LAC 2009 - Paper: Bowsense - A minimalistic Approach to Wireless Motion Sensing

2009·0.19 MB·English

by  Hans Wilmers

Bowsense – A minimalistic Approach to Wireless Motion Sensing
Hans WILMERS
Norwegian Centre for Technology
in Music and the Arts
(NOTAM)
Nedre Gate 5,
0551 Oslo – Norway
[email protected]
Abstract should be usable in real stage performance sit-
uations. In addition, the device should be easy
Anovelwirelessmotionsensingdeviceforcontrolof
to extend with external sensors such as push-
realtime electronics is presented.
The device consists of a small (20×40mm) cir- buttons and a finger pressure sensor.
cuit board including all circuitry and battery, and A first model shown in [1] contains a 3D ac-
contains a complete inertial measuring unit (IMU), celerometer, and a 2D gyroscope providing an-
measuring both acceleration and angular velocity in gular velocity. The second model now contains
3 dimensions. Data is transmitted wirelessly via a
a3rdgyroscope, enhancingthedevicetoacom-
bluetooth link and can be read from applications
plete IMU with 6 degrees of freedom. It is thus
either using bluetooth serial emulation, or using a
possibletocalculateagoodestimatefortheori-
smallstandaloneserverapplicationthatconvertsin-
entation of the device in respect to the gravity
coming data and sends reformatted data out via
OSC. vector.
Keywords 3 Electronics
Wireless, Bluetooth, Bow, Motion, Controller The first model contained a 1.5V AAA battery
and a step-up converter. This solution was re-
1 Introduction placed by a Li-Polymer battery, reducing both
weight and complexity of the design. A battery
In [1] a new sensorbow project was presented
charger was integrated into the device, and the
- a project that was started at the Norwe-
battery can be mounted in a way that enables
gian Academy of Music in colaboration with
“emergency exchange” in stage situations. The
NOTAM. The project is primarily focused on
device can be charged from a standard mini-
the artistic value of augmenting string instru-
USB cable.
ments, but some of our findings can be applied
Toreducenoiseintheanalogcircuitry,thede-
to other controlling tasks, and the sensor device
vicecontainsaseparate3V lowdropoutregula-
itself is versatile and flexible in design. In fact,
tor for the analog supply, and a 3.3V regulator
the hardware and software design itself is open,
for digital components, including the bluetooth
and can be retrieved from the author’s website.
module. With this circuitry, the device is able
In this article, the main focus is on the elec-
to run very near the cutoff-voltage of a typical
tronics used in the project, the technical chal-
Li-Polymer battery, and the resulting runtime
lenges encountered on the way, and the solu-
is well over 2h.
tions to them.
Function Component
2 Sensing the Bow
Microcontroller C8051F530 / Silabs
Wirelessmotionsensingdeviceshaveexistedfor Bluetooth Module RN41 / Roving Networks
quite a while. Many of these have been used Accelerometer ADXL330 / Analog Dev.
with bowed string instruments [2; 3; 4; 5], and Gyroscope, X/Y IDG300 / Invensense
severalhavehadtheaimofaugmentingexisting Gyroscope, Z LISY300AL / ST
instruments by adding new controlling possibil- Battery LPP402025 / Varta
ities [3; 4; 5]. Battery Charger MCP73831 / Microchip
For our project, a new device was devel-
oped that could be adapted closely to the task. Table 1: Components
It needed to be small and inobtrusive, and it
The device is built around a C8051F530
microcontroller from Silabs, which contains a
25MIPS core and a 12bit A/D-converter that
can be used with any of the unused port pins.
Withatotalof16portpins, and5oftheseused
fordigitalsignals, thisleavesupto11inputsfor
analog values. Of these, one is used to sample
the battery voltage, 3 for the 3D accelerometer,
and another 3 for the gyroscopes. The remain-
ing 4 unused inputs are made available at the
edge of the PCB, and can be used to connect
additional sensors to the device.
