wearable electronics - inertial elements · ieee iot/wearable technology tutorial, 2017 7th sep...

IEEE IoT/Wearable Technology Tutorial, 2017

7th Sep 2017, 10 AM to 1:00 PM

www.inertialelements.com www.oblu.io

Amit K Gupta Founder & CEO, Oblu.io

Wearable Electronics A Designer’s Perspective

Subhojyoti Bose Research Engr, oblu.io

Amey Karkare Assoc. Prof, CSE, IIT Kanpur

Outline

• Power Management

• Batteries

• Sensors

• Calibration

• Laboratory exercise

– Inertial Sensors’ calibration

Power Management

• Voltage supply / Power source

• Non-idealities, imperfections

• Power rating

• Real life design challenges

• Efficiency

• USB power supply

• Voltage regulator

Vout = ?

Question 1

Vout’ = ?

Vout’, IL (RL→0) = ?

Vout’, IL (RL→∞) = ? RL

IL

Question 2

• What is an ideal DC voltage source / power supply ?

• What are the common non idealities of a typical

DC voltage source ?

• Now go back to Q2 and give reason for drop in

Vout with load.

Question 3

Vout’ = ?

Vout’, IL (RL→0) = ?

Vout’, IL (RL→∞) = ? RL

IL

Question 2

• Why there is drop in voltage during peak summer / winter ?

• Why are you supposed to increase your meter wattage if you install an AC in your house?

Question 4

Vout’’ = ?

Vout’’, IL (IL→0) = ?

Vout’’, IL (IL→∞) = ?

IL

IL

Question 5

• You receive fixed AC voltage 240V (Ideal scenario)

• What about “current”. Is current also fixed?

• Who decides the upper limit of current?

• What parameters help in deciding the upper limit

of current?

AC Supply @ Your Home

• AC to DC conversion

• Cellphone charger is an excellent example

• SMPS of your PC: Accepts 240V AC input and gives

out +5, -5, +12, -12, 3.3V

– Is this information about an SMPS enough?

Power Handling in Electronic Circuits

Current rating is very important !

Power Rating of an SMPS

Vin

Iin

Charger / TV / PC

Input Power = Vin x Iin

Power Consumption

Vout

Icharging

Vin

Iin

Efficiency (Ƞ) = Power In

Power Out

Cellphone

Charger

Cellphone Charger

VUSB = ?

AC ~240V DC / AC ?

VUSB = ?

Wall Charger Car Charger

Q: Is there any limit on the current carrying capability of a USB i/f?

Cellphone Charger

• From where does the car charger derive its

power?

• DC / AC ?

• Battery chemistry?

• Terminal voltage?

Car Charger

Q: What is the power efficiency of the charger if the charging current here is 600 mA ?

VUSB

VIN

IIN

Efficiency of a Cellphone Charger

ABC GT Silicon

Q: What is the power efficiency of GT Silicon’s charger in this case ?

VIN

IIN

VUSB

IUSB

Benchmarking

• Provides voltage at certain level (1.8V, 2.5V, 3.3V, 5V etc)

• Maintains the voltage despite variation in input voltage, output current, temperature etc

• The max load current, maximum input voltage range and ambient temperature range for stable output are important specifications

Voltage Regulator

(Vin,min, Vin,max) Vout

Iout,max

Ƞ

Specifications of a Voltage Regulator

Specifications of CAT6219 (2.85V, 500mA)

Dropout characteristic Line regulation

Load regulation Vout vs Temp

• Voltage supply / Power source

• Variation in voltage due to current withdrawn

• Non-idealities, imperfections of a voltage supply

• Current rating (typical, max) of a system

• Real life design challenges due to imperfections

• Efficiency of a power mgmt unit

• USB power supply (current limit)

• Voltage regulator – Stability of output voltage, Range of input voltage

– Efficiency, Effect of ambient and operating conditions

Summary

Batteries

• Battery chemistry

• Non idealities

• Li-polymer battery

• Charging & Discharging

• Series & Parallel combination

• Protection circuit

• Application decides the primary source of power

• Non portable appliances use Mains as the main power source and battery for backup – Your TV / PC are not portable items. Those are meant to

operate on main supply. Battery/inverter is used as backup power. (Give more examples)

• Portable appliances use battery as the main power source and mains for charging or backup – Oblu (wearable sensor) runs on battery. It uses USB power

for charging battery and also as an alternate power source. (Give more examples)

Power Source: Mains / Battery ?

