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The Application of “Off-the-shelf” Components for Building IMUs for Navigation Research

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The Application of “Off-the-shelf” Components for Building IMUs for Navigation Research
  2014 International Conference on Indoor Positioning and Indoor Navigation, 27 th -30 th October 2014 The Application of “Off-the-shelf” Components forBuilding IMUs for Navigation Research Nimsiri AbhayasingheDepartment of Electrical and Computer EngineeringCurtin UniversityPerth, Western Australiak.abhayasinghe@curtin.edu.auIain MurrayDepartment of Electrical and Computer EngineeringCurtin UniversityPerth, Western AustraliaI.Murray@curtin.edu.au  Abstract —Inertial measurement units (IMU) are commonlyused in pedestrian and robotic navigation applications and re-search. Although many IMUs are commercially available, almostall of them are non-customizable and they process the collectedraw data before presenting them to the user. However, this createsa limitation for researchers due to the fact that they have to relyon a set of per-processed data. Further, available resources andfeatures such as SD card slots, wireless connectivity, availablein the IMU may not suit one’s research. This paper provides asurvey on availability and usage of different off-the-shelf devicesto build a custom made IMU. The authors considered open-source microcontroller platforms, low cost MEMS sensors andlow cost accessories in this survey so that the IMUs will beaffordable to many people. A range of sensors, their features,available processor options and different types of wired andwireless communication options available are discussed. Particularemphasis is made on the ability to modify or add functionalityto commonly available hardware. Possible technical issues inassembling the IMU and calibrating sensors are also discussedin this paper. Technologies available for constructing a housingand mounting systems for the IMU best suited to the applicationare also discussed in this paper. As an example, IMUs developedand implemented by the authors with different housing designsspecifically created for particular applications are presented. Thissurvey indicated that off-the-shelf components can effectively beused to build custom-made IMUs to suit the particular researchinterest or application best.  Keywords —  Human gait analysis; inertial measurement units;indoor navigation I. I NTRODUCTION Many researchers use Inertial Measurement Units (IMU) totrack movement of sections of body in human navigation andtracking systems. IMUs are commonly used in other areas suchas robotics too. Usage of bare sensor boards and microcontrollerboards is not uncommon in most cases in areas like robotics.However, securely enclosed devices will have to be used inapplications with human interaction and involvement, such asnavigation and tracking systems.Although IMUs are commercially available in a variety of packages, they are usually expensive and noncustomizable.Some limitations of using such ready made IMUs are thatthey often do some processing of data before presenting to theuser, availability of limited packaging options and availabilityof limited options (Bluetooth, SD card, etc.). If one requiresa different package or options than those currently owned,then purchase of a new module is required. In addition tothat, performing unknown processing inside the IMU beforeoutput is produced restricts the users’ access to raw data andthe flexibility to perform operations and computations desired.This paper presents the outcomes of a survey conducted toidentify the off-the-shelf resources available to build an IMUthat exactly suits one’s requirements. It also presents techniquesavailable for building custom made housings for the IMUsto suit the application. This paper also discusses two custommade IMUs that are designed and build by the authors to suitwith two applications as examples. The work discussed in thispaper is a part of a project conducted at Curtin University,Perth, Western Australia that develops a navigation aid forvision impaired people. Two commercially available IMUsare discussed in the “Commercially Available IMUs” sectionof this paper while available electronics circuit options arediscussed in the “Options Available for Electronic Circuitry”section. Possibilities for implementing custom made housingsare discussed in the “Enclosure Design and Implementation”section and the IMUs built by the authors, issues faced inbuilding those and how they could be overcome are discussedin the “IMUs Implemented by the Authors”.II. C OMMERCIALLY  A VAILABLE  IMU S Although many IMUs are available in the market, most of them are either sensor ICs or development boards. Details of these are discussed in “Sensors” sub section of this paper. Twoready to use IMUs available in the market will be discussed inthis section. These IMUs come in a casing and easy to use.One of them is the x-IMU by x-IO Technologies [1]. ThisIMU consists of a 3–axis accelerometer, a 3–axis gyroscopeand a 3–axis magnetometer. It has an SD card for data loggingand the IMU can be connected to a computer using a USBcable or Bluetooth. x-IO provides software needed to accessthe IMU and configure it and one can develop software for thisas the software is open source. However, their firmware is notopen source, hence one has to rely on its per-processed output.The x-IMU comes with an enclosure and a battery as shownin Fig. 1 and priced at £309.00.  