Commuter Booster - <1kg Friction Drive

What you described is a power limiting. Not current limiting.

If you just enforce a current limit, and ignore the speed, you would be fine from a slip perspective.

Power Limitting
1000W / 5 kph / 200 "torques" / 50 Amps
1000W / 10 kph / 100 "torques" / 25 amps

Current Limitting
1000W / 5 kph / 200 "torques" / 50 Amps
2000W / 10 kph / 200 "torques" / 50 amps

OR

500W / 5 kph / 100 "torques" / 25 Amps
1000W / 10 kph / 100 "torques" / 25 amps

So I would just set the maximum current below the slip/wear threshold. This also determines the maximum power the system consumes.

If you also want to control the maximum power the system puts out, then you might need mapping to speeds. As inefficiencies at low speeds will reduce the power out, and efficiencies at higher speeds will actually give you more power out. So if you were really anal about keeping to the "250w"/"750w" output power limits you may have to enforce a lower current limit at higher speeds.

Or put another way, you may actually be able to have higher current limits at low speed and still stay with in the output power limits. ( As long as you don't cook the motor, or shred the tire)

The only value I see in setting lower current limits at low speeds is to reduce the waste heat, not to manage tire slippage/wear.
 
Havent quite got my head around what you are saying but I think I understand your point in relation to a set current limit to manage tire slip.

The current limit mapped against speed was actually designed so that at low speed, you couldn't increase power to a point that wouldn't induce an out of sync condition. Once above this critical speed, the current limit is basically flat.

I have got my software partner working on the motor rpm pickup at the moment. Not sure if we can add it to the existing chip as this extra routine is quite resource hungry. Will see what he comes up with.
 
Yeah I think we have been talking at cross purposes a bit :lol:

Ah, so you reduce current to control sync issues. :idea: That makes more sense.

I wouldn't bother doing motor rpm pickup, if you don't already have it. Just get this thing on the market. Or at least into my hands. :D

Keep up the good work.
 
No need for 2000w, that is just what the motor can do out if the box. Since the RC controllers don't limit power.

All the electronics talk is around is actually trying to reduce the power to help improve range, reduce waste heat etc.

This bike is actually more powerful than my over volted hub motor at the moment. Really crazy since the bike is half the weight, with similar range.
 
Adrian,

I'm very interested in friction drives so I hope you won't mind my questions.

What power consumption would you get at 25kph on a flat road with no wind?

--
Bill
 
Same as any other ebike as far as I can tell. I get very similar average watt-hours/km as my hub motor when travelling at the same average speeds, taking the same route.
 
adrian_sm said:
Same as any other ebike as far as I can tell. I get very similar average watt-hours/km as my hub motor when travelling at the same average speeds, taking the same route.
Click to expand...

So, 125W to 150W. Would that be about right?

--
Bill
 
Not sure. If I am going that slow I am pedalling. :D

I can only really compare at my usual speeds of about 40kph on the flat, where I average about 12wh/km for my 15km commute that includes a few decent hills. Very similar to the numbers I saw for my hub motor when I had it set-up to cruse at similar speeds. Even when I had the friction drive wasting a whole lot of power burning up my tire, I was still seeing only about 14wh/km.

Hope that helps.
 
Tiverion said:
adrian_sm said:
Same as any other ebike as far as I can tell. I get very similar average watt-hours/km as my hub motor when travelling at the same average speeds, taking the same route.
Click to expand...

So, 125W to 150W. Would that be about right?

--
Bill
Click to expand...

No, more like 200 to 250W depending on the bike. My 20" folder is closer to 250W @25kph. Keep in mind friction drives take with one hand and give with the other. With the back wheel off the ground and full throttle, my drive draws 90W.

Out on the road on my folder, I have no problems getting 20kms out of 2 6S 5ah Lipo's. This is based on using a total of 8ah only (80% rule). This is around 9wh/km. I am not doing 40kph too often though on the folder. mostly between 25 and 35kph with a bit of lazy peddling thrown in.
 
