⚡peed boost

 A mighty new 48 V battery (13S4P Super Shark from EM3ev) has been powering the Firefly for the past 150 800 km.  Voltage means speed, and this leaves the 36 V original in the dust.  It's worth mentioning that 13 series cells exceed the motor controller spec, but an internet post said it was ok.
😉

 

The hole-pattern gods must have been smiling on me, because the new battery mounting rail fits the existing bolt holes exactly.  I did add a pair of large c-washers behind the rail, to distribute load onto the plastic part.

Frame mounting holes and back of battery rail.
The small washers are unused.

One of two c-washers between frame and plastic.  Without them, the raised metal rectangles at the bolt holes (prev. picture) would probably crush the small overlapping areas of plastic (less than 1 mm wide).

Aside from a galloping speed and oodles of range (about 70 60 km), the battery broadcasts more status information than most people care to know.  In a nod toward reason, I spliced four series diodes into the charge cable, limiting the Voltage to 52 V (80 % charge).  The Super Shark's higher mass and speed cause extra mechanical stress - inspect regularly!  More weight on the front wheel does help traction.

BMS Monitor shows battery status and cell voltages (via Bluetooth).  The BMS circuit prevents unsafe voltage, current (?) and temperature, and shuffles charge between cells to equalize their voltage.

Field weakening

The Firefly's wheel speed is limited by the voltage produced as the permanent magnets move across the motor coils (Faraday's law of induction).  The faster it spins, the larger is the induced voltage, which eventually equals what the battery can produce.  Then current and torque drop to zero, and the maximum speed has been reached.  For this reason, a fully charged battery with higher voltage can go faster than a depleted one.

I have been experimenting with Field-Oriented Control which, among other advantages, can use the motor coils to generate fields that counteract the fields of the spinning permanent magnets.  The technique is known as Field Weakening.


My tests with TI boards (LAUNCHXL-F28069M and BOOSTXL-8323RS) show a speed-up of up to 38 % compared to the original motor controller.



Original data where Kv0 = 10.46 rpm/V is the measured speed of the original controller:

Vsupply	Isupply (A)	Speed (rpm)	Kv (rpm/V)	Kv/Kv0	Field weakening (A)
32.23	0.91	350	10.86	1.038	0
32.23	0.97	371.1	11.51	1.100	2.5
32.23	1.24	394.6	12.24	1.170	5
32.23	1.67	422.2	13.1	1.252	7.5
32.23	1.75	431.6	13.39	1.280	8
32.23	1.9	441	13.68	1.308	8.5
32.23	2.35	466	14.46	1.382	9.0

The data were taken on a stationary test stand, where the wheel just spins in air.  It's not ready for vehicle testing yet.

New motor controller

The Firefly's motor controller (pictures below) energizes the motor coils in the right sequence to move the wheel according to the throttles.  It sits in the battery base and appears to be a variant of the S06 controller (schematic here) with second (reverse) throttle input.

For the past 65 km, I've been testing the more advanced Baserunner controller from GRIN and ASI in Canada.

Firefly with Baserunner taped to the side of the 11.6 Ah battery.  The rat's nest of wires near the bottom allows it to function (no speed display, headlight, and brake cut-off though) and the toggle switch between the handlebar and LCD selects forward/reverse direction.

The Baserunner is a lot smoother than the original controller due to its Field-Oriented Control algorithm, which the physicist in me appreciates.  FOC also allows about 15 % higher speed, and the Baserunner can integrate with higher voltage batteries to go even faster.

Other impressions:

• Makes the motor nearly silent, but the controller emits a slight high pitched whine.
• Torque-mode control is nice.  
• While maintaining speed at low torque, there is often a bit of chatter between zero and a little torque.  Perhaps my thumb is unsteady on the throttle.
• Sometimes the motor stutters when starting to move.
• Re-engagement is much smoother.  The old controller often gave a nasty kick when re-engaging the throttle after coasting (while still moving), but Baserunner re-engagement is silky smooth.
• I disabled regenerative braking and just use the mechanical brakes.   The efficiency cost is minimal, and this will be easier on the axle and dropouts, because motor torque reversal is much less frequent.  I dislike how the original controller applies motor braking for every little throttle decrease, because I generally want the Firefly to  pull forward (or push backward) when pressing the throttle and will pull the brake levers to slow down. 

Although the Baserunner has a Z910-type connector that the existing motor plugs into nicely, the rest of the Firefly doesn't connect so easily.  I obtained a special 9-pin JULET connector pigtail (JL-F-Z911AT, Thanks Mr. Wen!) to interface the Firefly signal wiring harness with the Baserunner's modular connectors, but there is only one throttle input and an extra toggle switch is needed to reverse direction (this reverse signal required customization at ebikes.ca because standard Baserunners do not expose it).  The Baserunner also doesn't communicate fully with the handlebar LCD and has no headlight connection or brake cut-off.  I've been customizing a microcontroller to address these shortcomings (and add new features), and will update the blog when there is more.

Update (10/30/2019): The Baserunner expects an extra 4th Hall sensor (that the Firefly motor doesn't have) to output speed information.  I'm moving on, to try a different Field-Oriented Controller (controller & driver).

Need for speed

Speed kills, but it also thrills.  The converse is also true:  Slow is safe, but oh-so-frustrating.  My manual wheelchair travels slowly because steering is problematic, shoulder power is limited, and obstacles abound.  Amazingly, the Firefly electric front wheel attachment solves these problems.  Power at the wheel is up to 350 Watts, similar to a professional bicyclist and over 10 times more than most people can get out of their shoulders.  The handlebar allows for proper steering, and the longer wheelbase improves handling on non-ideal surfaces, which are almost all of them in my city.  It brings out the joy of riding something fast, which must have been etched into our DNA long ago.

With speed and power comes danger!  Go too fast through a turn or cross a drop-off at an angle, and you gain a new appreciation for the tippiness of high center-of-gravity tricycles.  Someone more adventurous than me has maybe perfected the technique of balancing on two wheels to corner fast without the need for body English.  Hopefully they were wearing elbow pads.

Credit: Martin Pettitt originally posted to Flickr as Race Of Champions (CC BY 2.0)

But even straight-line speed is fun and helps eat the miles if you're going somewhere.  My first impression of the Firefly was that it's pretty darn slow.  Releasing the wheelchair's rear brake locks helped (duh), as did reading the instructions to understand that the device starts in 1st "gear", which has a low speed limit.  Setting the "gear" to 5 raises the top speed to 20 km/h (12 mph), already pretty fast if you're travelling on a sidewalk.  That's still an artificial restriction, which can be removed by adjusting the LCD controller.  Then the limit comes from the motor and battery voltage, and a fully charged battery reaches 24 km/h (14 mph).

There is a saying about all-wheel-drive cars, that they just get you to your accident faster.  Be that as it may, it would be nice to go faster than 24 km/h on smooth and clear pavement.  One option is the Phaserunner motor controller whose manual describes "Field Weakening for Speed Boost".  Apparently, the motor coils can be energized in a tricky way to gain 15-20 % in speed, at the expense of battery drain and maybe motor stress.

The Firefly's motor controller is a circuit board inside the battery base that is connected to the wiring harness.  All of those connections will make it hard to replace.  In the end, it might be more effective to replace everything (motorized wheel, the battery and maybe controller) with higher voltage parts that can go faster.  Update:  internet comments claim that most 36 V motors can operate with 48 V batteries.  I'm curious if anyone has done this already.  This could be a good project for the winter when I'm snowed in.  The cat's meow would be to also drive the rear wheels.