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[Reply #15 on:]

OK, I see the problem. What if you introduced a threaded shaft with a moving nut between the stepper motor and the capacitor to trip the home switch? The motor homes to said switch on start up giving an initial position just as a CNC machine would do.

[Reply #16 on:]

Another alternative would be to use a double shafted motor and connect a 10 turn pot to one shaft. No gears needed.

[Reply #17 on:]

So how is a 10 turn pot going to help when the total travel of this motor is 70 turns???

[Reply #18 on:]

Presumably, trying to move the capacitor beyond the limits of its travel will only damage it if you apply excessive force. Can you limit the stepper motor drive current so that the force is sufficient to move the capacitor but insufficient to damage it? If so, you can reset the system to a known state by driving it in one direction long enough so that you know it has reached the end of its travel (this is what some cheap inkjet printers to to reset the carriage to a known position). After that, you can keep track of the position by writing it to EEPROM, perhaps when you sense that power is about to go down (e.g. by monitoring the voltage on a capacitor on the input side of the voltage regulator).

[Reply #19 on:]

If so, you can reset the system to a known state by driving it in one direction long enough so that you know it has reached the end of its travel

For 70 turns of the motor! That is a long time for the system to take mechanical abuse. You would be better off trying to monitor the current and seeing when it reaches its stalled value.

by writing it to EEPROM, perhaps when you sense that power is about to go down

Sensing the power is about to go is an entire project in itself. I have done many of these when designing set top boxes and it is not easy unless the power supply is integrated into the system. I actually hold a patent (well it is my companies IP as I was working for them at the time) for a novel way of detecting a power outage on a set top box in order to write things to flash.

[Reply #20 on:]

For 70 turns of the motor! That is a long time for the system to take mechanical abuse. You would be better off trying to monitor the current and seeing when it reaches its stalled value.

I've done that for a DC motor, but never for a stepper motor. How practical is it? Does the current draw of a stepper motor vary significantly with rotation speed and/or load? I would have though that the average current wouldn't vary much, although the shape of the current waveform in each winding should vary with rotation speed. I take your point that 70 revs is a long time - maybe an optical sensor producing one or more pulses per revolution would be an alternative way of detecting that the end stop has been reached.

Sensing the power is about to go is an entire project in itself. I have done many of these when designing set top boxes and it is not easy unless the power supply is integrated into the system.

What I had in mind was monitoring the voltage at the input to the voltage regulator through a voltage divider - which means that at least the regulator is built in to the system, although the power supply could be a wall wart.

[Reply #21 on:]

What I had in mind was monitoring the voltage at the input to the voltage regulator through a voltage divider - which means that at least the regulator is built in to the system, although the power supply could be a wall wart.

Yes you can do that and it is the basis of many designs.  However, the tricky part is that the hold up times are determined by the reservoir capacitors, these tend to be bigger on the unregulated side than the regulated side. This means that when the mains is disconnected it holds up the voltage for a longer time, then when it sags it drops very quickly. What you need to do is to arrange a voltage comparator on the input side of the regulator that triggers an interrupt on the processor that then saves the parameters. In general with that sort of arrangement you had in the order of 50mS to do stuff. My patent involved detecting the actual mains and that gave a whopping 180mS warning. 

[Reply #22 on:]

Here is my idea
Take some resistor thread. In one ende connect a spring . As the motor turns the thread will be wound/unwound from the motor.
Now you can measure the resistans between the spring and the sensor placed halfway down the resitor thread.

I can see it is unpractical: if the diameter of the motorshaft is 10mm the length of the thread will be almost 2,5m with 80 turns. But that can be solved with more wheels, where the resistor thread could be zigzagging.

[Reply #23 on:]

So how is a 10 turn pot going to help when the total travel of this motor is 70 turns???

Ooops! I missed that. 

[Reply #24 on:]

Too much "hanger flying". The OP needs to clearly restate the goals of the project and clearly describe the mechanical requirements of his gizmo.

[Reply #25 on:]

Too much "hanger flying". The OP needs to clearly restate the goals of the project and clearly describe the mechanical requirements of his gizmo.