Asabluetoothtransmitter,theRN41module
made by Roving Networks is used. The RN41
has a max. range of 100m in open air. Data
Figure 1: Front view, showing processor and
is sent into the module by a serial port with
gyroscopes. The bluetooth module and the ac-
115kbaud. As the bluetooth data rate drops
celerometer are mounted on the back.
considerablywithincreasingdistance,hardware
flow control is used to avoid data loss.
In the choice of sensors, both technical spec-
colusedbyDanOverholtontheCUI[6]device.
ifications and resulting simplicity of the design
Every data item is sent as a 2byte integer (low
were important criteria. All sensors in this de-
byte first), and each packet framed by a hard-
sign can run from 3V, so a single analog sup-
coded sequence. The resulting packet length is
ply was sufficient. The sensor outputs are con-
constant.
nected to analog inputs of the microcontroller,
with only a few additional passive components. Header Data Footer
As accelerometer, an ADXL330 from Ana-
0x23 0x40 data0, data1, ... 0x24
log Devices is used. It measures up to ±3g
(1) along 3 axes X,Y (along the PCB surface) Table 2: Transfer Protocol
and Z (perpendicular to the surface). To mea-
sure angular velocity in 3 dimensions, two gyro-
scopes are used: An IDG300 from Invensense, Data can be sent with a rate of up to
measuring up to ±500deg/s around the X and 200samples/s.
Y axes, and a LISY300AL from ST, measur- Sending slip encoded OSC messages directly
ing up to ±300deg/s around the Z axis. The wastested,butasnumericaldatavaluesinOSC
battery used (Varta LPP402025) is a single cell are sent as 4bytes, this approximately doubles
Li-Polymerbatterywithacapacityof150mAh, the bandwidth requirement. It would certainly
and a weight of under 4g (2). The total weight be nice to define smaller OSC data types. Here,
of the sensor board inkl. battery is approxi- theefforttointroducetheuOSC[7]variantmay
mately 12g. beastepintherightdirection,althoughsmaller
The electronics layout was done with Eagle data types are not yet part of the specification.
5.3 professional, but as only two PCB layers are
5 Calibration
used, further work can be done with the free
edition Eagle Light. Special consideration has MEMS 3 components such as the sensors used
been made to make handsoldering of the board in this design typically reveal significant errors
possible,thenecessaryexperienceandtoolspro- in offset and scale. Especially the offset can be
vided. affected considerably by the mechanical stress
thatoccurswhenmountingthecomponent,and
4 Transfer Protocol
in some components by temperature. It is thus
Data is sent via bluetooth, using the Serial Port possible to calibrate the sensors on the finished
Profile (SPP). A very simple binary packet pro- board,andexpectlittlechangeintheoffsetover
tocol is used, which was derived from the proto- time, but only if temperature can be kept close
to the calibration conditions.
1with 1g=9.81m/s2
2with 1g=1gram 3Micro-Electro-Mechanical System
However, in some applications even a small cosα ∼ 1, with an error of less than 0.1%. This
offset in angular velocity must be avoided. We will lead to following simplification:
know that for most musicians, the mean rota-
tion speed of a device attached to the body is  1 0 0 
small, seen against the magnitude of the mo- R =  0 1 −α  (2)
tionsofinterest. Thegyroscopeoffsetcanthere- 0 α 1
fore be removed almost completely by applying
Thegeneralcaseofasmallrotationbyangles
ahighpasswithaverylongtimeconstanttothe
α,β,γ can be written as:
output values.
Removing the offset from the accelerometer
 1 −γ β 
output values is more complicated, as the in-
fluence of gravity in general prevents the mean R =  γ 1 −α  (3)
−β α 1
acceleration to become zero. We will later show
a method to estimate the direction of gravity in
If this is applied iteratively, the resulting vec-
respect to the sensor device, and to effectively
tor will grow slowly, and in addition rounding
calibrate offset and scale of both the accelerom-
errors can accumulate. This can be corrected
eters and the gyroscopes automatically.
by scaling the vector to unity length. Drift in
the orientation of the estimated gravity can be
6 Where is gravity?
corrected by adding a small portion †~a of the
Theorientationofanobjectinspacecanbecon- measured acceleration~a.
