IEEE Consumer Electronics Magazine - September 2018 - 10

The highest g r a d e s cannot be
reached
with current MEMS technoloFront Crash Sensor
Rollover Sensors
gy,
which
is confined to high-value,
Airbag
GPS
ESC Accelerometer
ECU
Navigation
low-volume applications, e.g., military
ESC Gyroscope
Gyroscope
and avionics. Due to bias, the estimated
ABS/Brake
position and orientation drift with time.
Stand-Alone
ECU
Peripheral
The errors on the estimated position
Sensor
due to accelerometer and gyroscope
ESC
bias increase as the second and third
ESC Unit
Gyroscope
ESC Gyroscope
power of time, respectively [7], and
ESC
grow to levels unacceptable for navigaTPMS
Accelerometer
tion (Table 2). To move current MEMS
Module
IMUs to tactical or higher grades, reliStand-Alone
able, low-cost, in-run calibration techMotion-Detector
Peripheral
niques are needed. Electronic calibration
Sensor
Module
compensates for the effect of temperaSatellite Crash Sensors
ture on bias and part of the effect of fabrication imperfections, for a limited
FIGURE 2. The motion MEMS already used in cars. GPS: global positioning system;
increase in complexity and cost. MeECU: electronic control unit. TPMS: tire pressure monitoring system.
chanical calibration requires a periodic,
controlled rotation along one or more
axes [8], but it is not a low-cost solution. Bias reduction can
Table 1. The IMU grades by bias value.
also be achieved through a better control of process variability. Inertial sensors are also affected by output random noise,
Acceleration Bias
Angular Rate Bias
expressed in terms of its noise spectral density, measured in
IMU Grade
(mg)
(°/h)
c/s/ Hz for gyroscopes and ng/ Hz for accelerometers.
Strategic
10−3-10−2
10−4-10−3
Noise causes the standard deviations of the estimated angles
−2
−3
and positions to grow as a function of time as t 1/2 and t 3/2,
Navigation
10 -1
10 -0.1
respectively [9]. Higher noise levels are linked to higher therTactical
1-30
0.1-30
momechanical losses in the MEMS device. For MEMS
gyroscopes, there is evidence that the current technology,
Consumer
>30
>30
based on the tuning-fork Coriolis (TFC) principle and capacitive driving and sensing, is getting closer to a hard limit of
2 $ 10 -3 c/s/ Hz (Figure 3).
Table 2. The 10-s position error due to sensor bias.
The future road map to very-low-noise gyroscopes is currently
not set, and several alternative approaches are being
IMU Grade
Acceleration Bias
Angular Rate Bias
researched. An improvement in the noise floor of TFC gyroStrategic
<0.5 × 10−3 m
<8 × 10−6 m
scopes has been demonstrated for piezoresistive detection
−3
−6
−3
[10]. However, piezoresistive sensing has issues of linearity
Navigation
0.5 × 10 -0.5 m
8 × 10 -0.8 × 10 m
and temperature stability. Capacitive (or piezoelectric) resoTactical
0.5-15 m
0.8 × 10−3-0.25 m
nant bulk-mode gyroscopes, operating at high frequency
(around two decades above TFC gyroscopes), exhibit an
Consumer
>15 m
>0.25 m
intrinsically lower mechanical noise, which is, however, only
a part of the total output noise. There is also research on
devices based on precession, the rate-integrating gyroscopes
MEMS to other automotive applications, such as navigation
(RIGs), whose output is proportional to the angle and not the
and positioning, with errors of a few centimeters, has proven
angle rate [11]. This feature reduces the impact of bias and
elusive. In inertial navigation systems (INSs), the accurate
noise significantly, as the integration of the output is avoiddetermination of position, velocity, and orientation relies on
ed. RIGs are still in their infancy, and their potential is still
the accuracy of the IMU around which the INS is built. Posinot clear.
tion and orientation are determined by the time of integration of IMU data. The IMU-based INS is typically combined
with a satellite-based system [7]. Each sensor shows an offMOEMS MIRRORS FOR LIDAR SCANNERS
set (or bias), with a temperature-dependent component. Bias
Lidars will play a leading role as essential components in
is given in milligrams for accelerometers and °/hour for
advanced driver assistance systems (ADASs), which are
gyroscopes. Based on bias, IMUs [7] are ranked in differdesigned to help the driver and increase car safety. Current sysent grades (Table 1).
tems, however, are not yet ready for autonomous (i.e., not
10 IEEE Consumer Electronics Magazine

september 2018

Table of Contents for the Digital Edition of IEEE Consumer Electronics Magazine - September 2018