2014 International Conference on Indoor Positioning and Indoor Navigation, 27 th -30 th October 2014Fig. 1: x-IMU [1]The second is 3-Space IMU series by YEI Technologies.They have several versions of the IMU with different featuressuch as SD card slot, Bluetooth and wireless. Each version hasits own set of features and there is no version which supportsboth wireless communication and SD card. These IMUs alsohave a 3–axis accelerometer, a 3–axis gyroscope and a 3–axis magnetometer. YEI Technologies also provide software towork with the IMUs and an API for software developing, butthe firmware of the IMUs are not open source. The Bluetoothversion of 3-Space IMUs is shown in Fig. 2 which is priced atUS$309.00.Further technical details of these two sensors are discussedin [3] and can be found in their websites.III. O PTIONS  A VAILABLE FOR  E LECTRONIC  C IRCUITRY There are many different options available for sensors,processing units, data storage, communication, batteries andchargers. Some of the low cost, easy to use options arediscussed in this section.  A. Sensors Although there are many different types of sensors andsensor technologies available in the market, the authors haveselected Microelectromechanical Systems (MEMS) sensors inthis survey as they have become more popular and affordablenowadays. Sensors that may be used in robotics and indoornavigation applications are listed with their usage in Table I.The authors have considered sensors that comes on a board(breakout board) in this study, so that they can be easily usedto assemble the IMU.Fig. 2: 3-Space Bluetooth [2]Table I: Sensors and Their Usage Sensor Parameter Measured by the Sensor Accelerometer AccelerationGyroscope Angular velocity (rotation)Magnetometer Magnetic field strengthPressure Sensor Atmospheric pressureTemperature Sensor Atmospheric temperatureAmbient Light Sensor Light level There are many MEMS accelerometers and magnetometersproduced by different manufacturers that come on breakoutboards in Sparkfun store [4]. However, there is a limited numberof MEMS gyroscopes available. The most common one is ITG-3200 by Invensense [5] for which Sparkfun has a breakoutboard. Although these individual sensors may be used in IMUs,the complexity and the size of the product increases when theyare used.One solution for this issue is to use a breakout board that hastwo or three of these sensors as necessary. There are some suchboards in Sparkfun. Although this option is better than usingone breakout board per sensor, larger size of such breakoutboards limits the miniaturization of the IMU. Further, usingseveral inertial sensors will introduce errors due to offset of sensors on the Printed Circuit Board (PCB). A solution for thisis to select a sensor that includes two or three inertial sensorsin a single electronic chip. One example for such a sensor isMPU6050 by Invensense [5], which has a 3–axis accelerometerand a 3–axis gyroscope in a single chip. A breakout board forthis is available in Sparkfun. When such a 6–axis sensor isused, one still has to use another sensor for the magnetometer.Depending on the application a better solution may be a 9–axis sensor that contains 3–axis accelerometer, gyroscope and amagnetometer. The authors of this paper have found four 9–axissensors during the survey. They are MPU-9150 and MPU-9250from Invensense [5], LSM9DS0 from STMicroelectronics [6]and BMX055 from Bosch Sensortec [7]. Breakout boards forboth MPU-9150 and LSM9DS0 are available in Sparkfun whilebreakout board for BMX055 is available in ThanksBuyer [8].All these sensor breakout boards are also available in on-linestores [9]. Prices of these sensor boards are shown in Table II.Some of the specification that has to be considered whenselecting a digital inertial sensor are resolution, measurementranges, sensitivity, zero-point offset and noise density. A com-parison of these parameters for the above sensors is shown inTable III. This indicates the the performance of each sensor of these IMUs are slightly different. All these have comparablemeasurement ranges. The sensitivity of the accelerometer of MPU-9150 and