Sempu Torque Sensor Help

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Jun 27, 2018
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10
I have installed a Sempu torque sensor bottom bracket with my new Cycle Analyst 3, Phaserunner and Crystalyte H3525 UFO Rear Motor. I’ve tried several different settings and just not having any luck getting the Sempu to work properly. Could sure use the wisdom of others here to help me figure this out. I followed the instructions in both of the videos at Grin, cannot see that I did anything wrong.

No matter what I do, it will not kick in while pedaling forward, and I get no readings on the CA screen for Hwt or RPM when I do so. But, and this is the interesting part, if I’m just sitting still and pedal backwards, the hub motor wants to kick in and move me forward. Also, the backwards pedal does generate readings on the CA, positive numbers for both Hwt and RPM. I have changed the polarity in the CA, it does this no matter which way I set the polarity. Here are my current settings:

PAS DEVICE SETTINGS
PAS Poles: 24 Pulses/Rev
PAS Signal Type: 2-Wire
PAS Direction Polarity: 5V Forward
Devise Sensor Type: Sempu BB
Zero Torque Offset: 1.50 V

PAS CONFIG SETTINGS
PAS Assist Mode: Torque
Assist Start Level: 5 HW (have even tried 0, 1 and 26 without success)
Assist Scale Factor: 4.00 W/HW
Torque Assist Averaging: 12 Poles
Start Threshold: 0.45 sec/pole
Stop Threshold: 0.29 sec/pole

This is installed on a recumbent trike. I installed the crank arm 90 degrees to the right on the bottom bracket per the video instructions. I should also add that I also have a GRIN Basic PAS and installed it to test all the wiring and equipment, it worked with no problem. I've rechecked all the Sempu wiring several times.

Overall I’m thrilled with everything about this new setup, just needing to figure out the Sempu.

Thanks for any advice you can provide.
 
You don't say which version of Sempu you're using. Maybe post a picture as naming differs quite a bit as well.
I got a T4 and it behaves strangely as well. But not as you write.
Wiring is straight forward as well, and not much to do wrong in my eyes.
I also can't imaging how you could produce HW while pedaling in reverse, as there should be a freewheel, so there is nothing you can pedal "against"?!?
I checked my bike, and in the "PAS" setup screen i can see P & D "arrows" moving when i pedal "forward" & "reverse". BUT my "PAS direction polarity" is 5V REVERSE and not forward. If i change that to forward, two things will happen a) it will not work when pedaling forward, but somehow when reverse, and b) it will change torque sensor type from "SEMPU" to "CUSTOM".
Hope that helps.
 
It is a Sempu T2.3, the current one they are selling at Grin (sure hope it is okay to link to their image). Not sure if it matters or if this helps, but I do have a Potentiometer for throttle power adjustment and a two button Digital Aux Input to adjust the PAS from 0-4.0 for the Sempu. Both of these from GRIN, so they connect easily, I have both of those at maximum settings.

sempu_68_-_115.jpg
 
No. It's a T4.3 type. I contacted Sempu and they gave tech support, and will send a replacement for free.
This is what they said:

"The bad reason is that the internal electronic components are not in good contact with each other.
In the future, we will strengthen the solder joint detection of electronic components. Thank you very much for your feedback.
"

So let's hope that the new hardware will solve the problem.
 
You should test by seeing if the CAv3 can show pedal RPMs and pedal torque (in newton meters). There is a calibration step to zero out the pedal torque. This will let you see if it is a programming issue with the torque assist in the CAv3, or just a basic sensor issue.

The installation of these is also unlike any other bottom bracket. It's really important to watch the Grin video and/or follow the written directions. It is installed backwards from a standard cartridge bottom bracket and requires the plastic lock to be threaded into place.

Sempu BBs don't seem to be the most reliable, I've had good luck with mine but some friends and local shops have had high failure rates. When they do work they work very well, but I miss the reliablity of the Thun BBs that came prior.
 
I'm aware of the calibration procedure of the CA. And this is why i assume that the BB is broken. Sometimes it shows the correct voltage and zero Nm at standstill. Sometimes it's way off. And the Sempu doesn't need calibration - it's a fixed and calibrated voltage for 0Nm. If you change that, the CA will switch to CUSTOM BB.
The issue i see is, that it sometimes works fine, sometimes give MASSIVELY wrong Nm, and shows 1000W+ while fake pedaling.

Btw: I received a free replacement from Sempu yesterday! That's great service at least. No discussion, no long questions asked! Very much appreciated.
 
It's okay for the CAv3 to flip to "custom". The Sempu settings just get you close, but every device will have slightly different components and require zeroing out.

I hope that your new BB solves the problems and that Sempu reliability goes up!
 
Installed the new (free replacement by Sempu!!) torque sensor, and it works fine so far.
As the old one was only installed for a short period i was really surprised to see rust inside it. I never ride in rain, and only rinsed the bike once, and dried it instantely after.

It seems the spring ring got rusty, and "infected" the bearing. I could remove the rust on the bearing by brushing it away. I now filled the cap with grease to maybe keep the rust away from the new one.


