilting of each wheel. As the frame of the mCar tilts when turning, the suspension changes such that each tire tilts in a corresponding direction—just as a four-wheeled motorcycle does.

Shafts suspending the driver cockpit also have angular sensors attached to them. This allows the driver to enjoy a smooth ride by controlling the behavior of the suspension.

According to MPS, the mCar is only a demonstration for the effective operation of a sensing and motion control for a demo Electric Vehicle but is not a high-precision application. For systems requiring high-precision applications, MPS has demonstrated a robotic arm that allows seven degrees of freedom.

With sixteen angular sensors inside it, the arm demonstrates the capabilities of the current generation of MPS angular sensors for precision applications.