LSM9DS0 is better than that of BMX055 whereas the sensitivities of the gyroscope of all three IMUs are inthe same range. The zero-point offset of the accelerometeris worst in MPU-9150 and best in LSM9DS0, but BMX055has the lowest zero-point error in the gyroscope. Althoughthe noise density of the accelerometer of MPU-9150 is worsethan BMX055, the noise density of the gyroscope of MPU-9150 is better than that of BMX055. This implies that noneof these sensors is a best: all have some parameters better  2014 International Conference on Indoor Positioning and Indoor Navigation, 27 th -30 th October 2014than the others. Because of this reason, there would not beany major advantage or disadvantage of selecting any of thesethree sensors.There are many digital temperature, pressure and ambientlight sensors available in on-line stores. Examples are TMP102digital temperature sensor, ISL29125 RGB Light Sensor andMPL115A1 barometric pressure sensor [4].  B. Processors There are many 8-bit microcontrollers available in the mar-ket. Most these microcontrollers suit the needs of an IMU,where the main task is to read sensors and either store them ontoa storage device or send them to a computer or both. For con-necting sensors, storage devices and communication devices,these microcontrollers are required to include communicationinterfaces such as I 2 C (Inter Integrated Circuit) and SPI (SerialPeripheral Interface). For one to implement the IMU withoutdesigning and producing a PCB, a development board has to beused. Although most microcontrollers have their developmentboards, authors have selected Arduino open source platform[10], as the best option because it is easy to use and there is anumber of different boards with different sizes and capabilities.The Arduino platform has several available boards witha reduced footprint, such as Arduino Micro, Arduino Nano,Arduino Pro Mini and Arduino Fio. All these boards are aboutthe same with Fio being slightly larger. Micro and Nano boardswork at 5 V while Pro Mini works either at 5 V or 3.3 Vand Fio works at 3.3 V. Although 5 V devices may be usedin wired applications, 3.3 V devices are better for wirelessdevices due to their suitability to work with battery voltages.Although Micro and Nano are programmable directly using amicro USB cable, an external programming cable or an FTDIbreakout board. The processors used in all these boards are 8-bit Atmel microcontrollers with comparable performance andall these have I 2 C, SPI and serial interfaces. C. Data Storage and Communication Options The most common storage option for embedded devicesis the Secure Digital (SD) memory card. The version of SDcards used in most small size devices is microSD. All SDcard versions support SPI bus mode and SD bus mode [11].Therefore, they can be connected with a microcontroller usingthe SPI bus and there is an Arduino library to work with SDcards [10]. microSD card breakout boards are available in bothSparkfun and in on-line stores.There are many communication options that one can use withan IMU. The wired options are serial (RS-232) and USB. USBTable II: Prices of 9–Axis Sensor Breakout Boards [4], [8], [9] Sensor Sparkfun/ThanksBuyer Price a. Ebay Price MPU-9150 USD 34.95 USD 11.50MPU-9250 – USD 10.00LSM9DS0 USD 29.95 USD 33.50BMX055 USD 16.36 USD 13.00 a. Prices at Sparkfun and ThanksBuyer are without shipping and prices at Ebay are with shipping to Perth, Australia. is a better option out of these two due to the fact that mostcomputers and laptops are equipped with USB ports and theserial port is rarely found in laptops nowadays. Arduino boardscan communicate with a computer through the programminginterface (FTDI). If RS-232 is opted, then a level shifter has tobe used. RS-232 level shifters are available in Sparkfun and inon-line stores.There are few commonly used wireless communicationoptions that can be used in an IMU. Two commonly usedalternatives are Bluetooth and NRF24 wireless transceivers.Both these modules are available both in Sparkfun and in on-line stores. Bluetooth transceivers communicate with the hostmicrocontroller using RS-232 interface while NRF24 uses SPIinterface. Using a Bluetooth transceiver will be advantageouswhen sensor data is to be sent to a smartphone or a PDA asthese devices are equipped with Bluetooth interfaces. However,for long rage (up to 100 m) communications, NRF24 will bebetter, but it needs a separate receiving device connected withthe computer. Details of this are discussed in “IMUs Imple-mented by the Authors” section. The Current consumption of the HC-06 Bluetooth modules after pairing is less that 10 mA[12] where as for NRF24, that is about 12 mA [13].  