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K5RPD
05-11-2010, 01:26 AM
I am trying to find the best solution for remote tuning of the air variable capacitor in my magnetic loop (small transmitting loop) antenna. I could use a stepper motor or a servo motor but I need to reduce the step/movement size. Any suggestions either way on stepper vs servo control? And I'm in need of suggestions for reducing the step size- such as a source for a good gear reduction drive or turns reducer.... I could use a simple 6:1 or 10:1 ball vernier drive but i dont know how repeatable/accurate they would be when used with a stepper motor control.
Any suggestions anyone?

Thanks in advance!
JJ
K5RPD

VK2HHS
05-17-2010, 12:55 AM
JJ,

Suggest you take contact with VK4AMZ who has done all this and more.

Please continue to post your results as I too am interested in your solution.

de VK2HHS


I am trying to find the best solution for remote tuning of the air variable capacitor in my magnetic loop (small transmitting loop) antenna. I could use a stepper motor or a servo motor but I need to reduce the step/movement size. Any suggestions either way on stepper vs servo control? And I'm in need of suggestions for reducing the step size- such as a source for a good gear reduction drive or turns reducer.... I could use a simple 6:1 or 10:1 ball vernier drive but i dont know how repeatable/accurate they would be when used with a stepper motor control.
Any suggestions anyone?

Thanks in advance!
JJ
K5RPD

KE5MC
05-17-2010, 01:12 AM
Stepper motor are quoted as if they are driven with 'full' steps. Depending on the stepper motor driver you can use 'half' step or 'micro' step. There are some trade offs for toque and speed, but you load does not have critical demands in that area.




...I could use a stepper motor or a servo motor but I need to reduce the step/movement size. Any suggestions either way on stepper vs servo control? And I'm in need of suggestions for reducing the step size- such as a source for a good gear reduction drive or turns reducer....

...

K5RPD
05-18-2010, 03:53 AM
I am familiar with the half and micro stepping aspects of the stepper motors so that shouldn't be a problem...
I have a 1.8 degree step bipolar stepper that I can half-step but I'm thinking I still want something with a smaller step (better "resolution" for the capacitor).

I did manage to find a 5V bipolar stepper with a 7.5 degree step that has a gear reduction already mounted on it of 1:50. The datasheet on it says that it works out to 0.15 degrees per step with two phases energized... maybe that will work...

JJ

KE5MC
05-18-2010, 12:50 PM
Another possibility is an electric screwdriver. Low cost, built-in gear reduction and reversible.

You asked about repeatable/accurate, is that an indication of closed loop control.

I intended to review QST for the last several months and forgot. I seem to recall that there was an article about your project.

I will try to remember and look tonight.

Mike

K5RPD
05-19-2010, 12:37 PM
Yes, I would like for it to have accuracy and be able to return to certain tuning positions with accuracy. I'm hoping by using the stepper motor that I can find the absolute positions that tune the loop for certain frequencies and then program those into a look up table of some sorts that I can call up and tune the antenna using a microcontroller of some kind (STAMP or PIC).
I hope I explained that well enough...

Thanks for the responses so far

JJ

KE5MC
05-19-2010, 01:42 PM
JJ

That can be done. There is a level of complexity added for returning to a known position. You have to establish alignment of the mechanical position to the electronics of the controller. This is called ‘homing’ the mechanism. Pick one end of the travel and place a mechanical switch or slotted optical sensor. This will be the home position. At power on the controller will read the switch to see if it is open or closed? If closed (home) the stepper is more away from home to allow the switch to open then reversed looking for the switch to close. At the moment the switch closes the position registers are set to zero.

From that point on the mechanical position and the position registers are in sync assuming no step loss during movement. Moving to a known position only requires the controller to send a string of pulses of a known value to get there. It can be calculated or found by manual positioning and reading the position register once you get to that position. The number is what you would store in a lookup table.

A stepper motor used in micro-step mode should have the resolution without gear reduction (direct drive) and still have the torque to move the capacitor.

I am sure you can get back to the same position with that kind of setup. What I am not sure of, does the tuned positions remain the same day in and day out. My experience as an example is when it rains my SG237 will tune my doublet but will be using a slightly different LC combination.

Instead of a lookup table it might be better to use a VSWR signal to control when the motor stops.

All of the above is a Cliff Notes summary of my experience working for a medical instrumentation company and we make extensive use of stepper motors in open loop control (home only) and closed loop control with encoders.

I failed again to look for the artical in QST. Maybe tonight???

Mike

VK4AMZ
06-30-2010, 11:10 AM
There are a few problems to consider and overcome when driving a loop tuning cap.

As you are using an air variable you should be able to physically drive it directly with a small stepper motor. If you were driving a large vacuum cap like the 40 turn 1000pf Jennings you would not be able to turn that cap using a stepper motor without using a reduction drive. (OK, you could use a REALLY big stepper motor)

Selecting the correct value of cap and keeping the loop operation to one band may relieve the need for tiny fractional adjustments. If the loop is very small for the band of operation then you may need a reduction drive.

A Loop covering a couple of bands will need a resolution close to 0.02pf per step to tune the loop on its lowest band.

A 20 turn 7 to 100pf vacuum cap on a 12 foot diameter 80m loop can be driven directly with motor having 1.8 deg per step. That is roughly 5pf per revolution of the cap which comes back to around 0.025pf per step. If you find you need substantially less resolution then you are either using a large loop for the frequency of operation or you have an incorrectly built or sited loop with high loss and low Q.

Another reason to keep the physical loading on the stepper motor low (by using a reduction drive) and especially with a vacuum cap is to allow it to run at HI speed to move across the band quickly with fast forward/fast reverse buttons. The final adjustment is best carried out using a low cost rotary encoder to single step as it allows you to rock the cap back and forward to overcome any play or flex in the drive, coupling or construction.

Once again as you are using an air variable you may get away without a gearbox if you are micro stepping and using a value of cap that just covers the part of the band you are interested in.

If the tuning of a loop is touchy or difficult you will find yourself tiring of it and not using it.

You can couple a high quality variable resistor (pot) to the shaft of the cap and use a micro controller to accurately read the position and return to this position. The problem with a HI Q Loop is any change in the surroundings or ground plane (if used) will cause a slight change in adjustment, so returning to the exact same position my not yield the correct tuning position. However it will be close and using a rotary encoder to rock the stepper makes final tuning a breeze.

The next step is an automatic tuning but that’s another subject.