(guidance navigation control) - analog devices

MARK LOONEY

MEMS IMUsfor GNC(Guidance Navigation Control)

24 June 2019 ©2019 Analog Devices, Inc. All rights reserved.1

What is an IMU (Inertial Measurement Unit)?

► Angular rate of rotation (spin rate) Delta-angle

► Linear acceleration Delta-velocity Orientation, with respect to gravity

(aka….tilt, incline angle)

► Triaxial measurements Mutually orthogonal

► Legacy IMUs also had magnetometers and barometers

► Primary use is in dynamic orientation tracking► Secondary use in short-term velocity and

position tracking

Analog Devices Confidential Information. ©2018 Analog Devices, Inc. All rights reserved.2

Y-AXIS

ωz

X-AXIS

Z-AXIS

ωxωy

1543

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az

axay

Y-AXIS X-AXIS

Z-AXIS

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Let’s start with basic pointing and noise influences


What are IMUs used for? Measure angles

► Accelerometers respond to their orientation, with respect to the earth’s gravitational field, according to a very simple trigonometric function

► Gyroscopes measure the angular rate of rotation, around their measurement axis, which enables an simple integration for estimating angle displacement


0709

6-00

8ay

θx

ax

θx

HORIZON

GRAVITY = 1g

∅ ·

x

yx a

aa tan

Sensor FusionReal-time

angle estimation

gaa x

x 1sin

Y-AXIS

ωz

X-AXIS

Z-AXIS

ωxωy

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8-02

9

What are IMUs used for? Measure angles

► Accelerometers also respond to linear vibration and changes in linear velocity (aka….linear acceleration)

► Gyroscope bias error translates into an angle error, which is proportional to time.


0709

6-00

8

ay

θx

ax

θx

HORIZON

GRAVITY = 1g

∅ · Y-AXIS

ωz

X-AXIS

Z-AXIS

ωxωy

1543

8-02

9

sin 1

MobileRobots Seekur®Improvement opportunities

► Full implementation of IMU function Terrain compensation of GPS location

data

► Implementing next-generation devices, which represent advancing performance. ADIS16265, ADIS16135, ADIS16385

► Using in-system calibration options. For example, running the robot around a 360° circle while integrating the output, enables application-specific accuracy correction, using a very simple pattern.

Vehicle Platform

Actual locationGPS thinks it is right hereKnowing the tilt angle & height of the receiver to ground enables

this distance to be corrected using simple trigonometery

Gyroscopes: feedback sensing element


Real-world example

► Optical inspection system► Problem: swinging motion

causes loss of resolution on theinspection surface

► Solution: mounting an IMU on the lens of the camera to track the orientation, sothat a servo motor can adjust the orientationof the lens in an equal but opposite manner

► The effectiveness of this type of system is directly dependent on howwell the IMU can track the orientationof the camera


Conveyor belt

Video Camera

Swinging motion

ωSW(t)

D

dSW

±φSW

Ideal center of camera view

Basic Feedback Control


ServoMotor

DigitalFiltering ADIS1647xIntegrator

-

ωG(t)

φE(t)

φCOR(t)

Mechanical Connection

CalibrationAlignment

φCMD(t)

ωF(t)

φP(t)

OPENING INPACKAGE LID

1543

6-04

6

Success!► Optical inspection system► Problem: swinging motion

causes loss of resolution on theinspection surface

► Solution: mounting an IMU on the lens of the camera to track the orientation, sothat a servo motor can adjust the orientationof the lens in an equal but opposite manner

► The effectiveness of this type of system is directly dependent on howwell the IMU can track the orientationof the camera.

► Primary limitation, in this case, isnoise, or Angle Random Walk


Conveyor belt

Video Camera

Swinging motion

ωSW(t)

D

±φRE

dREIdeal center of

camera view

Why does noise performance matter?