Thanks for the stats John. :D

BTW I rode in to work today on my hubmotor bike at 48v for a change (since I killed the LiPo booster pack :roll: ), this dropped the average speed down to very similar to my friction drive, but only peaks at about 800w. Allowing me to confirm the power consumption and ...... I used exactly the same watt-hours as I did the other day on the friction drive, for similar average speed. :D Even though the friction drive was peaking at well over 2000w, and wasting a lot on shreading my tire, sice I didn't have it set up right. :oops:

I also measured the hub motor draws about 80w no-load ~ 50kph, which is similar to Kepler's no load friction drive measurements.

The key difference is that the friction drives will have more losses at higher loads, due to the increased rolling resistance as the motor motor engages more with the tire. But this is offset by total disengagement of the drive when not in use, when for the hub motor you are pedaling against the back EMF (-80W at 50kph).

Due to the size and weight advantages of the friction drive, you can usually get away with a lighter bike, so you have added efficiency benefits. I would happily bolt the friction drive on a light weigh road bike, but for the heavy hub motors I ended up going with heavier MTB frames, suspension, and fatter tires. These all add up to more wasted energy.

This might explain why we are still seeing very similar efficiencies for the friction drive, even thought they have a higher transmission loss than a hub motor.

- Adrian
 
Thanks very much for your replies, guys. My interest is really in low power, sub 30kph assist systems. I need to maintain my fitness level so I neither need nor want a "No pedaling necessary" setup. I was hoping your experiences would extrapolate to the low end systems but that does not seem possible.

I've set some rigid criteria for a system that will suit my requirements, circumstances, and budget. I'm spending a fair amount of time researching and making notes at the moment. The plan is to come up with a configuration that will be a workable basic system out of the box,and add refinements progressively in a way that will not put the bike off the road for more than an hour at a time. The bike is my primary transport and the bus service here is crap.

Current focus is on a 40mm roller, timing belt driven by a pair of 24V 100W motors from Oatley. Sort of looks like Mickey Mouse ears & nose. The drive assembly slides on a rail, mounted on the seat tube below the seat clamp and extending horizontally over the rear wheel. May be able to incorporate a sprag clutch in the roller at a later time. Lots more things in the notes to be explored if the initial setup works.

Thanks again for your input, guys. I think it's about time I spent some money and started building.

--
Bill
 
For what you have described, you could probably just use my setup on a lower voltage. You could even get away with the smaller 5065 motor version (like in my avatar on the right), on a 4s LiPo pack. This would reduce the weight, speed, power, and waste heat to levels it would be very robust. Will fit on smaller frames too.

But personally I find it much nicer if you can keep the low power assist extending above the 30kph level. Otherwise the drive only ever would help on the hills. I like it to help me maintain a higher cruse speed, to get me where I am going faster. I still pedal as hard as a want from a fitness perspective, but don't have to bust a lung on the hills. Personally I have found 40kph is a nice balance.

Have you ridden many ebikes? You can still get a good work out on a 5-700w system. But once you started getting over 1000w, I find I help out a lot less and it is no longer about the exercise aspect, it is more about fun.
 
Update: Modified Watt Meter

I have modified a watt meter with a remote shunt, on/off switch, current limit pot, hall throttle input, and RC throttle output. I am running essentially Jonas's code, with a few minor timing tweaks to get the correct throttle signal out. But whenever I use it to control the throttle signal, it destroys the current reading.

The hardware mods I made to the watt meter are as per Jonas's schematic, but I also had to remove a series of resistors that appeared to tie the previously unused IOs together and ground from memory. I am now using those IOs for the current limit pot, and throttle IOs. I am using the ESCs 5v out to run the hall throttle, if that is relevant.

Here is a look at what the shunt voltage measured at the PCB is for various throttle signals.
From top to bottom is 0 through to full throttle
Left images are for using a servo tester to control the RC ESC.
Right images are for the watt meter controlling the throttle.


Would love to hear peoples opinion on what might be the cause of the strange shunt signal, and things to try to fix it.
 