• AC Mains (Converted to DC internally)

• Batteries

• USB (?)

Typical Power Source for Electronic Gadgets

• Fixed voltage – 5V DC

• Maximum current limit of a USB port (which also supports USB data transfer) – 100 mA

– 500 mA

• Cellphone chargers use USB connectors

• Those are just physical connectors for charging only

• There is no USB data transfer support in chargers

• Charger circuit controls the charging current

• Charging current is chosen as per battery specifications

• Use only the charger specified by phone manufacturer !

Image source: Internet

USB Port as Power Source (?)


How would you explain this to a 2 yrs old


Phone is eating food!


Do these appliances also eat food?

They do! But they don’t have stomach!


Do these appliances also eat food?

• A perfect analogy to understand • Form of energy – Chemical in both the cases • Source of energy • Energy storage • Charge rate and time • Discharge rate and time • Energy draining beyond a limit • Recovering some energy after some rest • Recovering from a fatal state

Batteries and Human Beings

• Commonly used rechargeable batteries

– Li-ion / Li-Poly (Lithium ion / Lithium polymer)

– Pb-Acid (Lead Acid)

– NiCd (Nickle Cadmium)

– NiMH (Nickle Metal Hydride)

• Different chemistries, different terminal voltages

• Somewhat similar characteristics (like humans)

• Li-ion / Li-Poly: most popular for portable and wearable IoT

– Highest energy density

– Low maintenance

– Ease of handling

Reference - http://batteryuniversity.com/learn/archive/whats_the_best_battery

Types of Batteries

• Small size

• High energy density

• Low price

• Longevity is least bothered

Important Factors for Wearable / Portable

• Typical terminal voltage of a unit cell (3.7V)

• Battery capacity (milli-Amp-Hours or mAH or C)

• Charging current

– Recommended C/2 for best performance

– Charging time with C/2 is ~2 hours

• Fast charging (2C, max limit)

– Typical is 2C

– Max limit of charge current

– Must not be used on regular basis

Li-ion Battery

Protection Circuit Module

• Over charging protection voltage (~4.2 V) • Over discharging protection voltage (~2.7 V) • Max discharging current protection • Over current protection • Short circuit protection

Charging Profile

• Terminal voltage profile – Varies nonlinearly. Faster variation near empty and slower at

near completion

– Approximated as linear for indication purpose

– Typically varies from 3.2V (full discharge) to 4.2V (full charge)

Constant Current

Constant Voltage


Discharging Profile

• Cutoff voltage (3.0 V) at room temperature • Faster discharge results in reduced capacity • Charging cycles of a battery are limited

C


• OCV or EMF (Li-ion): 2.7

(min), 3.7 (typ), 4.2 (max)

• Rint: ~500 mOhms

• Vbat = EMF – Iload * Rint

• Non idealities can be modeled as internal resistance

• Remember momentary current surge of 1A can cause terminal voltage drop by 0.5V !!

• Internal resistance limits battery backup of a system!

Internal Resistance

Series Combination

• For increasing the terminal voltage (V, 2V, …)

• Results in increased internal resistance (r1+r2+…)

• Capacity remains unchanged of the resultant battery (C)

• Use batteries from the same manufacturer, same model

EMF = ∑ EMFi Rint = ∑ Rint,i

Parallel Combination

Capacity = ∑ Capacityi 1/Rint = ∑ 1/Rint,i

• For increasing the capacity (C1+C2+…) • Results in reduced internal resistance (r1||r2||…) • Terminal voltage remains unchanged (V) • Use batteries from the same manufacturer • Defect in any one battery of the combination gets

distributed in the resultant battery system

• Tendency of battery to gain some strength given

rest from the normal operation.

• Measure terminal voltage when in operation. Stop

using it for few minutes. Measure the voltage

again.

