Electrical power measurements on my Agattu

[kitchenman]

Glad it didn't rain! ... Great work Mike - keep going! ...

 

[10mph]

How I got the breadboard working

4 posts ago I was stumped,

I got all the connections soldered, but the device is not working - the output should be sitting at half way between the 5v and 0V for no current flowing through the heavy current connectors - but it seems to be sitting at 5V, furthermore the Hall device is drawing no current from the 5V supply - it should be drawing about 10 mA.

Perhaps I have somehow wrecked the device. I have now ordered a tiny adapter board which will properly mount the SOIC style package and convert to an 8 Pin DIL which will match the holes on the strip board. I may have to order some more Hall sensors at £4.50 each.

It turned out that the output (pin 7) was a dry joint - continuity when I pressed the probe down on the pin, but when I released the pressure the joint opened.

The next thing that happened during bench testing was the filter capacitor which I installed on the bent up leg (pin 6) got knocked and the leg broke off at the package. So, until I get a new Hall chip and install it on a proper tiny header board which has now arrived, I have had to operate without this capacitor. A pity because I could see was reducing noise from the Hall sensor.

My next blunder happened while installing the Hall sensor on the bike. As I plugged the battery lead into the heavy connectors, one connector pulled the copper strips off the PCB. I scrapped these connectors, soldered to the board and connected them with some flexible copper desolder braid to prevent strain on the joint.

I have calibrated the Hall by comparing with my digital multimeter at 1 Amp . The Hall is very linear. When I tested it at 6Amps it was within 2%.

Everything fixed I have been out on the bike measuring the current. But the largest current I have seen with averaging by the program I have written for the Picaxe (Averaging of 100 ADC samples taken in 2 seconds) is around 8 Amps. 8A*25.5V = 204 Watts from the battery.

That is what I got in the original post of this thread.

So back to the experimental bench to see if I can work out why this is different from the larger currents I measured using my digital multimeter on the millivolt range measuring the drop across a current shunt and reported earlier in this thread, here: Flecc was right ~400watts. One of the measurements is wrong and I must find out which it is.

 

[tangent]

While riding back home into slight wind on the flat or slight downhill gradient (<1%), I watched the current consumption (averaged from 100 samples taken over 2 seconds and displayed on the LCD on my handlebars). I noticed how the current cut off as my speed varied from around 15 mph to 18 mph.

Max assist - top gear - gear 3rd gear on my 3 speed Agattu.
15 mph max current of 8 A available
17mph about 2 to 3 A available
17.5 mph some current available
17.9 mph no current available

I put the cut off at just above 17.5 mph which is 28.1 kph. This is 12% above the Euro limit of 25kph.

This agrees with my earlier observations where I saw the ramp down start somewhere above 14 mph. It is also in line with the typical type of ramp down shown in the latest Kalkhoff manual for an 8 speed model

All really interesting! It looks as though the 2011 Agattu is straining the limits of legality. I think you are allowed 25kph + 10% tolerance. However, the power assist at these speeds is fairly negligible so it would be silly to declare the bike illegal. The 35T chainwheel seems well matched for a reasonably comfortable cadence as well and I can see why it was introduced now. I calculate a cadence cut off of about 71 rpm (assuming 28 kph, 2190 circumference tyre, 35T/16T and 3rd gear ratio of 1.364).

 

[tangent]

I just noticed in the graph from the Agattu manual - Cadence cutoff 71!

Anyone know details of how the other crank drive systems work, e.g. Bosch. Do they all have similar power graphs to the Panasonic, based on rider torque with a power down slope between 2 cadences? Or do some have a power down phase linked to actual bike speed, e.g. by measuring wheel rotation rate?