trolled very intuitively by a musician, and may The algorithm for one iteration step is as fol-
therefore be a good control parameter for mu- lows (for simplicity, scaling to unity length is
sic applications. Also, knowing the direction of omitted):
gravity will make it easy to split the measured
acceleration into two parts, one part equaling  1 −γ β 
gravity, and the other equaling the dynamic ac- ~g0 =  γ 1 −α ~g+†~a (4)
celeration of the object. −β α 1
Todeterminetheorientationofthedeviceto-
wards gravity, the gravity vector can be esti- An algorithm based on this concept is imple-
matedfromthemeasureddata. In[8], agravity mented in the bowsense firmware.
estimator that is also used to correct for offset If the additional assumption can be made
in angular rate is constructed using a Kalman that acceleration and angular rate of the ob-
Filter. In [9], a gravity estimator that is used ject are not strongly correlated, the result of
to compensate for the effect of gravity is con- the gravity estimator could be used to calibrate
structed using rotation by the integrated angu- both the gyroscope and the accelerometer off-
lar rate. setautomatically: Theangledifferencebetween
The gravity estimator proposed here is based the estimated gravity vector and the measured
on continously iterated coordinate rotation, de- acceleration vector could be used to correct the
pending on the measured angular rate. It is so scaleofthegyroscopes. Eachofthecomponents
simplethatitcanbeimplementedinfixedpoint x,y,z of the estimated gravity vector could be
arithmetic, and without the use of trigonomet- used to correct the offset of the corresponding
ric functions. accelerometer when small, and to correct the
A coordinate rotation by an angle α around scale, when big.
the X-axis can be defined by following rotation A tested and finetuned algorithm for auto-
matrix: matic calibration is subject to further investi-
gation.
 1 0 0 
R =  0 cosα −sinα  (1) 7 Firmware
0 sinα cosα
The firmware resources are constrained by the
With a data rate of 200 samples/s, and if the internal 256byte RAM and 8kbyte FLASH
assumption can be made that the typical an- memory of the C8051F530 microcontroller. In
gular rate does not exceed 360 deg/s, the ex- the most recent version, less than 2kbyte
pected angle differences from one sample to the FLASH is used for program storage, leaving
next are small, and we can write sinα ∼ α and space for future enhancements.
The firmware is written in C, and compiled [6] Dan Overholt. Musical interaction de-
with Keil C51. It could probably be ported to sign with the create usb interface. Proc.
the SDCC compiler suite with little effort. ICMC2006, International Computer Music
Conference, New Orleans, 2006.
8 Conclusions
[7] Andy Schmeder and Adrian Freed. uosc:
A compact 6 degrees IMU system has been pre- The open sound control reference platform
sented, that can be used as a wireless controller for embedded devices. Proc. NIME2008,
for realtime music applications. The hardware New Interfaces for Musical Expression,
is easily extendible, either by connecting sen- Genova, 2008.
sors to the existing board, or by extending the
[8] H. J. Luinge and P. H. Veltink. Measur-
design based on existing material.
ing orientation of human body segments us-
A simple algorithm that calculates an esti-
ing miniature gyroscopes and accelerome-
mateforthegravityvectorhasbeenshown,and
ters. MedicalandBiologicalEngineeringand
the algorithm may be used for automatic sensor
Computing, 43:273–282, 2005.
calibration.
More information can be found at: [9] ManuelaBandalaandMalcolmJoyce. Wire-
http://notam02.no/proj/bowsense less inertial sensor for tumour motion track-
ing. Journal of Physics: Conference Series,
9 Acknowledgements 76, 2007.
Special appreciation goes to the Norwegian
Academy of Music, namely Knut Guettler, Vic-
toria Johnson and Haakon Thelin for inspiring
cooperation in the sensorbow project. Thanks
also to Dan Overholt, Anders Vinjar and Giso
Grimm for participating in the implementation
and debugging of the transfer protocol and con-
version to OSC.
References
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