20 Revolution Position Sensor?

[MikeMl]

(Also posted to AAC.)

I am building an Arduino-controlled remote L-C antenna tuning network with a motorized inductor and capacitor on a common shaft. I have a 99uH roller-inductor that takes 20 revolutions of a 0.25" shaft to adjust the inductance from ~1uH to 99uH. Total travel of the shaft: 20rev*360°/rev=7200°. The DC gear motor that will drive the shaft has no internal position sensor.

I need an absolute shaft position sensor that would tell me the present position of the roller tap in the inductor. Absolute means that I can read it on power-up to determine the position of the shaft. I need sufficient resolution to determine which revolution (1 of 20) and at least a few steps within that revolution. Quantizing the position with a 10bit AD would be sufficient.

A 20Turn sensor could couple directly to the end of the rotating shaft. I could also use 2:1 gears or cog belt to drive a 10Turn sensor.

Any ideas and USA sources of supply of the sensor and/or the drive train (gears/belt) .?

 

[jpanhalt]

Two items come to mind: a 20-turn pot and optical or mechanical encoder. The pot would give you absolute position; however, with a 20:1 gear reduction, I suspect passive movement may be nil. Thus, with an encoder, you could simply save the last position. You didn't mention price, but I got several optical encoders from this source that seem reasonably priced: **broken link removed**

John

 

[kubeek]

Or you could measure the inductor.

 

[Les Jones]

Hi Mike,
I think John's idea is the simplest using a 10 turn helipot with a 2:1 reduction. I have never seen a 20 turn helipot.

Les.

 

[jpanhalt]

les,

Or simply use an inexpensive planetary gear drive for models with a pot that has fewer turns. I had seen some "20-turn pots" on my last visit to a vendor, but on further checking, they were trim pots and probably not suitable because of durability.

John

Edit: spelling

 

[MikeMl]

Or you could measure the inductor.

Yes, but it complicates things because the tuner is being driven with a few Watts of RF at the operating frequency while being tuned. The Arduino will have the ability to measure the frequency of the excitation frequency (1.8 to 18MHz), and will slew the tuner to the (previously determined) approximate position as fast as possible (continuously reading the sensor), and then switch to a "search" mode to find the final angular position (jog the shaft) based on reflected power on the feedline.

I thought about switching the inductor into an oscillator whose frequency is a function of the inductance, but this would take time, and I would have to turn off the excitation in the middle of the slew phase of tuning...

 

[MikeMl]

Or simply use an inexpensive planetary gear drive for models with a pot that has fewer turns.

Are you suggesting using a 20:1 gear reduction so as to be able to use a single turn pot?.

I looked around, and nobody seems to make a 20 Turn pot except as a trimmer, which as you said, likely wouldn't take as many cycles as I will likely subject it to.

I'm off looking at gears and timing belts.

 

[jpanhalt]

Hi Mike,

Belts and gears are easy to envision, but a 20:1 ratio or higher might be a challenge with belts. Of course, a 10-turn pot only needs a 2:1 ratio. Setting up a gear train requires fairly accurate machining.

I suggested planetary drives as being compact and simple to use. All you need to do is attach the driving spur gear to your shaft and the output to a suitable pot. I suspect such a drive will be less than $20. HobbyKing (NB: it has US warehouses) would probably be the cheapest, and you may get a "free" motor in the deal. Here is what I am referring to after a very short search:

**broken link removed**
https://www.amazon.com/Tamiya-Planetary-Gear-Box-Set/dp/B00061HIDU

Wrong ratio, but cheap (doesn't appear to be planetary, but is still compact):
https://www.hobbyking.com/hobbyking...tor_15mm_6V_20000KV_w_75_1_Ratio_Gearbox.html

Regards, John

 

[Les Jones]

Another advantage of a helipot is that the linearity is normaly specified to be more accurate than a normal pot. A toothed belt could be used for the 2:1 reduction. The reduction would not need to be exactly 2:1 so it would be easier to find some gears to do the job. Even if you only used half the travel of the pot (4:1 reduction) it would probably still be accurate enough. A helipot I have in stock is marked with 1% linearity.

Les.

 

[jpanhalt]

While browsing eBay for something else, I found a 20-turn pot: **broken link removed**

Not cheap ($60 for lot of 3), but they are or were made. John

 

[alec_t]

If you can find a suitably sized ~30-tooth pinion gear then a worm drive would give a ~30:1 reduction for driving a single-turn pot through most of its resistance range (assuming its rotation is limited to ~240 degrees).

 

[jpanhalt]

Hello again, Mike.
Does the tuning slug turn? If not, you might couple it directly to a linear pot. If it does turn, you could simply use a nut and appropriately threaded screw for the coupling. Here's the type of pot I am thinking about:

Or something like this: https://www.mouser.com/ds/2/54/048-776406.pdf (500,000 cycles).

John

 

[rsfoto]

Hi MikeMi,

What about using a stepper motor with 20 steps per turn. The question here is also how hard is it to turn the pot. OK, a stepper motor can loose steps and therefor some limit switches would be required.

OTH you could also use a stepper Motor with a high reared output and send for each actuation several tens of steps which from my guts tell me would be less lost steps due to the fact of a high torque at the putput of the gear.

I have been using for my telescope focuser high quality stepper motors from www.micromo.com They are not cheap but very good quality and a huge amount of different gear boxes.

Here you can see one of my projects with a stepper motor with 10mm diameter and a 256 : 1 gear box. They even have smaller stepper motors in diameter ...



Below a datasheet of a 8mm diameter stepper motor on which you can even put encoders for exact positioning.

**broken link removed**

 

[MikeMl]

Thanks for all the suggestions.

I thought about a stepper, but I really want a system that knows where it is when it wakes up! An un-geared stepper would likely not have enough torque. I also want a faster slew rate than a stepper would likely support.

The way that the roller inductor is made, there is no way to couple a linear pot to it.

I found that 20T pot on Ebay before John linked it. But at that price, I will go with 10T pot and a 2:1 gear train. I already own a 10K 10Turn 0.5% linearity precision pot. The links to the Robot stores were very helpful.

 

[KeepItSimpleStupid]

Mechanical parts: https://www.sdp-si.com/ The website is strange to navigate.

 

[rsfoto]

Thanks for all the suggestions.

I thought about a stepper, but I really want a system that knows where it is when it wakes up! An un-geared stepper would likely not have enough torque. I also want a faster slew rate than a stepper would likely support.

The way that the roller inductor is made, there is no way to couple a linear pot to it.

I found that 20T pot on Ebay before John linked it. But at that price, I will go with 10T pot and a 2:1 gear train. I already own a 10K 10Turn 0.5% linearity precision pot. The links to the Robot stores were very helpful.

Hi Mike,

Could you please post an image af the part you want to drive ? Perhaps I have more ideas.

What is the speed you want ?

In how much time do you want to move the 20 turns ?

What is the pitch of the threaded rod ?

I could compare with my focusers and tell you how fast the desired distance would be made ...

thanks

 

[rsfoto]

Hi Mike

Is it a piece like this ?

https://www.rfparts.com/chokes-inductors/chokes-inductors-roller/755017a4530.html

 

[MikeMl]

Hi Mike

Is it a piece like this ?

https://www.rfparts.com/chokes-inductors/chokes-inductors-roller/755017a4530.html

Yep, except that it is 98uH end-to-end. Made by Gates, model 105-VB-3735. I'm building a remote automatic antenna tuner for the 160m band.