What's wrong! What's wrong! This time the unlucky is the "local tyrant" Apple: just released the new iPhone5s and all M7 sensors are exposed.
64-bit A7 and M7 are the highlights of Apple ’s iPhone5s. M7 is an assistant of A7. It is designed to measure motion data from acceleration sensors and gyroscopes. Without it, fitness apps that track physical activity must be read from A7. Retrieving data, with M7, there is no need to continuously access A7, thereby reducing power consumption.
For example, A7 is the boss, M7 is, M7 can handle some major specific tasks to share the pressure of A7.
Unfortunately, the assistant is in trouble now. What is the specific aspect? Because our company is doing the sensor attitude fusion algorithm, let me make a bold guess:
The types of sensors in the iPhone: acceleration sensors, gyro sensors, electromagnetic compass sensors, generally referred to as the 9-axis inertial sensor group.
Three factors that affect the accuracy of attitude output, the accuracy of the device's original data, the calculation accuracy of the attitude solution, and the calibration accuracy of the device.
A major innovation of the new iPhone5s and 5c is the M7 sensor hub, sensor hub, which is a good concept. The sensor hub is responsible for all sensor input and output, power management, attitude calculation, etc., which can effectively improve accuracy and reduce energy consumption. Reduce the pressure on the main processor. However, the problem that we have seen so far speculates that the problem of great possibility lies here.
The recent problem of sensor output inaccuracy is more complicated to evaluate. Let ’s talk about the conclusion first, and then elaborate: It is most likely that there is a problem with the calibration algorithm in the built-in algorithm of M7, or the calibration step is ignored during the production of the foundry.
The reason for the inaccurate attitude output of the 9-axis sensor group was just mentioned. It may come from three aspects: raw data output, algorithm, and calibration strategy.
What is the specific situation of Apple M7, let us analyze it in detail:
With the current information, we know that both 5s and 5c use:
1.3-axis accelerometer: BMA220 accelerometer produced by Bosch;
2.) 3-axis gyroscope: currently determined to be a gyroscope produced by ST; dual sources are not excluded, and InvenSense also provides gyroscopes for Apple.
3.3 Axis magnetometer: AK8963 magnetometer produced by AKM.
The accuracy of these sensors is reliable and no problem, so the original data output should be no problem.
In addition, the inaccuracies pointed out in the current media reports are basically concentrated on the static accuracy. The accuracy of the static accuracy is relatively easy from the perspective of the calculation algorithm, and the possibility of problems here is not very great.
The most suspicious at present is that there is a problem with the calibration strategy:
For the mems sensor, the manufacturer leaves the factory with a rough calibration, but the manufacturer's calibration is generally done in the sensor output linearity, range, resolution, etc., the actual terminal manufacturer needs to perform a second calibration
At present, the two major criticisms of the output of the Apple mobile phone are that the level is not accurate, that is, the level is flat, but the output still has several degrees of deviation. This problem lies in the calibration of the acceleration sensor.
The other is that the direction is not accurate. This problem lies in the calibration of the electromagnetic compass sensor.
It should be noted that many media directly point to problems with the gyroscope. In fact, this is wrong. If the gyroscope is not properly calibrated, it is more likely to cause insufficient dynamic accuracy and "posture flick".
According to the current Apple hardware architecture, the calibration parameters and calibration algorithms are written in his M7 sensor hub. Disassembly revealed that the M7 processor in the iPhone 5s is actually NXP's cortex-M3 MCU LPC18A1.
To sum up the analysis: I made the following guesses about where the problem is:
1. There is a bug in the calibration part of the M7 processor, and the calibration function is not enabled. If this is the reason, maybe it can be repaired by a simple firmware upgrade;
2. There is a bug in the calibration parameter storage part of the M7 processor, and the calibration parameters are lost. If this is the reason, you must return to the factory for re-calibration, or Apple produces a rough self-calibration software to allow genius employees or users to calibrate themselves;
3. The foundry has neglected the calibration procedure. QC did not detect this and must return to the factory for recalibration;
4. There is a problem with the foundry process. Calibration is performed before product assembly. After the characteristics of the finished sensor are changed due to factors such as assembly stress, magnetization or thermal stress, the calibration parameters become invalid. Must be returned to the factory for recalibration;
5. There is a problem with the calibration matrix algorithm of NXP. You must upgrade the firmware and return to the factory for calibration.
All in all, the problem of sensor calibration is very troublesome, it is difficult to train customers to solve it by themselves, I think it may need a large number of recalls. Prayer is one of the reasons mentioned above.
Even big brands like Apple have such a big problem. The lesson we learned from this is that the sensor can be used without the board. The calculation accuracy and calibration accuracy are very important.
Author: Daryl plow PhD, MPhil, SPIE Member, ASME member, Teng Technology Co., Ltd. Beijing Connaught also Chief Technology Officer / Co-Founder
Dr. Dai Ruoli graduated from the University of Science and Technology of China (BEng, 2002) and the Chinese University of Hong Kong (MPhil, 2004, PhD, 2007) and currently serves as the Chief Technology Officer of Beijing Nuo Yi Teng Technology Co., Ltd. Prior to participating in the establishment of Nuo Yiteng, Dr. Dai had served as the technical director of Anse Asia Pacific Hong Kong Branch and the chief technical officer of Metano Intelligent Technology Co., Ltd.
Dr. Dai Ruoli has rich experience in the development of cutting-edge technology products. His research and work mainly focus on motion posture capture technology, somatosensory interaction technology, virtual reality technology, wearable device development and other fields. Dr. Dai has published more than ten academic papers in international journals and academic conferences, participated in the writing of one academic monograph and has more than ten patents. The "wireless high-speed full-body motion capture system" developed by his leadership has reached the international top level and is produced in film special effects , Animation production and game interaction industries have high influence.
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