Capture.JPG
 
Anyone got any ideas on how to replace the bearings on a sempu torque crank? I got mine out today in the carpark of a local bike shop and took it inside and we were all a little stumped. The bearings aren't visibly rusty but the chain side ones move like they're full of sand. It looks like an easy repair if I could just work put how to get them off...

I've looked a fair bit across the net and found 0 posts on replacing these, in fact little about the sensor overall really. Guess most people are treating their ebikes like modern cars and just taking them straight into the shop.
 
moonglow said:
Anyone got any ideas on how to replace the bearings on a sempu torque crank?
If there arent' any circlips (probably not), then you'd probably need a bearing puller with "arms" that can be wedged in the thin gap between the bearing and the body, and a tip that can push on a crank bolt's center in the end of the crank (or inside the crank bolt hole itself)...or you could try freezing the unit, then warming up the bearing's inner race while pulling it. Or just use something like CO2 to chill just the axle at that end, while pulling the bearing.

Sometimes a bearing ends up having to be cut off; you have to be careful with this on plastic-housinged devices. Generally I would make a thin metal barrier to go between the bearing and the rest of the device (old soda can, etc), then slowly dremel (you may not be able to use discs, but might have to use machining tips) thru the outer race of the bearing in two places (or one and pry it open but that may be difficult), making sure you never let anything get hot enough to damage anything the bearing touches, keeping in mind that hte shaft itself is going to conduct some of the heat directly into the rest of the unit (you can cool the shaft from either end with "ice rags" if necessary). Once the outer race is off, the inner race can be cut thru; this should only take one cut, as once it is cut it won't be an interference fit with the shaft anymore.

Keep in mind that if you slip or heat stuff up, you may make the rest of the unit unsalvageable. ;)


The bearings aren't visibly rusty but the chain side ones move like they're full of sand.
If one of the balls failed, it could have a chipped surface that has left debris in the path of other balls that then were chipped and left more debris...eventually you don't have much in the way of bearings anymore, but it feels like you describe. Sometimes you can actually see the damage if you remove the plastic seal-ring on a bearing.
 
Thanks Amberwolf, damm I miss the days when I worked in an engineering workshop with all sorts of crazy pullers and gizmos. no real room to grip these though so it would need something to either force itself behind or as you say cut it off. I'll see what the local retailor says, but think its a whole new crank which doesnt seem like something I want to do every 2 years. Might order another from aliexpress and then play with this old one and see what I can do once I've a backup handy.

Any thoughts on clamping the bearing in a vice and taking a rubber mallet to the crank shaft?
 
moonglow said:
I've looked a fair bit across the net and found 0 posts on replacing these, in fact little about the sensor overall really.

I had all generations of the Sempu sensors on my workbench and documented them quite well:
First gereration
Second generation
Third generation

Normally there should be no problem to pull off the bearings, they aren't pressed on, if the sensor is new you can pull them off by hand. If there is rust, you should use something like WD40 to loosen them. But be careful not to wet the strain gauge with the oil.
Good Luck!

regards
stancecoke

index.php
 
Fab thanks Stancecoke. Any top of mind thoughts on the differences between gen 3 and 4?
 
moonglow said:
the differences between gen 3 and 4?

I don't know a 4th generation.
first generation: Torque measurement by bending of the shaft, speed by magnet ring
second generation: Torque measurement by twisting the sleeve between shaft and chainwheel, speed by magnet ring
third generation: Torque measurement by twisting the sleeve between shaft and chainwheel, speed by light barrier (link corrected in the post above ;))

The third generation is called T4, I don't know why :)

regards
stancecoke
 
lol, thanks Stancecoke, it was the T4 designation that was catching me out.

Thanks a lot.

Dean
 
Where can these be purchased now that Grin isn't carrying them? Do other suppliers includes a CAv3 compatible cable?

I understand why Grin stopped carrying them, but prefer the Sempu to the other options that I've seen and used. I have a 6 year old Thun with lots of miles that will need replacement soon and would like to get a Sempu as it's replacement.
 
Alex W said:
Where can these be purchased now that Grin isn't carrying them?

Why do you want a Sempu?! There where many quality issues with them, I think this is the reason, why grin doesn't offer them any more.
My recommendation is the E-Rider sensor, you can get it from aliexpress or from Grin...

What kind of motor controller are you using? It is possible to use the torquesensor directly with a Lishui Controller, no need for a CA...

regards
stancecoke
 
Why do you want a Sempu?! There where many quality issues with them, I think this is the reason, why grin doesn't offer them any more.
My recommendation is the E-Rider sensor, you can get it from aliexpress or from Grin...

The E-Rider sensor doesn't work with decent cranks. I don't want wide and low quality cranks on my bike and the Sempu lets me use low q-factor cold forged Ritchey Logic cranks. I've had good luck with the Sempu, I do know friends who have had issues but that could almost always be connected back to installation issues.

