Andreas Pfingstl, Marketing Manager- Encoders at austriamicrosystems explains how a new generation of magnetic Hall encoders are enhancing motion sensing in automotive applications
Many automotive applications today require the measurement of linear or rotary motion. Typical examples are the brake, clutch and gas (accelerator) pedals, steering wheel, transmission, gearbox, throttle and valves. Multi-axis sensing is also deployed in the joystick used as a control knob in navigation and infotainment systems.
Automotive electronics designers are finding that established position-sensing technologies for these applications are highly compromised: vehicles would benefit if measurement systems could provide improved measurement performance and/or superior reliability.
The common way to accomplish linear or rotary sensing today is with the sliding potentiometer, which has the advantage of being simple and inexpensive in low-speed applications. The potentiometer provides a resistance value proportional to distance or angle of rotation. While it provides fairly good mechanical and thermal stability, it is prone to interference caused by contaminants entering the sensor housing; mechanical wear also limits its operating lifetime.
Since dust, moisture and other contaminants are prevalent in vehicles, the sensitive parts of the sensor need to be tightly encapsulated. This raises the cost of manufacturing, and introduces an unwanted element of risk, requiring the manufacturer to account for the probability of the seal being breached and contaminated.
Automotive sub-system suppliers have therefore experimented with contactless sensing technologies. One such is the optical encoder: the effect of mechanical wear on such systems is almost negligible, but they are still prone to contamination, and thus in many automotive applications tight sealing of the optical elements will be necessary as a result.
Automotive designers, require a position sensing technology with the speed and precision of optical encoders but offering high reliability and complete immunity to contaminants. Precise rotary inductive sensors (or resolvers) have been adopted in industrial applications, but their high cost invalidates their widespread use in vehicles.
Magneto-resistive sensors have been touted as a solution for automotive applications, but their sensors can be corrupted by stray magnetic fields, and integration of the sensing elements with signal conditioning circuitry on a single chip is both complex and expensive.
Magnetic encoders (available until now as separate magnetic rotary encoders and magnetic linear encoders) have emerged as the best response to the challenges of the automotive environment. These devices combine an array of four or more Hall sensors with signal conditioning and signal processing electronics on a single chip.
Of course, simple Hall elements have been used for decades as contactless switches, triggered by a magnetic field. But it was the integration of Hall sensors in standard CMOS blocks that enabled the technology’s widespread deployment in position-sensing applications. Crucially, these Hall sensor-based devices can operate in harsh or dirty environments without the need for encapsulation.
Differential amplification of two opposed sensors produces two 90° phase-shifted signals with double amplitude. These two analogue signals are digitised and then processed. In the case of magnetic encoders from austriamicrosystems, an on-chip CORDIC (COordinate Rotation DIgital Computer) converts the variable magnetic field strength data into a simple angular displacement value, supporting speeds of up to 80,000rpm and up to 12-bit resolution (0.09°).
By implementing differential amplification of the sensor signals the encoders gain two important benefits:
• The measurement result is not derived from the amplitude of the signal, but from phase changes. This means that variations in the magnet’s flux due to temperature changes or variation in the gap between the magnet and the IC have no effect on the sensor’s output. Similarly, temperature drift in the Hall elements has no resulting effect.
• They are also immune to stray magnetic fields, and therefore require no shielding.
Existing magnetic encoder implementations measure movement in a single plane. Robust, precise, easy to interface to system controllers, and free of encapsulation and shielding, these devices are already superior to older established technologies.
But these first-generation 2D magnetic encoders still have limitations. For instance, they require precise assembly, with an air gap of 0.5mm-2mm specified between the magnet and the surface of the IC. In addition, the range of linear measurement is limited to a maximum of 10mm. This is an absolute physical limit, the result of the characteristics of two-pole magnets and the arrangement and sensitivity of the Hall elements in 2D magnetic encoder ICs.
The new 3D HallinOne sensing elements offer much higher sensitivity than the Hall elements in previous generations of magnetic encoder. This gives the new 3D devices an extended range, including absolute linear measurement range beyond 40mm, depending on the magnet used. The ICs can also be daisy-chained to extend measurement range over longer distances.
They provide their precise and accurate outputs as a 16-bit SPI or a 12-bit PWM/analogue signal.
The new AS54xx also brings improvements to the production of position-sensing sub-systems: the required air gap between the IC and its magnet has now been extended to >10mm. This makes for more flexible production arrangements with less scope for error and less requirement for precise assembly.
Finally, the provision of on-chip configuration capability in EEPROM memory gives automotive designers more flexibility to change magnet specifications, or to switch from an on-axis to an off-axis measurement arrangement, without having to change the IC.