Step Motor feedback and Worm drive?

garyg

Hi
I've been thinking about building an electronic shifter for one of my bicycles.
Shimano has one called the Di2.
The Di2 uses step motor of some type to control position of the derail mechanism.
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My plan calls for the shifter to a short length of cable and position the derail mechanism pretty accurately.
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The plan I have would use a Propeller mini to set up the positions.
I like the idea of a step motor, but when powered down, I believe vibration and pounding would cause it to lose position.
regular RC servo motor might work, but I'm thinking again when motor is powered down and then powered back up
that the RC servo would twitch or not hold its position under no power.
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I've been looking for a small stepper motor with some type of position feedback, but info appears to be quite scarce.
All sites I end up on are industrial-very precision-very costly.
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A Di2 shifter costs about $1500 to $2000 US.
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There must be a reason the Di2 shifter types are not being used on common bikes, at a relatively low cost.
Some of the bicycle racing teams are using them.
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Any comments, suggestions, pointers etc. will be greatly appreciated.

Thanks

erco

Copy a downtube shifter design. Rotary drum on a ratchet with pawl does all the cable work, servo just has to advance or release ratchet one tooth.

Edit: Add a spring assist to reduce the force to downshift.

lardom

I like simple stuff. My first consideration would be a geared DC motor and a worm gear. As a position sensor my first thought is a light proof enclosure with an ir LED for every sprocket. I guess a rotary encoder would work too.
I would try to eliminate the need to return to "home" every time the circuit initialized. I would also want to eliminate the need to write the "last position" to memory.
In solving those two problems my first thought, as an example, would be to pulse the 5th led for the 5th gear 5 times.
I love bikes. I'd probably try to design one with a rocker switch to select gears. Please post your progress.

erco

Not sure about the latest gen, but the first gen Shimano electric shifter had an oscillating cam, driven by the small sprockets, obviously powered by the chain and ultimately the rider. There was negligible power consumed with no load. To shift, a solenoid was momentarily energized, which extended a pin which the cam would hit on its next oscillation to change the gear up or down. That's a clever way to minimize electric power consumed by the shifter, since the shifting power ultimately comes from the rider.

garyg

Hi
Thanks for your input on the shifter subject.
It just happens that I have an old bike hanging in my garage that has a downtube shifter.
I think I will really look into the downtube shifter ratchet design.
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I also like the idea of using a wormgear drive or possible a wormgear drive with a motor brake.
I'll keep looking for wormgear motors that are very small and at least relatively inexpensive.
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and according to an article about the Shimano Di2 shifter, it appears that they now use a small stepper motor
in their design.

erco

As you know, there is a lot of force/torque on the shifter, which might be hard to overcome and induce a lot of wear & tear. Here's an offbeat method that would be very easy to implement. On most downtube bikes, the shifter cable is unsheathed along the downtube between the shifter and bottom bracket cable guide. If you pull the cable sideways, it pulls the cable in with very little force. You could put some sort of servo or stepper motor there pretty easily and reduce the forces involved.

garyg

Hi
It's interesting that you mentioned the unsheathed cable.
On my Tandem - With which I have trouble getting the shifting to work correctly - I usually thwack the cable under the frame
to be sure it's not hanging up.
I really don't think thwack is a word, but that's what I do.

Tandems have quite a bit of trouble with adjusting things because the cable lengths are very much longer than on a single
person upright bike.

Possibly deflecting the cable at midpoint would be a thought.
Ideas are starting to come.
Last thought was to embed a nut into a piece of square tubing, sliding inside of another piece of square tubing, which would
be attached to the derail mechanism cable.
Use the bolt matching the nut, driven by a step motor or continuous servo to push and relax the cable.
It seems like, if I'm pulling the cable that I only need about 1" total movement.
This would be possible even using a 3mm screw or 2-56 screw.
..
I believe I need to use my sketch pad to start working on a napkin sketch.

Thanks
Garyg

garyg

I just realized that I do not know how to edit my last post.
I was going to add that If I use the square tube actuator I described in my last post, I could maybe
use irLEDs to give a general Idea of what gear has been selected.
..

Suggestions of how to edit a post would be greatly appreciated.

Thanks again

erco

Editing: Not real obvious, changes have been requested but no improvements yet. There's an invisible gear icon in the upper right of your text box. You have to know where it is, then hover over it, then click on it to get an EDIT icon to pop up, then click on that... kinda Rube Goldbergy IMHO.

Yup, we have a Santana tandem too. Those long cables are a pain, we stuck with 8 speed, less finicky. We have some big hills around here, and I like to go downhill fast. As every tandem captain knows, it's all over when your normally-brake free stoker learns how to apply brakes by grabbing and yanking on the cable like this.

