Monday, November 18, 2013

Custom Electronics from the EE Team


The EE team this year is working on some custom electronics in addition to all of the COTS (commercial off the shelf) work we do on the larger electric vehicles. Our responsibility has grown to not only include the interface and safety circuitry for the large motor controller, battery pack, and system logger for the trike but also a variety of custom electronics.
We have two (new) main projects):

  1. Electronic actuation of a brake system for the NY bike trailer
  2. A custom BMS (battery management system) for small (bike) to medium (moped/go-cart) electric vehicles
In addition to these two tasks, its also my job to teach electronics to the EE-team; the engineering goals of the projects have to balance cost, effectiveness, and the potential to teach electronics design principles.

Our lab space was recently upgraded with a new electronics test bench (courtesy of the MIT Edgerton Center) to accommodate, in part, our new role.

Brake system

We needed a way of actuating a pair of rim brakes on the bike trailer, and while a mechanical linkage was preferred initially, mocking up the requirements for quick disconnects and cable pull routing lead us to shift to electrical actuation. The system uses a worm-driven motor to pull and release the brake cable; because this can not be back driven (a property of the work gear) the motor does not need to stall to continue apply brake force. While the ME's worked on enclosures and mounting, team produced a schematic and a board layout for this task using an Attiny85 micro-controller at the center. 
Layout for NY-bike-trailer-brakes

Battery Management System

We are working on a battery management system that is fully modular. Someone who wants to build an EV, once our system is complete, could connect several 4s2p modules together with a central controller to form a 'smart pack' capable of self balancing and charging. We are still at the system design level and currently working on a digital communications bus to communicate between all of the individual modules.

More updates are coming your way in the future.
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Saturday, November 9, 2013

Mock-up of the Drive Train config

Busy day today at EVT!

The MechE team built an awesome mockup of the drivetrain design out of laser parts, sprockets and zip ties.

yes... our final version will have acrylic sprockets!
lauren and erich dismounting a wheel

Sunday, November 3, 2013

Drive Train and Brake Drive Design

This weekend there was a lot of progress in the drive train design as well as in the trailer brakes.

We arrived at a motor decision, we'll be using the LMC-170 motor.  It is a bit smaller than the motor used in the eSuperbike but it is still an axial flux PM DC motor, it can give us a lot of power (...a lot). We ran some simulations to determine the gear ratio we should use.

Speed Time curve for Trike at 0% grade

Speed-Time for 10% grade

screenshot of simulink model (made by Abraham, Roberto and Nigel)
The plots above are at a voltage of 50V (duty cycle of ~50% in the controller), also at the speeds shown above our speed is limited by the no load speed of the motor (which is of course dependent on the voltage) this means we have the freedom to increase our speeds considerably if we increase the voltage a bit. We'll be limiting the motor speed via the motor controller output voltage to stay within legal limits.

From the data shown and practical considerations of the size of the sprockets we arrived at a gear ratio of 8. Erich and Alex are currently busy designing/buying the sprockets using the #35 chain specs. Jacob and Will were busy all day yesterday CADdding out the mounting of the motor. Here is a picture of the current design.

notice the eccentric motor mount that allows for tensioning the chain
We have put in the order for the motor and as soon as we get some more detailed cad for the motor we can go ahead and manufacture the mounts.

The brake team had a very productive day, we decided to try out some work-gear driven motors (like windshield wiper and car window motors) to actuate the brake cables. These motors have huge gear reductions which makes them very hard to back drive which is great for us. The simplicity of the motor will allow us to drive it open-loop to open and close the brake calipers.
c-clamp holding the motor for testing

jack and j-rod testing out the motor driven brakes

We powered the motors from a power supply and the mechanism worked well. The addition of these brakes will give us lots of stopping power and keep us safe in the road.

The EE team also had a big day yesterday but I'll let them give you guys an update in a separate blog post.