Sorry guys for being late to the discussion. I was thinking the same thing, clearly stating requirements just as for any project.

Problem description: Capacitor shaft turns 70 times between lowest and highest capacitance. It is coupled to a copper pipe loop for use as a transmitting antenna. Voltages/current developed are as high as 4kV and 32A and antenna is extremely sensitive to ohmic losses. Cap shaft has a hard stop at one end and pushing beyond the limit at the other end may knock the shaft lose from internal plate couplings. Also, the capacitor is expensive - USD 200-300 - so tolerance to cap damage is very low. Capacitor range is from 7.5 to 350 pF - roughly 4.9pF per rotation. The antenna is sensitive to 1pF changes in capacitance.

Requirements:
1. Remotely rotate the capacitor shaft.
2. Stop the capacitor from hitting ends on either side to prevent capacitor damage. If #3 is accurate/reliable enough, we may rely on the operator (yours truly) to not turn the shaft beyond predetermined turn numbers.
3. Remote visual tracking of shaft absolute position - both - the turn number and angle (0-360) to allow the operator to figure out which direction to turn the shaft in for matching to a certain frequency.
4. Turn the shaft to allow for 1pF capacitance changes, that is, assuming ~5pF / rotation, comes to a minimum of 72 degree stepping. For safety, lets say 60 degrees.
5. Weather-proof the components since the antenna is to be placed outdoors.
6. No computer (as in a laptop/desktop) control since the antenna is expected to be portable.
7. Cap shaft control mechanism should not connect to the antenna/capacitor electrically in anyway to avoid changing antenna characteristics.

Did I miss anything? Hope not. I suggestion I got was connect identical motors to the same driver. Couple the cap shaft to one via appropriate gear to allow for 1pF stepping and couple the other motor with the exact same gear to a mechanical counter. Identify the lowest and highest counter readings for cap shaft's 70 turns. Is it practical?

Thanks! Really appreciate all the responses to the thread.

[Reply #26 on:]

How much force is required to turn the shaft (can you turn it with two fingers)? Might be possible to use a servo to turn the shaft so you would have simple position control. Couple of ways using gearing and such that might work.

http://www.servocity.com/html/gears___sprockets.html

[Reply #27 on:]

How much force is required to turn the shaft (can you turn it with two fingers)? Might be possible to use a servo to turn the shaft so you would have simple position control. Couple of ways using gearing and such that might work.

http://www.servocity.com/html/gears___sprockets.html

Yes, I can easily turn the shaft with two fingers or index finger and thumb. Not sure how to measure required force.

[Reply #28 on:]

I would still suggest a stepping motor, this will give you the precision you need and as the load is light there should not be any worry about needing positional feedback, you can just use dead reckoning.

The only difficulty is in determining a reference position for this system. There have been many suggestions for this, however as you have the physical system then you can determine what would be the easiest way to do this.
It could be an optical slot switch, a micro switch or even a proximity switch sensing something about the thrust of the capacitor core.

The only thing you omitted is the speed you want to change this tuning at. However, I suspect this is slow given the application.

[Reply #29 on:]

I would still suggest a stepping motor, this will give you the precision you need and as the load is light there should not be any worry about needing positional feedback, you can just use dead reckoning.

The only difficulty is in determining a reference position for this system. There have been many suggestions for this, however as you have the physical system then you can determine what would be the easiest way to do this.
It could be an optical slot switch, a micro switch or even a proximity switch sensing something about the thrust of the capacitor core.

The only thing you omitted is the speed you want to change this tuning at. However, I suspect this is slow given the application.

After looking at quite a few options, I am leaning towards stepper motors now. As you said, it gives the right control over motion.

Here is what I am thinking, please correct me if I am wrong. A controller like this:
http://www.electronickits.com/kit/complete/motor/CANUK1130.htm

Seems to give control over stepping size with variable frequency. Along with that, any stepper motor with sufficient torque should do. Can I connect two motors in parallel to the signal from this controller? One inside the shack connected to some sort of counter and the other outside that drives the capacitor.

Also, how do I dial the number of steps I want the motor to make in one go?