We have a short ride video online from when I had just discovered those mini keychain DVRs. The wife was way pregnant with our twins and we were going way too fast in retrospect. One last crazy irresponsible ride before the joys & responsibilities of full-time parenting!

lardom

"thwack" is a word I remember from comic books.
I wish I could picture the devices you and erco are talking about. I have 'thought' about electronic gear shifting. I have also thought about a hub mounted brushless motor.

garyg

Hi
I sort of skipped my usual napkin sketch when starting on a project.
Went to True Value hardware store and a local model train store.
I purchased a few nesting square tubings.
Sizes available say that my 1st mechanical design part of this say that I'll be sliding brass tubing into
brass tubing. -- If things work out I may end up with a small linear actuator.
I'm using the parallax StepperMotor as my motor model.
They had a drawing that I used specs from.
I'm a long way from successfully building my small actuator, but need to try some things.
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Lardom- cable stretch is a pretty large problem on My tandem.
I have a Trek tandem which is more than likely a bit shorter in wheelbase than Erco's Santana.
If I can eliminate about 6ft or so of cable, I think my tandem would shift better on the rear shifter.
Anyway, the general idea behind this potential project was the Shimano Di2 electronic shifter.
Design of the Shimano Di2 shifter has been in development for quite a few years and is available.
The Di2 shifter package costs far more $$ than my entire Tandem Bike and accessories.

garyg

I made some progress on this shifter thought.
I started doing a drawing using the Parallax Step motor that is no longer available.
I'll attempt to attach a screen shot
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Now that I found out the Parallax stepper motor is not available, I started searching.

I found a company called SparkFun that might be able to supply a step motor, coupling and StepDriver.
I picked out a few part# and sent their tech dept a request if the three parts are all
compatible.
My 1st thought, although a bit premature, is to use a Prop-mini to control the stepper driver circuit.
I've used Prop-mini in a timer project and liked it's compact size, completeness of design and
was able to interface it with an LCD display.
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Things I build, usually become very large in size and I would like this project to be of a small scale.
I usually like building out of modeling plywood, but I'm thinking that I'll use 3D printing for needed small
frames and adapters.

As usual
any comments, suggestions, words of wisdom would be greatly appreciated.

Gary

erco

Gary: Have you tried teflon coated cable and a rollamajig? Makes shifting much improved.

Edit: Discontinued! http://backboneracing.blogspot.com/2007/09/avid-rollamajig-death-of-classic.html

lardom

My views are untested. I just want to contribute to the dialogue.
The stepper from Parallax will serve as proof of concept. It doesn't have much torque.
I built a digital camera slider in 08' that used a 12v unipolar stepper. That motor had the precision and torque I needed. I love the precision of steppers.
My thought is that shifting gears on a bicycle will likely cause you to miss steps over time. I think you'll need a feedback circuit especially on a tandem.

garyg

Hi
I'm looking at info about cable types and quality.
I did not know about Rollamajig.
If it were available, I would buy one.
It seems like what the Rollamajig does is pretty much exactly why I cannot quite get
consistent shifting.
The company that made Rollamajig was purchased by Sram and Sram quit building it.
That happens quite often.

However: While I'm looking into how to improve the shifting system,
I believe I'll continue with my step/screw thoughts.
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lardom
Since I'm also concerned about missing steps, I think I'm going to continue building
my simple as is possible linear positioner and see how it does.
I have a stepper motor template cutter I built in the 1990's.
It would run for hours cutting templates very accurately, but whenever I powered it up,
I needed to zero it.
There were limit switches that were zero position.
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On this linear positioner, because of how the bicycle shifting works, I'm not sure how to verify that
the step motor has not lost steps.
I'm almost tempted to add in a Potentiometer for reference.
I have a robot platform that uses Potentiometers for positioning of the steering.
The steering system on that robot is not near the precision needed with this little linear actuator.
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I'll post again when I hear back from SparkFun people.
The step motor I'm looking at is NEMA 17 size, 68oz.in and they say 3V 1.7A/Phase
Motor couplings have always caused me concern, even in a past life.

Thanks again
Garyg

erco

Used Rollamajigs on Ebay

Also, change to a heavy duty derailleur return spring

lardom

garyg,
"I have a robot platform that uses Potentiometers for positioning of the steering."
I'm interested in understanding the feedback circuits of servo's. Some servos can be expensive. Geared dc motors are much cheaper.
At some point I'd like to build an arm with maybe an elbow and a gripper. Once I understand servo circuits I could scale things up.
At the moment I just ordered a pair of nrf24L01 modules to study because I want to learn wireless control and I think standard rc radios are too limited.