“Wearables”,
the word that defines the next revolution in consumer electronics, gadgets are
moving from being portable or mobile to being simply…wearable, either embedded
inside your shirt fabric or around your wrist, in your shoes or in the necklace
you are wearing, and even inside your sunglasses, anything you wear or touch
your body can be employed as a mount for such gadgets to either measure your
biological vital signals like body temperature, pulse rate, blood pressure,
motion & physical activity, muscles, heart & brain activity, and/or provide
you with real-time assistance like geo-navigation, healthcare monitoring,
personal assistance (appointments calendar, to-do list…etc.).
In this
article we will discuss main guidelines of designing and prototyping for wearable
devices, the basic rules for prototyping doesn't differ much than the ones
discussed previously in the first two articles in the series “Introduction to
Rapid Prototyping, adding to them the following guidelines to complete the full
picture:
- It has to be light weight (both the gadget and power source).
- With very small physical profile.
- Ultra-low power consumption.
- Easy user interface.
- Flexible, so it can be shaped on human body.
- Easy to install and setup.
- Material made of has to be skin-friendly and never cause any biohazard (e.g. skin irritation, allergies…etc).
- The gadget has to be electrically safe (no electric shocks, short circuits, dissipated heat).
 |
| The three stages of a wearable device: 1. Sensing, 2. Signal processing, digitization, data processing & storage, 3. Data transmission to a remote host for further analysis and visualization. |
Sensors:
Sensors are
electronic devices that convert physical quantities like temperature, light,
motion…etc. to electric signals so to be easily interfaced to a computer, since
computers don’t understand anything except electric signals (this simple
definition of sensors targets non-technical readers).
Let’s make a
list of the physical quantities a wearable gadget would measure and sensors
that can be used in each case:
Physical
quantity
|
Example
|
Sensor
|
Suggested
mounting
|
Motion (for humans or objects)
|
Walking, running, physical exercising.
|
Accelerometer can measure object
acceleration.
Gyroscope can measure tilting/angle of inclination.
|
Wristband, embedded in shoes or
fabric.
|
Heart rate
|
Heart pulse rate per minute
|
Pulse rate sensor
|
Wristband, ear clip.
|
Fatigue and exhaustion
|
Heart activity
|
Electrocardiogram (ECG)
|
Embedded in fabric.
|
Body temperature
|
Skin surface temperature, ambient
temperature
|
Thermistor & thermocouple sensor.
|
Wristband, embedded in fabric
|
Shock
|
Physical impact
|
Piezoelectric shock sensor
|
Embedded in shoes/footwear.
|
Force
|
Weight, muscle strength/force
|
Strain gauge
|
Embedded in shoes/footwear.
|
Touch
|
Finger tips touch
|
Capacitive pads
|
Embedded in fabric.
|
Light
|
Ambient light, reflected light from a
certain source
|
Photo-diode.
|
Wristband, finger clip.
|
Surface bio-signals
|
Muscles and limbs activity
|
Electromyogram (EMG)
|
Embedded in fabric/cloth.
|
Mental stress/activity
|
Brain activity like concentration,
stress, emotion
|
Electroencephalogram (EEG)
|
Embedded in eyeglasses or headwear.
|
Example
of off-the-shelf sensors and modules:
· MXR7900: 2-Axis accelerometer from MEMS IC
can measure up to ±0.5g acceleration in X- and Y-Axis.
· LPY503AL: 2-Axis gyroscope from ST Microelectronics can measure up to ±30° degrees in yow- and
pitch-directions.
· TMP36: Ambient temperature sensor IC from
Analog Devices.
· OPT101: Ambient light sensor IC from Texas
Instruments.
Power
storage
The wearable
device requires a power source to keep it running for enough time to do the
required measurement or assistance to the user; however the known constraints
of physical profile and weight of the wearable device must be taken into
consideration when choosing the power source.
So far
batteries are the best option for wearables in terms of physical profile and
portability, however they aren’t sustainable and require replacement or
recharging on regular basis, there are three different kinds of batteries
available in the market for wearables:
- Lithium Ion (Li-ion) batteries: the most commonly used type of batteries used in electronics, with the advances in technology it comes in different shapes and forms (flexible and solid).
- Solid-state batteries: made of semiconductor materials, usually meet the size require but have a very low capacity compared to Li-ion ones.
- Super-capacitors: a high capacity electrochemical capacitors, their size and leakage problem are the main obstacles to use them in wearables.
Energy
harvesting
Another way
of providing electric power to wearables is energy harvesting (AKA. energy
scavenging), it is the process of converting waste energy from the surrounding
environment found in the forms of heat, motion or light to usable electric
power, this field is fairly new in comparison to batteries and research didn’t
achieve to a revolutionary design that can replace batteries or any other form
of energy storage till the date of the article. There are three types of energy
harvesting based on the waste energy they harvest:
- Thermal energy harvesting: harvesting waste heat from the surrounding environment or from human body, Thermoelectric Generator (TEG) is used for this purpose.
- Vibrations energy harvesting (VEH) is the process of harvesting waste kinetic energy resulted from motion and vibrations into electricity by the means of electromagnetic, electrostatic or piezoelectric transduction.
- Solar energy: as most of readers have read or saw it before working, Photovoltaic (PV) cells are used to convert sun light to electricity and by far it is the most successful form of energy harvesting, solid glass solar panels aren’t easy to use with wearables but flexible solar panels are more convenient and design-friendly.