It looks like Q-factor with the E-Rider will be in the 185-195mm range.

Thun BB works great too, but only measured torque on one side and the one that I have is showing some pretty good bearing wear. I know that Grin was trying another similar design but they don't list it anymore either.

Both of my bikes have CAv3 and Baserunner controllers. I'm not looking to change that, I love the silent drive and torque that I get with the Baserunner on a double reduction geared hub (one bike has a Bafang G311, the other is Bafang G370).
 
Alex W said:
The E-Rider sensor doesn't work with decent cranks.

Yep, I'm looking at the same thing. I have the E-Rider and it does work well, but the available cranks aren't as good as my original and the Sempu T2 let's me use my original crankset. If I have to replace the bottom bracket every 2 years, I can live with that. If the Sempu is going to require replacement every 2 months, that is a different story.
 
Hopefully Sempu will fix these quality assurance/quality control (QA/QC) issues with soldering and corrosion-resistance. It may be a good product at the basic level. The tiny "strain gauge" is a well-understood device, and I'm glad to see there are more options today.

I'm a bit shocked the sensor is not potted. But, you can coat it with clear nail polish from the dollar-store before installation.

"Liquid Nails" is a construction adhesive. It is a polyester base "rubbery" glue, which would be slightly more removable in the future, if that's a possibility...

I've recently used clear "Gorilla Glue" as a potting compound, and I like the result. Harder than clear silicone, but still "just" rubbery enough that is it very unlikely to develop cracks, which has happened to me when using clear epoxy as a sealant.

Glue1.jpg

As far as rust on the inner parts, again I'm a bit shocked that stainless steel wasn't used. That being said, you could pack the part in grease when assembling onto the bike. You can get a large 13-oz tube of "automobile high temp synthetic disc brake bearing grease" for under $20. This would not be for the bearings, it's for an assembly lube to prevent rust on the tightening rings. The synthetic grease does not get thin when it's hot, so it doesn't get "runny" and flow away over time.
 
ramstadt said:
Alex W said:
The E-Rider sensor doesn't work with decent cranks.

Yep, I'm looking at the same thing. I have the E-Rider and it does work well, but the available cranks aren't as good as my original and the Sempu T2 let's me use my original crankset. If I have to replace the bottom bracket every 2 years, I can live with that. If the Sempu is going to require replacement every 2 months, that is a different story.

This is an old thread, but the data point may be useful. I just replaced a Sempu after over 2 years of daily use. The failure was nothing mechanical, the data cable was pulled a little hard through the frame and rubbed bare. I better protected it with the new one and expect to get more than 2 years. I live in Seattle and ride a lot in the rain.
 
Alex W said:
This is an old thread, but the data point may be useful. I just replaced a Sempu after over 2 years of daily use. The failure was nothing mechanical, the data cable was pulled a little hard through the frame and rubbed bare. I better protected it with the new one and expect to get more than 2 years. I live in Seattle and ride a lot in the rain.

Excellent. I have a Sempu in my trike. It isn't a daily rider, but I am getting use and I've been pretty happy so far. I really like being able to use my original crank set.
 
spinningmagnets said:
"Liquid Nails" is a construction adhesive. It is a polyester base "rubbery" glue, which would be slightly more removable in the future, if that's a possibility...
Note that LN contracts as it dries, so it is possible for it to either crack or pull away from the edges of whatever it is contacting if it doesn't adhere fully to it.

I used it originally for sealing up teh small gaps left after building SB Cruiser's cargo seatbox out of narrow planks...and even after letting the first LN application dry fully for weeks, and then adding a second layer, it still cracked. I ended up carving all the LN out and using RTV silicone (the acetic-acid based GE Silicone I version), and that hasn't failed yet.
 
stancecoke said:
Normally there should be no problem to pull off the bearings, they aren't pressed on, if the sensor is new you can pull them off by hand. If there is rust, you should use something like WD40 to loosen them.

My 3rd gen Sempu (T4) for 100mm BB is acting up and after removing it I discovered it was moist and filled with dirt. No rust thought. After attempting to pull, hammer and apply heat and WD40 to the bearings, they simply refuse to move a single millimeter. Using the bike for some 1500km has locked the bearing up pretty well, although I bet they are press fit.

The tubing closest to the bearings is plastic, so my next idea was to cut two small holes to it in order to fit a bearing puller behind both of them. Giving some more thought I abandoned this idea because this would likely destroy the bearings.

Now, I'll simply remove a fair share of the plastic tube (which houses some electronics), so I can reveal the PAS photodiode and clean it up. After servicing, I'll glue the photodiode housing back with epoxy. Not an optimal solution, but as my Sempu is practically a paper weight, I'm not really losing anything else than my precious time...
 
Hm, strange, maybe Sempu changed the design, or the tolerances are bad.
For my sensor and the one of another user it was no problem, to pull the bearings off...
https://www.pedelecforum.de/forum/index.php?threads/integration-des-sempu-sensorlagers-in-den-fc.51648/post-929461

index.php


regards
stancecoke
 
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