IPIN 2014

October 28th, 2014

I (John-Olof) is cur­rently attend­ing the IPIN con­fer­ence in Busan, South Korea. The pre­sen­ta­tion of today is avail­able at:


Tomor­row I will also give a demo at the con­fer­ence. See you there!

Clarification — calibration body

October 2nd, 2014

Just a small clar­i­fi­ca­tion con­cern­ing the cal­i­bra­tion body. Both the MIMU22BT mod­ules and the MIMU4444 boards (as well as the old MIMU3333 boards) fit in the body. (The do slide in fur­ther com­pared to the pho­tos.) The wire­less link of the MIMU22BT mod­ules is really nice when you do the cal­i­bra­tion. Just be sure to pair with the right mod­ule. Once I spent tenth of min­utes try­ing to fig­ure out why my cal­i­bra­tion script had stopped work­ing until I real­ized that the mea­sure­ments did not come from the mod­ule in the body.

Calibration procedure

August 22nd, 2014

Our recent mod­ules con­tain mul­ti­ple single-chip iner­tial mea­sure­ment units. To get the best per­for­mance out of them, they should be cal­i­brated. How­ever, the “IMU array” setup means stan­dard cal­i­bra­tion pro­ce­dures will not work. We have there­fore devel­oped array cal­i­bra­tion meth­ods for the mod­ules. These have lately been pub­lished in the let­ter below. Mat­lab code for per­form­ing the cal­i­bra­tion and and STL-file and cor­re­spond­ing SCAD-code are also provided.

In short, the mod­ules are inserted in the cal­i­bra­tion body seen above, sta­tic mea­sure­ments are take from all sides, the cal­i­bra­tion para­me­ters are esti­mated in a blind sys­tem iden­ti­fi­ca­tion fash­ion, and code is gen­er­ated which should then be loaded onto the respec­tive module.


John-Olof Nils­son, Isaac Skog, and Peter Hän­del, Align­ing the Forces—Eliminating the Mis­align­ments in IMU ArraysInstru­men­ta­tion and Mea­sure­ment, IEEE Trans­ac­tions on , vol.PP, no.99, pp.1,1

Repro­ducible research: code, STL/SCAD

Hardware in the loop algorithm testing

May 29th, 2014

Algo­rithm imple­men­ta­tions live their grown-up life in C, but fre­quently they are devel­oped with tools like Mat­lab. Also, exper­i­men­ta­tion, tun­ing and char­ac­ter­i­za­tion are con­ve­niently done in Mat­lab. Con­se­quently, I con­stantly find myself hav­ing to port code from Mat­lab to C and hav­ing to check the con­sis­tency between Mat­lab and C imple­men­ta­tions. In the case of the Open­Shoe mod­ules, the embed­ded plat­form makes this work worse than usual. Orig­i­nally (back in 2011) we had some frame­work (found on source­forge) for test­ing the C imple­men­ta­tions of the algo­rithms but it was not main­tained and it is now hope­lessly out of sync with the cur­rent run­time frame­work. With the new mod­ules we have renewed exper­i­men­ta­tion, tun­ing, and char­ac­ter­i­za­tion needs. There­fore, I have now added built-in sup­port (to ensure it’s main­tained), in the run­time frame­work, for push­ing iner­tial data over the USB, run­ning some pre­con­fig­ured pro­cess­ing func­tions (con­fig­ured by com­mands), and get­ting some requested states back. This I use to check the con­sis­tency between Mat­lab and C imple­men­ta­tions of the pro­cess­ing func­tions and related logics.

A side note is that it’s a pain check­ing the con­sis­tency between C code and Mat­lab code when using float­ing point vari­ables, espe­cially with sin­gle pre­ci­sion. C and Mat­lab sim­ply don’t give iden­ti­cal results and then it’s hard to know whether this is due to some small actual error or if it’s just numer­i­cal lim­i­ta­tions. The insta­bil­ity of the iner­tial nav­i­ga­tion makes it even worse. There­fore, the code has been para­me­ter­ized such that it can be com­piled with dou­ble pre­ci­sion for off-line test­ing (the 32-bit uC means that using dou­ble pre­ci­sion is too slow for real­time processing).

Status report: testing and integration

May 20th, 2014

The test­ing and inte­gra­tion of the new MIMU22BT mod­ules are slowly pro­gress­ing. So far so good! Above you see one of the mod­ules with a cas­ing printed in white by www.beta-prototypes.com. My feel­ing is that the new mod­ules give sig­nif­i­cantly improved per­for­mance but this far I have not made any rig­or­ous per­for­mance tests. Before I did this I wanted to increase the inter­nal sam­pling fre­quency (see pre­vi­ous post) and write soft­ware such that I could log all the raw data to an Android phone rather than a lap­top. Car­ry­ing around a phone instead of a lap­top is way more con­ve­nient when col­lect­ing a lot of data. The for­mer is done while the lat­ter remains.

We have now essen­tially replaced the old ADIS16367-based mod­ules in the TOR sys­tem with the new mod­ules. The wire­less con­nec­tions are a great improve­ment com­pared to the old cabled con­nec­tions. I don’t miss bro­ken cables, unplugged con­nec­tors, USB hubs, and the trip­wires the made up at all. (The wire­less con­nec­tion to the Android phones is up and run­ning but to log all raw data in order to carry out a per­for­mance eval­u­a­tion the wired con­nec­tion need to be set up as well.)

In the test­ing and inte­gra­tion we have not encoun­tered any major hard­ware issues this far. There are a bunch of small things I would like to change such as place­ment of LEDs, exchange the power switch with a push but­ton with a latch cir­cuit, and so on. How­ever, over­all the mod­ules seems to work reli­ably, for our purposes.