D. Batteries and Battery Chargers Lithium-ion polymer batteries (LiPo) are the commonly usedbatteries in embedded and robotics applications. LiPo is wellsuited due to the availability of capacity and dimensions - suitshousing design. One can select the shape, size and the capacitydepending on their design and time of run required. Small sizeUSB powered LiPo chargers are also available in the marketso that one can use a USB port or a USB charger to chargethe battery. Some battery and charger options are discussed in“IMUs Implemented by the Authors” section of this paper.IV. E NCLOSURE  D ESIGN AND  I MPLEMENTATION Prototyping with filament type 3D printers has now becomeaffordable. One main advantage when using a 3D printer toprint the IMU enclosure is that a custom made enclosurecan be made that suits the application. Although high end3D printers that give much better qualities are also available,they are not affordable as home and hobby use 3D printersdo. Some low-end 3D printer makes are Leapfrog, Makerbot,PrintrBot, Afinia, Rostock, Ultimaker, Reprap, Bits from Bytes,Makergear, Airwolf3D and Bukobot. Although 3D printers areavailable for prices under USD 1000, a decent quality printerwill be in the range of USD 3000 and the filament price is inthe range of USD 30-60 per 1 kg roll. The authors of this paperhave designed three different enclosure designs and printed witha Leapfrog Creatr Dual Extruder printer, which are discussedin “IMUs Implemented by the Authors” section of this paper.V. IMU S  I MPLEMENTED BY THE  A UTHORS Two IMUs implemented by the authors for different require-ments are discussed in this section. Devices used for each IMU,the enclosure designs, some concerns when implementing IMUand remedies for those are discussed in relevant subsections.  2014 International Conference on Indoor Positioning and Indoor Navigation, 27 th -30 th October 2014Table III: Comparison of Key Specifications of the Inertial Sensors [5], [6], [7] SpecificationAccelerometer Gyroscope Magnetometer  MPU-9150 LSM9DS0 BMX055 MPU-9150 LSM9DS0 BMX055 MPU-9150 LSM9DS0 BMX055 Resolution N/A N/A 0.98 mg N/A N/A 0.004 °/s N/A N/A 0.3  µ TMeasurementRanges±2 g, ±4 g,±8 g, ±16 g±2 g, ±4 g,±6 g, ±8 g,±16 g±2 g, ±4 g,±8 g, ±16 g±250 °/s,±500 °/s,±1000 °/s,±2000 °/s±245 °/s,±500 °/s,±2000 °/s±125 °/s,±250 °/s,±500 °/s,±1000 °/s,±2000 °/s±1200  µ T ±200  µ T,±400  µ T,±800  µ T,±1200  µ T±1300 µ T (x,y),±2500  µ T (z)Sensitivity 16384 LSB/g ,8192 LSB/g,4096 LSB/g,2048 LSB/g16384 LSB/g ,8197 LSB/g,5464 LSB/g,4098 LSB/g,1366 LSB/g1024 LSB/g ,512 LSB/g,256 LSB/g,128 LSB/g131 LSB/°/s,65.5 LSB/°/s,32.8 LSB/°/s,16.4 LSB/°/s8.75 m°/digit,17.5 m°/digit,70 m°/digit262.4 LSB/°/s,131.2 LSB/°/s,65.6 LSB/°/s,32.8 LSB/°/s,16.4 LSB/°/s0.3  µ T/LSB 0.008  µ T/LSB,0.016  µ T/LSB,0.032  µ T/LSB,0.048  µ T/LSB1  µ T/  µ TZero-pointOffset± 80 mg (x, y),± 150 mg (z)± 60 mg ± 70 mg ± 20 °/s ± 10 °/s, ±15 °/s, ± 25 °/s± 1 °/s ±1000 LSB N/A ± 40  µ TNoise Density 400  µ g/  √  Hz  N/A 150  µ g/  √  Hz  0.005 °/s/  √  Hz  N/A 0.014 °/s/  √  Hz  N/A N/A 0.6  µ T  A. Strap-Mount IMU  The requirement for this IMU was to record inertial data of the lower human body (legs). Logging of data from multiplesensors was also a requirement.The sensor selected for this application was MPU-9150mainly due to the fact that it contains all 3 sensors (accelerom-eter, gyroscope and magnetometer) in a single chip and hencethe size of the boards is smaller. A picture of the MPU-9150board is shown in Fig. 3a.A clone of Arduino Pro-mini 3.3 V was selected as themicrocontroller board because of its small size and as it isworking at 3.3 V. Although the 3.3 V version runs at 8 MHz,it is sufficient to cater the demands of the IMU. A picture of theboard is shown in Fig. 3b. This clone of Pro-mini was selectedbecause of its pin placement is helpful in assembling the IMU.nRF24L01+ RF transceiver was selected to communicatewith the host device because it can communicate simultane-ously with 6 slaves. A “dongle”, which is connected withthe computer, was designed using Arduino Uno board andnRF24L01+ to communicate with 6 IMUs and transfer datato the computer. All IMUs are synchronized to the time of thedongle, so that the relation between the movement of differentsections of the leg can be studied.A 3.7 V 900 mAh flat LiPo battery of size of 48 mm ×  30 mm  ×  5.5 mm was used so that the circuit can bemounted directly on the battery and the finished IMU to have (a) MPU-9150 (b) Pro-mini (c) nRF24L01+ Fig. 3: Devices used for strap-mount IMUproper aspect ratio. The battery selected has a built-in protectingcircuit. An external USB charger was used to charge the battery,so that the size of the IMU and internal heating is a minimum.The charger contains a charging controller to avoid over chargeof the battery. The battery and charger are shown in Fig. 4.The assembled IMU is shown in Fig. 5a. The enclosure wasdesigned to have loops to strap down. The 3D printed enclosureis shown in Fig. 5b. The program of the IMUs was designed tosample all sensors at 100 samples per second and send data tothe dongle and the program of the dongle was designed to readdata from 6 IMUs and send them to the USB port. A serialreader software was used to read data from the USB port andlog them. Although the dongle theoretically support 6 IMUs,it was observed that 4 IMUs is the practical limit for reliableoperation due to collisions and subsequent packet loss.The dimensions of the strap-mount IMU are 55 mm × 41 mm ×  23 mm without the loops and the total cost was aboutAUD 35.  