Increased sampling frequency

May 20th, 2014

Due to some legacy code from the first MIMU3333 test boards, the inter­nal sam­pling fre­quency of the board was pre­vi­ously set to around 400Hz while the inter­nal sam­pling fre­quency of the IMUs was a 1000Hz. The main rea­sons for the low sam­pling fre­quency was that the IMUs (of the MIMU333 test board) were orig­i­nally con­nected to two dif­fer­ent ports on the uC and that the uC was run­ning on a sin­gle inter­nal PLL, which lim­ited the inter­nal clock fre­quency since the USB needed 48MHz. These hur­dles have now been cleared and the sam­pling fre­quency of the board is now set to 1000Hz, just like the inter­nal IMU sam­pling frequency.

Unfor­tu­nately, the 1000Hz sam­pling fre­quency is not with­out prob­lems. For the MIMU22BT boards every­thing works fine. How­ever, for the MIMU4444 array boards, the sam­pling fre­quency works fine inter­nally but the USB can­not keep up in case you want all raw data from all 32 IMUs. If you try to out­put all raw data from all IMUs at every sam­pling instant, the sam­pling fre­quency will drop to around 500Hz since the uC will be wait­ing for the data to be sent. I’m not sure whether this is a lim­i­ta­tion of the USB pro­file, the USB stack, or our code.

The mech­a­niza­tion and the fil­ter­ing of the aided iner­tial nav­i­ga­tion can be run at 1000Hz. This means that the clock fre­quency, i.e. power con­sump­tion, can be traded for sam­pling fre­quency. Cur­rently, the uC makes up around 2/3 of the power (cur­rent) con­sump­tion of around 100mA. Its power con­sump­tion will scale rather lin­early with the clock fre­quency. Con­se­quently, the power con­sump­tion could, in prin­ci­ple, for exam­ple, be reduced by 1/3 by halv­ing the clock fre­quency at the cost of halv­ing the sam­pling frequency.


April 1st, 2014

Finally, the new gen­er­a­tion of the IMU array boards, the MIMU4444v1, arrived yes­ter­day. These boards mea­sure 49.3x26.6[mm] and con­tain 32 MPU9150 IMUs each, which are all sam­pled in par­al­lel. The embed­ded soft­ware for the boards is achieved by com­pil­ing the code on source­forge with the argu­ment –DMIMU4444. The design-files for the boards are avail­able under Massive-MIMU. This is not an April fool mockup! The multi-IMU boards are rather a major side project to the main foot-mounted INS devel­op­ment. The same prin­ci­ples are used in the MIMU22BT boards but with 4 IMUs.

Casing STL files

March 30th, 2014

The cas­ing STL files can now be found under:
Sys­tem repro­duc­tion
The cas­ing is 1.5[mm] thick all around.
The outer dimen­sions of the cas­ing are 23.2x31x13.5[mm].
The cas­ing has a snap fit solu­tion and does not require any screws or sim­i­lar. This far, we have had the cas­ings printed on a Maker­Bot 2 by

Brief technical summary

March 25th, 2014

Below is a brief tech­ni­cal sum­mary of the new boards/modules

  • 4 MPU9150 9-axis IMUs sam­pled in parallel
  • Atmel AT32UC3C2512C 32-bit float­ing point microcontroller
  • ST SPBT2632C2A Blue­tooth module
  • Bat­tery pow­ered with charg­ing via USB
  • 4 layer 22.5x20x5.5mm PCB (exclud­ing pro­trud­ing USB connector)
  • Mod­ule size, includ­ing 150mAh bat­tery, circa 31x24x14mm (cas­ing not final)

In addi­tion we have also thrown in a flash mem­ory (up to 64Mb) and a pes­sure sensor.

If you have looked at the pho­tos below you may be won­der­ing why there are two micro-USB con­nec­tors. The one which has a slot in the cas­ing is a proper USB used for com­mu­ni­ca­tion (if you don’t want to use the Blue­tooth or sim­ply want more data than the Bluetooth-link can han­dle), charg­ing, and repro­gram­ming with the boot­loader. The one hid­den inside the cas­ing actu­ally con­tains a JTAG inter­face for debug­ging and repro­gram­ming. Micro-USB con­nec­tors are sim­ply con­ve­nient small foot­print connectors.

    Design files released

    March 24th, 2014

    Since we finally got a mod­ule work­ing, I have now pub­lished the PCB design files under

    Sys­tem reproduction

    Basic embed­ded soft­ware for the new mod­ules is already avail­able on source­forge. How­ever, sig­nif­i­cant updates, espe­cially to the wire­less inter­face, is expected in the near future. The STL-files for the cas­ing I will pub­lish as soon as it has gone through some more iterations.

    By the way, the new boards goes under the name MIMU22BTv2 (MIMU=multi-IMU, 22=number of IMUs on front and back, BT=Bluetooth, v2=version 2). This is how it’s referred to in all config-files in the software.

    Note that for some fea­ture such as the mem­ory and the pres­sure sen­sor there is still no soft­ware sup­port for and they have not been tested at all. On the other hand they are not nec­es­sary for the core func­tion­al­ity or the boards, i.e. pedes­trian tracking.