• Fully drained out cellphone also becomes alive for

few minutes, after sometime

Elasticity (The ability to self recover)

• Form factor, energy density, charge cycles

Cylindrical Prismatic Coin Cell


Battery’s shape

Image source - http://spectrum.ieee.org/tech-talk/consumer-electronics/portable-devices/ces-2017-panasonic-shows-off-bendable-lithiumion-battery-for-iot-wearables

Flexible Li-ion Batteries for IoT, Wearables

Image source: www.amazon.in

Universal Battery Charger

We were shocked when we measured thickness of this coin cell using this vernier caliper, without much thinking. Can you tell why did it happen?

Flex Your Neurons

Flex Your Neurons What is this?

Electrodes (Cathode, Anode)

Insulator

Li-ion battery unpacked!

Flex Your Neurons What is this?

• Similar to human beings

• Energy storage (food)

• Internal resistance (weakness, immunity)

• Li-poly rechargeable battery

– Capacity 3.7V XXXXmAH (milli Ampere Hours)

• Terminal voltage’s variation with charge/discharge

• Max. charging & discharging rates (2 hrs charging)

• Serial & parallel combinations

• Protection circuit – Min and max cut-off voltages

Summary

Sensors

• Technology Trend

• Sensors’ characterization

• MEMS sensors:

• Pressure Sensor

• Accelerometer

• Gyroscope

• A case study of a motion sensing application

• Shoe-mounted inertial navigation

• A sensor typically measures or identifies a particular

physical quantity.

• Sensors convert the physical properties to electrical

signals understandle by machines.

• Sensors are found everywhere.

Source of images: Internet

Sensors

Changing Trends in 21st Century


Wearable Devices


Internet of Things

• Touchscreen

• Light

• WiFi

• Wind speed

• Bluetooth

• GPS

• Proximity

• Barometer

• Tilt

• Magnetometer

• Accelerometer

• Gyroscope

• Temperature

• Humidity

What is a sensor ?

Smartphone – A Sensor Hub


Is watch a sensor?

Time Sensor??


Trend Enabling Technology

VLSI Technology & Moore’s Law

The observation made in 1965 by Gordon Moore, co-founder of Intel, that the number of transistors per square inch on integrated circuits had doubled every year since the integrated circuit was invented. Moore predicted that this trend would continue for the foreseeable future.

Source: Wikipedia Image source: Internet

Trend Enabling Technology

Micro Electro Mechanical Systems (MEMS)

Accelerometer Gyroscope


Micro Electro Mechanical Systems

• Miniaturized mechanical and electro-mechanical elements

• Moving structures fabricated on a Silicon substrate

• Made using techniques of microfabrication.

Micro motor Gyroscope Accelerometer


MEMS v/s CMOS

MEMS fabrication is same as IC (CMOS) fabrication, except • Mechanical Properties

• Feature Size

• Unconventional Materials

• Release Process


Different Kind of MEMS Sensors

• MEMS Inertial Sensors (Accelerometers and Gyroscopes)

• MEMS Pressure Sensors

• MEMS Gas Sensors

• MEMS Humidity and Temperature Sensors

• MEMS Chemical Sensors

• MEMS Bio sensors

MEMS Pressure Sensors


Silicon Substrate

~10um thick membrane

MEMS Pressure Sensors


• Pressure results in change in shape

• Change in shape changes resistance

• Change in resistance changes electrical signals

Pressure Sensor Full Bridge Configuration

• Half Bridge and Quarter Bridge configurations also possible

• Sensitivity: Full Bridge > Half Bridge > Quarter Bridge

• What is Sensitivity of a sensor ?


Micrograph, Sensitivity

Courtesy: Prof K N Bhat, CeNSE, IISc

-

+

R2

R2 R1

R1 Vout = ?

V-

V+

Flex Your Neurons

-

+

R2

R2 R1

R1 Vout = ?

V-

V+

Vout = (R2/R1) (V+ - V-)

Flex Your Neurons

-

+

R2

R2 R1

R1 Vout

V-

V+

-

+

+

-

Amplifier with Voltage Follower

-

+

R2

R2 R1

R1

V-

V+

-

+

+

-

Rf

Rf

Rg

Flex Your Neurons!

