REV Dyno Project

This is my record of our dynamometer project. By saying "our" I mean REV (Rensselaer Electric Vehicle). Our mission is to design, build and race electric cars, and the goal for the racing is efficiency. Therefore, having a device that can measure the physical output of our vehicle is crucial for the development: you need to know how much your car outputs in order to know how efficient it is.

The device that measures the physical quantity (speed and torque) of our vehicle is called dynamometer. It's basically a treadmill for the car. Since our car is custom built, our dyno has to be custom built as well: normal car dyno is way too expensive and way above our needs, yet a bicycle roller trainer is too simple and fragile for our car.

So here comes the design specification: a two-roller dynamometer system that can handle up to 150kg axle weight, 400rpm wheel speed, and 80Nm torque. The system also needs to dissipate approximately 2000w heat produced by the vehicle's physical power output.

Eddy current brake system, after our selection process, is thought to be the most suitable design concept for our need: contactless resistance, adjustable resistance level, and nice heat dissipation.

For the frame, 2x6 wood will be used; these woods can support up to 1 ton of weight under our configuration; it should be good enough for our application.

Here is the frame design:

frame

Yeah I know it looks simple, but simplicity usually means strength and reliability. This wood frame, according to our calculation, can support 1 ton of weight, which is more than enough for what we need. 

Here is the real thing in our shop:

building in progress

finished frame (me on the right)

Even though the work for building the frame seems tiring and tedious, I actually found this part quite interesting. Personally, I love to hand drill holes and cut things (whether wood, plastic or metal) into appropriate shapes; it feels like you are infusing some form soul in the process of building, and the finished product is more than just the sum of its building blocks. 

The next part is the roller part. The rollers are the parts that directly contact the car's rear wheel, and it also serves the function of supporting the car's rear axle. Here is the design:

rollers on frame

As you can see, the rollers are short and thick, this is because we want it to be stable under high rotational speed (400+rpm). The roller material, currently, is stacked basswood plates (0.25 inch thick), since that's the most accessible material for us and it is very easy to manufacture (laser cut). The wood discs will be glued together and there will be 6 steel bolts (see the 6 half-inch holes on the plate) that go across them for extra support. Since the roller is made of stack wood discs, the surface for wheel contact is the outer edge of the discs. This configuration gives us an additional advantage such that we can customize the shape of the outer edge such that we can add extra texture to the surface of the roller, which provides us additional friction for better traction.  

Another suggestion from our mechanic team is using a single piece of the aluminum block -- machine this block down to the shape of a roller plus a shaft. This design will provide additional structural integrity. Personally, I think it is hard and difficult to machine a single piece of aluminum into a specific shape, but I think the mechanic folks have their expertise so I will see how it goes. 

Back to the original roller design. The shaft here is 1-inch steel shaft, which has a theoretical critical speed of 2400rpm under this configuration. At 400rpm wheel speed, the roller speed will be around 1600rpm, which stays safely under 2400rpm. 

The manufacture of this part is not yet done. The result will be posted in later blogs. 

Now let's talk about the eddy current breaking unit. This is the unit that actually produces resistance and eats the power from the vehicle. I will not discuss theory here, and let's jump right into design and manufacture. 

The frame of the unit is a pair of acrylic sheets, which hold all the magnets in place: 

assembly with one frame to show the inside

complete assembly

This is the unit that provides the magnetic flux. There are 12 electromagnets in the unit, each of which can produce up to 0.5T magnet field at 3mm distance. 

system assembly

Two 12 inch aluminum discs (or car brake disc, if we found an appropriate one) will be used as the rotor of this resistance unit; they are the actual handler of the load from the vehicle. The vehicle will drive the rollers, which spin and transfer the kinetic power to these metal discs through an adjustable chain system (not shown in the CAD, but is easy to build), and eddy currents will be induced in the spinning metal discs, which provides the resistance. 

The building of these parts is not yet done. Any progress on the development will be posted in the later blogs. I really want to see this thing running in our shop, testing our vehicles to their very limits. The electronics team and mechanic team will work together to push this project forward.