B. IMU for White Cane The second version of the IMU was designed to be attachedto the white cane to study the synchronization of the legmovement with the white cane movement. Therefore, it wasdesigned to be longer, but narrower. Sample data was to besent to a smartphone for further analysis. Therefore, HC-06 Bluetooth was selected as the communication interface assmartphones are equipped with Bluetooth. A longer (58 mm ×  19 mm  ×  7 mm) 600 mAh battery was selected so that itmatches the design of the IMU. The battery and HC-06 are (a) Battery (b) Charger Fig. 4: Battery and the charger  2014 International Conference on Indoor Positioning and Indoor Navigation, 27 th -30 th October 2014 (a) Assembled IMU (b) IMU with enclosure Fig. 5: Completed strap-mount IMUshown in Fig. 6. All other units are as same as the units usedfor the strap-mount IMU.Two enclosure designs were used, one for the white cane andthe other for strapping to the thigh. The completed IMU andthe two enclosures are shown in Fig. 7. The dimensions of theIMU for white cane are 91 mm  ×  25 mm  ×  25 mm withoutthe mounting mechanism and the total cost was about AUD 35. C. Sample Outputs In one of the experiments, two strap-mount IMUs wereused to check the synchronization of the two legs. In thisexperiments, two strap-mount IMUs were attached to the twothighs of the subject and the subject was instructed to walk on flat surface. The output of the two IMUs were logged inthe computer and the thigh angles were computed by fusingthe accelerometer and the gyroscope. The outcome is shownin Figure 8. This is similar to the thigh angle waveformdiscussed in [14], [15] and [16] that has been captured using a smartphone. However, it was observed that there are packetlosses sometimes, which makes the waveform non-smooth atsome points.Further results obtained using both these IMUs are discussedin [17] with more details on development steps of the IMU forWhite Cane.  D. Concerns and Remedies The main concern when building both IMUs was the align-ment of the sensor with the enclosure of the IMU. Threeremedial measures were taken to reduce and compensate thealignment error. The first is to align the sensor board with themicrocontroller board as much as possible while assemblingthe IMU. Solid connector pins were used to mount the sensoron the microcontroller board so that there is no free movementbetween them. Suitable holding mechanisms were designed inthe enclosures to avoid any movements of the circuit inside the (a) Battery (b) HC-06 Fig. 6: Devices used for IMU for white cane (a) AssembledIMU(b) White cane mounted IMU (c) Thigh mounted IMU Fig. 7: Completed IMU for white caneFig. 8: Thigh angle of the two legs computed from data of twostrap-mount IMUsenclosure. As these two solutions cannot avoid misalignmentfully, an axis calibration was done after building the IMU, sothat the output of the IMU is aligned with the enclosure.VI. C ONCLUSIONS This paper presented a survey of available off-the-shelf devices for building IMUs for navigation research and possiblecustom-made enclosure making technologies. This also pre-sented two IMU built for two different applications with differ-ent requirements. It is concluded that off-the-shelf componentsand devices can effectively be used, without great difficulty, tobuild IMUs well suited to each application with an enclosuredesigned specifically for the application .R EFERENCES[1] x-io Technologies.  “x-IMU”  [Online]. Available: http://www.x-io.co.uk/products/x-imu/, [July 17, 2014].[2] YEI Technology.  “YEI 3-Space Sensor”  [Online]. Available:http://www.yeitechnology.com/yei-3-space-sensor, [July 17, 2014].[3] R. Chandrasiri, N. Abhayasinghe and I. Murray. (2013, Oct.). “BluetoothEmbedded Inertial Measurement Unit for Real-Time Data Collection,” in Forth International Conference on Indoor Positioning and Indoor Naviga-tion (IPIN2013) [Online], Montbéliard, France, 2013, pp. 510–513. Avail-able: http://ipin2013.sciencesconf.org/conference/ipin2013/eda_en.pdf,[Nov. 19, 2013].[4] Sparkfun. [Online]. Available: https://www.sparkfun.com [July 17, 2013].[5] Invensense. [Online]. Available: http://www.invensense.com, [July 17,2014].
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