±φRE

-0.0006

-0.0004

-0.0002

0

0.0002

0.0004

0.0006

0 0.005 0.01 0.015 0.02 0.025

Inte

grat

ion

Erro

r ()

Integration Time (seconds)

ARW = 0.17°/√hour

What messes up your measurements?

► REFERENCES: http://www.analog.com/en/technical-articles/critical-noise-sources-mems-gyroscopes.html http://www.analog.com/en/analog-dialogue/articles/low-noise-feedback-control.html


What nobody talks about……


ie: Vehicle-mounted antenna, camera, other optics, etc.ie: Vehicle-mounted antenna, camera, other optics, etc.

Rough conditions cause angular vibration (±10°/sec) in the y-axis (pitch). High cross-axis sensitivity (GCAS) will cause angular jitter on the x-axis (roll).

ØROLL xθPITCH

PIN 1PIN 23

aY

mY

gY

Y-AXIS

gX

X-AXIS

aX

mX

Z-AXIS

aZ mZgZ

1027

7-01

7

Typical Device Spec: +/-2% ADI Spec: <0.087%

Cross-AxisSensitivity

PIN 1PIN 23

aY

mY

gY

Y-AXIS

gX

X-AXIS

aX

mX

Z-AXIS

aZ mZ

gZ

1027

7-01

7

Rough conditions cause up/downvibration (±2g‐rms) in the z-axisHigh Linear-g sensitivity (GL) will cause angular jitter on all three gyroscopes.

ØROLL x

Typical Device Spec: 0.1 o/s/g ADI Spec: 0.015 o/s/g

Linear-gResponse

14

Vibration/Cross-axis on top of noise

ConsequenceNoise/Jitter Increase

Analog Devices Confidential Information. ©2018 Analog Devices, Inc. All rights reserved.

What matters?


Stability/Repeatability (long term drift; scale and bias)

Noise (angle random walk)

Vibration rectification

Hysteresis

Non-linearity

g-effect error (linear accel)

Offset / Bias

Scale / Gain Error

Tempco’s

Cross Axis Sensitivity

Correctable through test and calibration, to the limits of resolution and stability

Theoretically capable of being corrected through test and calibration, to limits of resolution and stability

Inherent to sensor performance (limited opportunity to calibrate)

ADI D

esigned-in Performance

at MEM

s Sensor LevelAD

I System-Level

Calibration Focus

o Architected to Reject Performance Limiting Errors

o Sensor Conditioning and Filtering Optimized for Reliable Precision at Application Level

o Packaging Minimizes Stress; avoids long-term drift (ie: from overmold/moisture)

o Sub-System Test & Calibration Ensures out-of-box best Precision, & Reliability

o Fully, Conservatively, Specified

Guidance Navigation Systems


Ground Vehicle Example


2

1

t

tH dt

Axis of rotation

rategyro K

Independent steering & drivecontrol for all4 wheels!

What are IMUs used for?


TrajectoryPlanner

ServoControl

&Motor

ServoControl

&Motor

ServoControl

&Motor

ServoControl

&Motor

Forward Kinematics

Encoders Encoders Encoders Encoders

Inverse Kinematics

GyroIMU

GPSor

Laser

TrajectoryPlanner

Localization (position correction)GPS for outdoor systemsLaser range finding for indoor systems

► It is easy to get wrapped up in the entire system

► While remembering that we are looking to support accurate motion representation….

► Look for context with each system, environment, as we help customers understand their performance needs

► For those who seem to “start cheap,” the discussion is in how complex the other inertial observers need to be

► For those who are moving from more expensive technologies, it is about what they have to add/understand, to make ADI IMUs work

Example Robot System Steering control

The forward kinematics system employs Ackermann Steering on all four wheels to help minimize tire slip and improve efficiency.

Each tire will have a different steering angle.

The example shown is for a rack and pinion system, which employs the Ackermann relationship mechanically.