Vout = ?

Vout = (1+2Rf/Rg)(R2/R1) (V+ - V-)

Flex Your Neurons!

-

+

R2

R2 R1

R1

V-

V+

-

+

+

-

Rf

Rf

Rg

Vout = ?

Types of MEMS Accelerometer

• Capacitive

• Peizoelectric

• Tunneling

• Peizoresistive

• Thermal

MEMS Accelerometer

Working principle


Operating Principle


Capacitive MEMS Accelerometer


MEMS Gyroscope


Types of MEMS Gyroscope

Tuning Fork Gyroscopes

• Rotation causes the proof masses to vibrate out of plane

• The vibration is sensed capacitively with a CMOS circuit


Vibrating-Wheel Gyroscopes

• Capacitive sensing under the wheel

• Can be used to detect two in-plane rotational axes


Types of MEMS Gyroscope

ax

az

ay

x0, y0 , z0

x1, y1 , z1

x1 = 2.t

xa

2

1.t

xv

0x

y1 =

z1 =

2.ty

a2

1.t

yv

0y

2.tz

a2

1.t

zv

0z

Motion Sensing With Accelerometer

Accelerometer is for linear motion

x

y

z

Roll =

Pitch =

Yaw =

Where is the angular rate

readings from gyroscopes

trx

trztry

r

Motion Sensing With Gyroscope

Roll

Pitch

Yaw

• Gyroscope is for angular motion

• Real objects have finite shape

• Real Motion is a combination of linear and angular motions

Current Trends in Sensor Technology

•6-axis IMU •3-axis Accelerometer •3-axis Gyroscope •Motion Processor •4mm x 4mm

MPU-6050

•9-axis IMU •3-axis Accelerometer •3-axis Gyroscope •3-axis Magnetometer •Motion Processor •4mm x 4mm

MPU-9150

•9-axis IMU •3-axis Accelerometer •3-axis Gyroscope •3-axis Magnetometer •Motion Processor •3mm x 3mm

MPU-9250

Case: Inertial Measurement Unit (IMU)

Image source: www.invensense.com

Invensense MPU-9X50

Image source: www.invensense.com

• (Non) Linearity

• The quantity to be sensed

• Ambient conditions

– Temp, Humidity etc

• Operating conditions

– Voltage, Current

• Response time

– How quickly a sensor can report changes

• Hysteresis

– Sensing while ascending vs descending

Sensor Characterization

Perfect Motion Sensors

Perfect Motion Model

Perfect

Path

Estimation

(Motion model not required) (Motion sensors not required)

Working with Low-cost Motion Sensors

Low-cost Motion Sensors

Motion Model

Improved

Path

Estimation

Fusion

Example: Pedestrian Navigation

Motion model: Zero Velocity Update (ZUPT)

Summary

• Current trend

• Enabling technology

• MEMS pressure sensor

• MEMS accelerometer and gyroscope

• Motion estimation with accelerometer and gyroscope

• Sensor characterization

• Fusion of low-cost sensors and motion model

• Motion model (ZUPT) for pedestrian navigation

Calibration

• Why is calibration required ?

• What is calibration ?

• What are the ways to calibrate inertial sensors ?

• What is the outcome of calibration process ?

• How is calibrating one sensor different from

calibrating sensor array ?

Some random errors in sensors

• Randomness in mother nature gets manifested in the

sensors’ structures during fabrication and packaging

• Some errors may come after prolonged use

• Further there are some errors that occur based on the

environment/operating conditions.

These errors are corrected by calibrating the sensors.

Why is Sensor Calibration Required

• Variation during fabrication & packaging are random in nature

• Each sensor from the same lot or same fabrication house would be different

No two fabricated sensors are same !

Each sensor must be calibrated before use !

Does Each Sensor Require Calibration

• Sensor parameters are compared with any standard reference to find the error.

• The error in any measured parameter can be modelled as gain and bias

Where k is the gain and b is the bias

• The process of correcting sensors output with gain and bias known as calibration compensation.

