IMU sys­tems

They feed in­for­ma­tion to your ECU to con­trol elec­tronic rid­ing aids... but how do they work?

Motorcycle News (UK) - - Garage -


Mo­tor­cy­cle elec­tron­ics guru Bob Gray has 16 years’ ex­pe­ri­ence as a data and en­gine­m­an­age­ment tech­ni­cian and BSB crew chief.

In­er­tial mea­sure­ment unit (IMU) is a term that of­ten crops up on new mo­tor­cy­cles. IMUS are de­signed to pro­vide a con­stant stream of mea­sure­ments to the ECU, which it can then process to keep track of the bike’s at­ti­tude. The ECU uses this in­for­ma­tion to mod­ify its con­trol strate­gies – like trac­tion con­trol and anti-wheelie – which is where we ben­e­fit.

IMUS typ­i­cally use two types of sen­sor; ac­celerom­e­ters and gy­ros. Ac­celerom­e­ters mea­sure ac­cel­er­a­tion, while gy­ros mea­sure an­gu­lar ve­loc­ity. To cap­ture ev­ery way a chas­sis can move, there are typ­i­cally three ac­celerom­e­ters in­side an IMU (mea­sur­ing lon­gi­tu­di­nal, lat­eral and ver­ti­cal ac­cel­er­a­tion) and three gy­ros (mea­sur­ing roll, pitch and yaw). These are re­ferred to as six-axis IMUS, although some only use two gy­ros (roll and pitch), in which case they are five-axis IMUS.

In­er­tial mea­sure­ments don’t de­scribe the chas­sis’ at­ti­tude, only its move­ments. For ex­am­ple, the pitch gyro of a bike rid­ing along a level road will have the same read­ing as a bike do­ing a per­fect bal­ance-point wheelie

(0°/s). That’s be­cause the gyro mea­sures move­ment, not an­gle. To cal­cu­late an­gle, the gyro mea­sure­ments must be in­te­grated with re­spect to time. Rewind a few sec­onds and you might find the pitch gyro on the wheelie bike had an av­er­age read­ing of 90°/s for 0.5 sec­onds, from which the ECU can work out the bike is cur­rently do­ing a wheelie at 45°.

In terms of rid­ing your bike, though, three things mat­ter. First, the ac­cu­racy of the IMU mea­sure­ments. Sec­ond, the qual­ity of the fil­ter used in the ECU to turn them into us­able values like lean an­gle and wheelie an­gle. And third, the qual­ity of the con­trol strat­egy. If the qual­ity of any one of those things is be­low par, you’re go­ing to curse at some point be­cause there’s ei­ther in­ter­ven­tion when you don’t want it; or worse still, there’s no in­ter­ven­tion when you need it. So, IMUS are great, but only if they’re part of a good sys­tem.

‘Ac­celerom­e­ters mea­sure ac­cel­er­a­tion while gy­ros mea­sure an­gu­lar ve­loc­ity’

az= This mea­sures any side­ways drift from the front or rear wheel

ax= This mea­sures the lean an­gle of the bike

ay= This mea­sures the ac­cel­er­a­tion and brak­ing of the bike

Suzuki’s new GSX-R1000R uses a six-axis IMU to col­lect data

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