The Mobile Robots Seekur employs this individually on each wheel, using separate servo motors

MobileRobots Seekur®Kinematics Basics, Dead Reckoning via Odometry

D(Tire Diameter)

LLength/Displacement

NC = Number of encoder counts per rotationNE = Number of encoder counts read into Inverse Kinematics

C

EN

NDL

The basic idea is that each wheel uses an optical encoder to measure the wheel rotation, which can be translated into distance traveled, using the following relationships

MobileRobots Seekur®Odometry error sources

D(Tire Diameter)

LLength/Displacement

NC = Number of encoder counts per rotationNE = Number of encoder counts read into Inverse Kinematics

C

EN

NDL

Tire diameter is dependent on: Temperature, tread wear, air pressure

Gear backlash in the drive and steering system Vehicle geometry: relative tire positions Tire slip Non-uniform surface Flex in the structure of the robot BOTTOM LINE: With well-calibrated tires, the odometry-based dead reckoning

provides useful position data over short distances, up to 20 meters

MobileRobots Seekur®GPS: Outdoor position/velocity measurement & correction

GPS receiver receives coded messages from 3-10 satellites, out of a constellation of 24-32.

Flight times are used to measure distance to each satellite. Positions of each satellite are fixed Triangulation techniques lock in on the position Differential GPS uses fixed receiver to reference platform measurements Challenges: Requires line of sight, only provide fixed time position, 1-4SPS

MobileRobots Seekur®Laser: Indoor position measurement & correction

Seekur comes with the SICK LSM111 Laser Range Finder system.

270° scan angles 25-50Hz scan rates 20m range Some conditions requires a stop to

get stable measurements.

What are IMUs used for?


Indoor: Gyroscope feedback

Indoor: Poor or no gyro feedback Massive improvement, expansion of robot capability

Outdoor:- Uncertain terrain- GPS obstruction

management

http://www.analog.com/en/analog-dialogue/articles/inertial-sensors-and-autonomous-operation-in-robots.html

MEMS GyroscopesTypical Block Diagram

Axis of rotation

rategyro K Gyroscopes are angular rate sensors They provide a simple relationship

between angular rate and the electrical output signal.

Assuming a single-axis (yaw/z), the heading estimate is calculated by integrating the angular rate.

2

1

t

tH dt

iSensor®

Understanding specifications – mathematical representation

Axis of rotation

rategyro K

MEMS Gyroscope ImplementationImportant performance parameters, scale accuracy

Sensitivity error directly translates to heading error when the gyroscope is rotating.

2

1

2

1

2

1

1t

t

t

t

t

tH dtKdtKdtK

Error term from sensitivity error

Correct HeadingSensitivity Error

MEMS Gyroscope ImplementationImportant performance parameters: bias accuracy

► Noise is dependent on bandwidth and will impact bias estimates.

► Alan Variance curves provide a relationship between bias accuracy and averaging time.

► For the Seekur system to get the best bias accuracy out of the ADIS16135, the Allan Variance shows that an average of 100 seconds will provide ~0.002 °/sec of bias accuracy.

► The Allan Variance curve also provides accuracy for lower average times, in case the application doesn’t have 100 seconds

► In-run bias stability is the minima on the curve. This time also sets the optimal integration time (t2-t1) for the heading calculation.

► In the case of the ADIS16135, t2-t1 = 100 seconds will provide the best accuracy.

0888

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7

1 10 100 1k0.001

0.1

0.01

10k

(°/s

ec)

Tau (sec)

2

1

t

tH dt

Quick reference to typical cause/effect


de Ψe

de

Ψe

Heading error = Ψe

Position error = de

Sensitivity error causes heading and position errors during a turn

Nonlinearity (2nd order) causes a heading error that can show up in patterns like an S-turn

Bias causes a drift in heading and position errors, even when there is zero turning.

de

Ψe

Proper path, heading position in blueError-burdened path in red ADIS16495:

+/-0.2% error over temperature0.05% end of life!

Presented By:Mark Looney

Application Engineer

Analog Devices, Inc.One Technology WayNorwood, MA 02062PHONE [email protected]

(guidance navigation control) - analog devices

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