Tiverion said:
Thanks very much for your replies, guys. My interest is really in low power, sub 30kph assist systems. I need to maintain my fitness level so I neither need nor want a "No pedaling necessary" setup. I was hoping your experiences would extrapolate to the low end systems but that does not seem possible.

I've set some rigid criteria for a system that will suit my requirements, circumstances, and budget. I'm spending a fair amount of time researching and making notes at the moment. The plan is to come up with a configuration that will be a workable basic system out of the box,and add refinements progressively in a way that will not put the bike off the road for more than an hour at a time. The bike is my primary transport and the bus service here is crap.

Current focus is on a 40mm roller, timing belt driven by a pair of 24V 100W motors from Oatley. Sort of looks like Mickey Mouse ears & nose. The drive assembly slides on a rail, mounted on the seat tube below the seat clamp and extending horizontally over the rear wheel. May be able to incorporate a sprag clutch in the roller at a later time. Lots more things in the notes to be explored if the initial setup works.

Thanks again for your input, guys. I think it's about time I spent some money and started building.

--
Bill
Click to expand...

I am always keen to see another friction drive design so good luck with your build and make sure you do a build log :) There are quite a few excellent friction drive threads well worth reading through. Twin motors driving a roller through a timing belt and sprag clutch sounds interesting. Those motors are super cheap especially considering they include controller and throttle http://secure.oatleyelectronics.com//product_info.php?cPath=53&products_id=190

Keep in mind, the main reason why we use motors that can handle 2000W is not so much for speed and power, but for reliability. I will be very interested to see how these motors stand up even in a light assist application. Good luck with your build :)
 
Kepler said:
I will ask my software engineering partner to have a quick look at the data. He is a wiz at interpreting this sort of info :)
Click to expand...

Thanks mate. But don't distract him from more important work. We all want to see your interface out in the wild. :D
 
No, definitely want keep him focused. :)
I got a new software revision yesterday to test. This revision has a type of cruise control now added. This cruise is not designed to be a full on "set speed and forget" setup but more a speed trim function.

The previous software matches motor power input with the given speed and was tuned to a power level that would that maintain speed on a slight encline. Although this works quite well, the problem with this was that if activated on flat ground, you would endup speeding up a bit or slowing down on a steeper gradient.

This new revision, matches motor power with speed in the same way but then checks if you are slowing down or in increasing speed. It then resets the power input to suit. However, this is a one shot speed re set and as such its not constantly chasing that speed like a conventional cruise control does. Every time the button is pushed the interface re matches the speed in the same way.

Keep in mind, current limiting is still the main over riding factor so the interface will can only trim power input to this limit.

Cant wait to get this out there for you guys to try. :)
 
Kepler said:
Cant wait to get this out there for you guys to try. :)
Click to expand...

Well today is your lucky day then. I was thinking of going to Jells Park with the kids for a ride......... I could quite easily drop by and grab one if you have it spare...... ?

Please.
 
Kepler said:
By the way, testing the 50 series motor to see how it reacts with the interface today. See if current limiting will keep it alive.
Click to expand...

Awesome. This is what I will be really interested in. The 50 series motor, with current limiting, and throttle interface that works on drop bars, would be a stellar combination.
 
I think you are right Adrian.

I had given up on the 50 series motor but thought I better re visit it since the interface was close to release. Glad I did. I forgot how nice it was to run a smaller diameter motors with a bit more RPM. Set it up with the low power setting on the interface. This is the "200W" config. Reality is that it controls between 200 and 300W with the ocasional peak to 500W just like any other "so called 200W system"

Motor performed beautifully. Much smoother then the larger motor. Rode the folder around my area which is quite hilly (Adrian, you know my riding area) Sucked up 180W/hrs and returned to base to take some temps. Motor was warm but not hot. measured 50 deg C with the temp gun. More then happy with the results. Will do some testing on the medium power setting which is basically 850W US spec. I think this will be the limit of this little motor but it will be interesting to see how it handles it.

Motor looks tiny on the drive now.
 
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