• On the fly calibration-suitable feedback mechanism.

bkpp measuredcalibrated *

Calibration

oblu – A Shoe-mounted Indoor GPS

• The MEMS IMUs are placed in an array of 2x2

• Within every IMU, 3-axis accelero and 3-axis gyros are present

• Within IMU, accelerometers are orthogonal to each other

• Within IMU, gyroscopes are also orthogonal to each other

• Orientation of all the IMUs is same

– Accelero in x/y/z direction in all the IMUs are aligned by design

– Gyroscopes in x/y/z direction in all the IMUs are aligned by design

oblu – A Multi-IMU Platform

Randomness during fabrication get embedded as following:

• The axes x-y-z within an IMU may not be orthogonal to

each other

• One IMU’s coordinate axis may not align with rest of the

IMUs

• Other obvious errors may appear in the form of gain/bias

in individual sensors.

All the above non-idealities can be modelled as gains &

biases !

Error in oblu

• Accelerometer are calibrated using 3 axis-Shakers Table

• Gyroscopes are calibrated using 3 axis-Rate Table

• Both these instruments cost thousands of dollars.

• A low cost and less time consuming solution is required

Sophisticated Calibration

• A known standard reference acceleration is available

everywhere

– Acceleration due to gravity

• z-axis accelerometers will sense “g” when oblu is placed

horizontally.

• x and y axis will also sense g when placed vertically.

• All three axis will measure components of “g” if placed at

certain angle.

Gravity as Standard Reference

• A 3D printed icosahedron with cavity to hold oblu

• Compare “g” as obtained from oblu and with the expected value

• Repeat the process for 20 different angles at which oblu is placed

Icosahedron: Calibration Device

• oblu’s data is collected and compared against the expected output for 20 different cases

• Only 12 cases are enough for calculating the gain and biases

Calibrating “oblu”

• No low cost standard reference for gyroscopes is

present.

• So only bias (offset) of the gyroscopes is estimated

• At static condition the angular velocity reading is

considered as bias.

The offset error can easily be estimated and eliminated !

Estimating Bias of a Gyroscope

• The errors that arise due to noise is eliminated by the Zero

Velocity Update (ZUPT) Algorithm.

• Human stride shows a standstill moment or Zero Velocity

• The standstill phase of stride is called “step”.

• Imperfect sensor measure non-zero velocity at standstill

• Non-zero velocity at standstill is due to noise in the system

• Fine tune the computation model based on this information

On-the-fly Calibration

• IoT, Sensors and Calibration - Whats the relation? by Amit and Subho – https://www.linkedin.com/pulse/iot-sensors-calibration-whats-

relation-amit-k-gupta

• Story of a Shoe-mounted IoT Sensor Calibration by Amit and Subho – https://www.linkedin.com/pulse/story-shoe-mounted-iot-sensor-

calibration-amit-k-gupta

Refrences

https://www.linkedin.com/pulse/iot-sensors-calibration-whats-relation-amit-k-gupta















https://www.linkedin.com/pulse/story-shoe-mounted-iot-sensor-calibration-amit-k-gupta

















• Random variations in fabrication process introduce random errors

• Each sensor has different characteristic due to randomness • All the errors can be modelled as gain/sensitivity and

bias/offset • Process of identifying gain & offset is known as calibration • Each and every sensor must go through calibration • Sensor’s output is calibration compensated at post

processing • Calibration needs to be performed periodically • Non-orthogonality of axis is also a source of error in motion

sensors • Misalignment of IMUs’ coordinate system is source of error

in multi-IMU system • Step detection (ZUPT approach) is used for on the fly

calibration and compensation for shoe-mounted PDR sensors

Summary

Thank You

Contact Us

[email protected]

R&D Centre: 171 MIG, Awadhpuri, Block B, Lakhanpur, Kanpur, India, PIN – 208024

www.inertialelements.com www.oblu.io

Internal Resistance

Rint

EMF

• OCV or EMF (Li-ion): 2.7 (min), 3.7 (typ), 4.2 (max)

• Rint: ~500 mOhms

• Vbat = EMF – Iload * Rint

Vbat

Iload ≡ (Ityp, Ipeak)

wearable electronics - inertial elements · ieee iot/wearable technology tutorial, 2017 7th sep...

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