Interface
& Communication:
Microcontroller
Unit (MCU):
The
microcontroller is the processing unit of the wearable device responsible of
reading the signals from sensors, process them to become useful data, display
it to the user and finally communicate it if required with any external device
via wired (USB as an example) or wireless means (Bluetooth, Zigbee…etc.). The
microcontroller unit has to meet the power requirement of low power consumption
since the whole gadget is running on a limited power supply like a battery, the
following microcontroller families are good candidates for wearable designs in
the sense of low-power consumption and low cost:
- Texas Instruments MSP430, 16-bits RISC CPU.
- Microchip PIC18 and PIC12, 8-bits RISC CPU.
- ST Microelectronics STM8, 8-bits RISC CPU.
- Atmel ATmega, 8-bits RISC CPU.
Data
display and communication:
Another
important feature a wearable gadget must have is data display, either by
sending it to an external device or on the gadget itself using a small LCD
display or LED indicators, and because of the of limitations in power supply
and physical profile design most wearable designers prefer to send data to an
external device for display. There are two ways to communicate data to external
world:
- Data is stored on an on-board memory then the user can retrieve it by connecting the gadget to a computer or a smartphone wired or wirelessly using USB, Bluetooth, Wi-Fi or Near Field Communication (NFC).
- Data is communicated in real-time to a nearby device via short range wireless technologies like Bluetooth, Wi-Fi, ZigBee or similar protocols.
- Blend Micro: Bluetooth Low Energy (BLE) enabled Arduino development board from RedBear, very small form factor and low power consumption for 35$: http://redbearlab.com/blendmicro/
- Texas Instruments CC3200 SimpleLink Launchpad: Wi-Fi enabled microcontroller kit for 30$, has on-board 3-axis accelerometer and temperature sensor: http://www.ti.com/tool/cc3200-launchxl
- ESP8266: a very low cost, small form factor, low power Wi-Fi module with serial interface for wearables and IoT applications, can't operate independently, requires a microcontroller unit, starts from 5$ only: http://www.seeedstudio.com/depot/WiFi-Serial-Transceiver-Module-w-ESP8266-p-1994.html.
- Arduino Mini Pro: low cost, small form factor, Arduino compatible microcontroller kit for 10$: http://www.arduino.cc/en/Main/ArduinoBoardProMini
Wireless
communication between the wearable and display device is the preferred option for
designers because it meets the portability and simplicity requirements easily,
however this comes on the expense of power consumption since wireless
communication modules are relatively power hungry when compared to the rest of
the wearable components (MCU and sensors), however designers were able to
overcome this problem by either lowering the data rate (speed), range and
non-continuous transmission, low power technologies like Bluetooth Low
Energy (BLE) and ZigBee use these techniques to lower power consumption and
extend battery life.
Other
design ideas
- As discussed previously wearables have to be small and light, also “flexible” if possible to take the shape of the human body/part it is worn on, and here comes the benefit of “flexible printed circuit board” (or Flex PCB), it is made of special polymers to be physically flexible and bend easily without any impact on the electric connections or components mounted on its sides.
- Conductive threads/plates: can be used to detect electric connectivity from an outside source or help transmitting a specific signal through clothes fabric.
- 3D printing: to accelerate prototyping of wearable gadgets 3D printing is a good option and much lower in cost in comparison of similar alternatives like mold injection and casting.
 |
| Flexible PCB (Source: http://edablog.com/2009/03/10/utcp-wearable-electronics/) |
Components
manufacturers
This is a
list of the most known manufacturers of sensors and semiconductors that can be
used in wearable designs:
- Inertial sensors (accelerometers & gyroscopes): MEMS IC, STMicroelectronics, Analog Devices.
- Temperatures sensors: Maxim Integrates, Texas Instruments, STMicroelectronics, Analog Devices, Microchip Technologies.
- Light sensing: Texas Instruments.
- Analog Front End (AFE), Analog to Digital Converter (ADC), Sensor interface chips: Texas Instruments, Linear Technologies, Maxim Integrates, Analog Devices, Microchip Technologies.
- Energy harvesting solutions: Linear Technologies, Texas Instruments, Cymbet.
- Low-power microcontrollers: STMicroelectronics, Texas Instruments, Microchip Technologies.
- Flexible solar cells: http://www.flexsolarcells.com/PowerFilm-Solar-OEM-Components.php
For board
design the following software tools are quite helpful for PCB layout, schematic
capture and 3D design & modelling:
- PCB layout and schematic capture tools: DipTrace, KiCad, Altium, PCB Artist, OrCad, Mentor Graphics Pads.
- 3D design & modelling: SolidWorks, AutoCAD Inventor.
And since
you have reached this point in the article then possibly you are thinking of
building your own board, these companies offer PCB manufacturing services at
low cost for prototyping purposes:
- Seeed Studio (also offer 3D printing services): http://www.seeedstudio.com/service/
- APCircuits: http://www.apcircuits.com/
- iMall (flexible PCB): http://imall.iteadstudio.com/open-pcb/pcb-prototyping/2layer-flexible-printed-circuits-fpc-5cm-x-5cm-max.html
Last but not
least, this article is meant to be an introductory material for hobbyists and
developers interested in learning rapid prototyping for wearable gadgets but it
doesn’t cover everything; this article is meant to be your first step, meant to
make you more hungry and curious to know more about wearables, so good luck.
--
Karim El-Rayes
March 11, 2015
Vancouver, Canada
--
Karim El-Rayes
March 11, 2015
Vancouver, Canada
