November 30, 2013

CNC Everything

Just before last summer, a lab donated their old CNC mill to MITERS, in a mostly-not-working state.  Until now, I have not had a project that required any CNC machining, and even for this project I designed the parts for the belt drive reductions to be water jet cut, rather than CNC milled.  However, over this break my source for water jet access was out of town, so I decided to learn how to operate the MITERS CNC mill instead.

Here is one of the motor mounts, just after machining:

And here is a part in progress:

The parts look beautiful, with practically polished edges, compared to the extremely coarse edges left by an abrasive water jet.

so shiney
To make the standoffs, I used my old technique of making a clamping jig, so that I could face the ends off all the standoffs simultaneously.

Wherever possible I used countersunk stainless fasteners, for added shininess:

One great thing about CNC milling parts, rather than water jetting them is that I can make precise press fits for bearings without any fiddling with tool offset.  If I CAD my part with a hole .001" too small for the bearing, the mill machines a hole exactly that size.

And here's the assembled unit.  It is still missing the attachment for the linkage on the output 60 tooth pulley.

November 7, 2013

Electric Tricycle: 9 Month Service, Painting, and Data Collection

After almost nine months of use, the trike has gotten pretty gross.  Since all the mechanical bits are directly behind the front wheel, they get sprayed with everything the front wheel goes through, be that water, dirt, snow, sand, or anything else.  There was also some bare steel on the frame, which got pretty rusty.

Much of the grime came off with the help of rags and solvents.  I could not get the differential really clean without completely taking it apart, so I just wire-brushed as much of the gunk out as possible and regreased it with some thin teflon lubricant.  The chains were cleaned by putting them in a cup full of acetone and leaving them on an agitator platform.

I stripped down the steel part of the frame, and removed all the old paint and rust with an angle grinder.  I primed the frame with self-etching primer, and spray painted it red.

Before painting the frame, I briefly reassembled the trike to take it to the Swapfest garage, and finally collect some power and time data to compare it to the rest of the MIT small electric vehicle fleet.  For some reason the Kelly controller started to cut out almost immediately, making the trike useless.  Somehow during the reassembly process the sensor timing seems to have gotten messed up, despite the fact that I know the sensor positions did not change.  This, plus the continual other Kelly controller problems I've had (inability to max out the current limit, over current shutdown in the higher gears) lead me to buy a high-speed version of the same Kelly controller.  According to Shane, the high speed controllers have higher PWM frequency, which means less current ripple.  The (assumed) reason the Kelly's sometimes shut down rather than current limiting as they are supposed to is that the current control loop is not fast enough to prevent current spikes from exceeding the hard overcurrent shutdown.  Faster PWM means smaller current spikes and a faster current control, which should prevent the over current protection from being tripped.

The controller upgrade turned out to be a mixed bag.  It does seem to have fixed the overcurrent shutdown.  I can now full throttle in eighth gear from a stop without shutting off the controller, which I could never have done before.  However, acceleration felt slower than with the old controller.  A clamp meter on a motor phase read a max current 0f 75-80 phase amps during acceleration, which is well shy of the 120 peak amps the controller is rated for.  Unfortunately, I never did the same test with the old controller, so I can't directly compare numbers.  It feels off though.  More testing and diagnostics will come later.

Science Time
Before the cleaning and controller upgrade, Jaguar and I took the trike to the Stata parking garage to do some test laps with a wattmeter inline with the battery.  Using the pillars and dividers in the parking garage as obstacles, we've come up with a fun track involving a combination of tight turns, slalom, and straight-aways.

We each did at least one test lap in each of the eight gears, as well as some laps using all the gears.  The results were a bit surprising.

The first graph is Watt-hours consumed vs lap time, a metric taken from Charles and Shane's garage vehicle testing.  The number by each point corresponds to the gear used.  My laps are the red x's, and Jaguar's the blue o's.  First thing you notice is that gears 1 and 2 use way less energy than the others.  I'm fairly sure that this is because in gears 1 and 2, you can basically full throttle the entire lap without ever needing to brake.  Constant high speed operation is where electric motors are the most efficient.  Times for first and second gear were not as slow as I expected either.  I think this can be attributed to the shorter line you can drive at lower speed.  In first and second gear the trike as an incredibly tight turning radius, so you can take all the corners sharply and travel less distance over the lap.

Also interesting is that using all eight gears is barely any faster than using just using 4, 5, or 6th gear.  My best time with all 8 beat my 5th gear time by .75 seconds out of ~30, which is definitely within normal lap-time deviation.  Just fifth gear was significantly more efficient, using over an entire Watt-hour less energy over the lap.

Now Watt-hours*seconds vs gear.  The difference between my results and Jaguar's comes mostly from weight.  Results past 5th or 6th gear I don't think are completely valid, because in those gears it was impossible to max out the throttle without the Kelly controller cutting out.  So power consumption was lower and times higher than they could have been in the top gears.

Something else interesting was that the watt-meter never recorded battery side current greater than about 65 amps, or 2.6 kW.

The only conclusion I can really make from the data is that (for driving around this test track) if you are not really good at shifting, trying to use the gears will make you slower.  Your best bet is to just stick to 5th gear.  I've been riding the trike for a while now, and my multiple gear time was barely better than just 5th gear time.

Also, new riders seem to instinctively shift when they can't go any faster in a particular gear.  Which is a terrible time to shift if you want to maximize power.  If you go back to my first electric trike post ever. you can see some power vs gear ratio curves overlaying the 8 ratios on the Shimano gearbox.  The optimal shifting point is actually when the motor is a little past 1/2 its no load speed.  What I should do is make some indicator lights to tell you when you should be shifting up or down based on the motor speed.

Next up:  more robot arm.