Feature Automotive Special 3D position sensing for automotive market
Andreas Pfingstl, Marketing Manager- Encoders at austriamicrosystems explains how a new generation of magnetic Hall encoders are enhancing motion sensing in automotive applications
any 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.
M
Automotive electronics designers are finding that established position-sens- ing 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 inexpen- sive 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 contam- inants entering the sensor housing; mechanical wear also limits its operat- ing lifetime.
Since dust, moisture and other con- taminants 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 con- tactless 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 posi- tion 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 appli- cations, but their high cost invalidates their widespread use in vehicles.
16
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 envi- ronment. These devices combine an array of four or more Hall sensors with signal conditioning and signal process- ing 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 sen- sors in standard CMOS blocks that enabled the technology's widespread deployment in position-sensing appli- cations. 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 austri- amicrosystems, an on-chip CORDIC (COordinate Rotation DIgital Computer) converts the variable magnetic field strength data into a simple angular dis- placement value, supporting speeds of up to 80,000rpm and up to 12-bit reso- lution (0.09°).
By implementing differential amplifi- cation of the sensor signals the encoders gain two important benefits:
The magnetic encoder IC is
mounted parallel to a two-pole magnet in many rotary position-sensing systems
• 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, temper- ature drift in the Hall elements has no resulting effect.
• They are also immune to stray mag- netic fields, and therefore require no shielding.
Existing magnetic encoder imple- mentations 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 mag- netic 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 lim- ited 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 ele- ments offer much higher sensitivity than the Hall elements in previous gen- erations of magnetic encoder. This gives the new 3D devices an extended range, including absolute linear meas- urement range beyond 40mm, depend- ing on the magnet used. The ICs can also be daisy-chained to extend meas- urement range over longer distances. They provide their precise and accu- rate outputs as a 16-bit SPI or a 12-bit PWM/analogue signal.
Andreas Pfingstl is Marketing Manager of Encoders at austriamicrosystems AG
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 con- figuration capability in EEPROM memory gives automotive designers more flexibility to change magnet speci- fications, or to switch from an on-axis to an off-axis measurement arrangement, without having to change the IC. austriamicrosystems AG
www.austriamicrosystems.com Enter 207
JULY 2011 Electronics
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44