Rick, I am no guru, but maybe I can help. See below.
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Rick Brashear Sent: Friday, June 30, 2006 11:01 AM To: [email protected] Subject: [AMRadio] Motorized Variables Calling all Vacuum Variable Gurus... I am in need of education on vacuum variables and how they are motorized: >Is it necessary to use "stepper" type motors or will a high gear >reduction type configuration work as well? Stepper motors need a point of reference such as a switch or optical flag somewhere in the midst of travel, such as at one extreme. They are dumb since they are given a set amount of pulses to move a set amount of degrees rotationally per pulse. If the torque load is too high they will slip, and then the position is unknown. Using a stepper motor is an option, and I've seen kits in electronic stores (Ramsey kits??)for a PCB based stepper motor drive. >How are the limit switches accomplished? At both ends of travel, either a mechanical limit switch or a optical sensor with a flag will work. The idea is to limit further motor movement in that direction only, and still allow for travel the other way. >Is a friction type clutch safe maybe if the speed is not too high when hitting the stop. This should not be your first choice. >enough or are the capacitors too delicate for that? I don't like ramming a servo into the mechanical stop. I'd rather have the slip clutch (or motor current limit control) and mechanical limit for situations when things are out of control. >What is the best speed (RPM) to use when turning the capacitor? I think that depends on the tuning rate, circuit 'Q', etc. You clearly want to be able to pick a tuning peak or null without going too fast, and passing it. Going too slow, and you might burn up a tube while looking for the plate current dip. >Should I be concerned with interference if I use 115 vac motors? No not really, but if you look at 115 vac motors, look for ones with a 2 phase winding, and an external phase shift capacitor, and a gear reduction gearbox. These can be had with high torque at low speed, and with some switching of the phases, be reversed in direction. This type of motor is best for a move CW or CCW (while pushing a button) until you are where you want to be. A CW or CCW switch, and push button, and limit switches will work here. This will be the simplest approach to take, but might be unpleasant to operate. Some switches, relays, and relay logic to deal with the limit switches are all that is needed. No ic chips, power supplies, etc. This approach uses 1930's technology quite well. I would personally prefer to use a DC motor, and a gear box with a output shaft position pot. Then at the control end use a operator tune pot (use a 3 turn precision pot), and a servo feedback amplifier where the two pots are part of a resistance bridge (is that a weinbridge circuit?). You turn the pot, and the motor tunes automatically in the correct direction until the bridge is nulled. The hard part will be getting the position sensing pot to track the capacitor position over it's full range. Consider a linear pot instead of a rotating pot. You can do this all analog, go switching, stepper, or even with a microprocessor embedded controller with an encoder to sense position. All said, I have never tried any of this on a vacuum variable capacitor.... Now maybe a real guru will step in, and set me straight.. How would you do it? For those familiar with B-29 gun turret control systems, you could use vacuum tubes, mag amps, and a 3 phase ac resolver!! That technology was state of the art, and even by today's standards are very good indeed. This was position control by wire of the gun angle, and turret rotation position. My 2006 Mazda 3 has throttle by wire, whereas the B-29 had this in the early 1940's for the gun turrets. Absolutely amazing. When I was in tech-school in the mid 1970's we had a fully operational gun turret in the lab. We had to analyze every circuit, and were tested on how it worked. This thing would take your head off easily if you were standing in the way of the gun barrels. The servo control, precision, backlash, response, overshoot specifications were all excellent, and would be a challenge to achieve today with modern components. Regards, Jim WD5JKO As you can tell, I'm dumb as a stick about these things. Any advice and help would very much appreciated. I'm sorry for the duplicate posts to those who are on multiple lists. Thanks, Rick/K5IZ ______________________________________________________________ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:[email protected] AMfone Website: http://www.amfone.net AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb -- No virus found in this incoming message. 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