Wednesday, August 22, 2012

AP2 Module and Design Changes

I’ve decided to migrate to the AP2 module for V3 as previously alluded to. So that means I need to mount the AP2 module via connector or soldering to the new board. I initially choose soldering because as seen below the AP2 module just barely fits inside the BB torque tube.DSC_8919

So I designed up the surface mount connector for the PCB according to the AP2 module datasheet. It ends up being a little bit bigger than the module. It ends up being 22mm wide, while the BB torque tube measures 21.27mm. It just barely won’t fit as seen by my paper print off cut to scale.


Look at that bowing of just a piece of paper. I could trim it back, and I’m not sure how close the PCB places will allow me to be to the edge. Can I have copper right to the edge of the board? I’ll find this out shortly.


However I’ve been thinking about my V2 design. The trace antenna inside that torque tube is really doing a number on garbling my data causing a lot of failed signals. I’ve ordered new gauges, this time from Omega. Honestly, I prefer Vishay. I was at their facility, I got to see around and talk to applications engineers, and learn some “secrets” to a good strain gauge setup. However they don’t make a double shear gauge as small as I want and I’m not yet willing to pay the setup and production fee for a new design.

I’m working on alternative placements for the microcontroller and transceiver board. I’m still planning a shear gauge inside the BB torque tube so I figure it might be wise to place the board near the other gauges. Namely mounting it to the back of the right crank arm or between the the crank spider arms next to the crank arm.

The V2 Prototype is based on an SRAM Rival crank before they switch to a hollow forged. The new one on Order, like shown in the last post which is on my current bike, is the newer hollow forged. These have a flat back which would be ideal for mounting on.

While I wait for new parts, I am planning to take a stab at another project. Making a pseudo power meter for cycle trainers based on a combination of the regression curved for power dissipation combined with the flywheel mass used to calculated the additional force due to acceleration.

P = C1 * V + C2 *V^2 + C3*V^3 + (m_o * alpha)*omega

Where C1 – C3 is the regressed curve constants, V is rotational velocity, m_o is the polar moment of inertia, alpha is rotational acceleration in rad/s^2 ( m_o * alpha = Torque in N-M), and omega is mean rotational velocity during that acceleration period in rad/s.

The tricky part is getting m_o. Two methods – Calculated it based on size and weights or materials for the flywheel and roller and guess the wheel, or try and figure out how to calculate it based on a coast down time from a given speed.

What will be needed is a microcontroller, an ANT+ transceiver and a magnet pickup and magnet. I’m curious if the Garmin speed cadence sensors are using an MSP430. They could potentially be hacked and reprogrammed to do this. With so much stuff on order I’m not quite willing to buy another ANT+ device only to intentionally rip it open with no hope of getting it back together so this will be DIY with my existing stuff.

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