| Electronics > Projects, Designs, and Technical Stuff |
| Aircraft instrument servo motor |
| (1/2) > >> |
| georges shed:
Hi all, I have a smiths radio altimeter that i wish to convert for use with a flight sim. It is driven using a 26v motor that has 3 coils. 2 are labelled control and one is labled reference. the 2 control coils are connected together so there are 3 pins. I have no idea how i would run this? any suggestions greatly appreciated. The control coils are 24 \$\Omega\$ each and the ref coil is 46 \$\Omega\$ |
| jmelson:
Yeesh! This can be messy. A very common device from WW-II era was the "selsyn". These had a wound rotor and a stator wound just like a 3-phase motor. You connected some excitation (typically 120 V 60 Hz for Navy ship applications, and 26 V 400 Hz for aircraft applications) to the rotor, and just connected the stator windings 1:1 together. If you mechanically drove one shaft to some position, the other unit would move to the same position. I think this could be a similar scheme, but with just two stator windings at 90 degree relationship. This is commonly called a "control transformer" and is actually identical to what is now called a resover. If the rotor is excited, the stator windings produce a singla which is either in phase or 180 out of phase with the excitation, depending on shaft angle. Their amplitude varies as the sine and cosine of the shaft angle. If you were to apply a sine wave that matches this relationship, it should work as a weak motor, enough to turn the instrument. So, you need to convert the shaft angle you desire to sine and cosine, and then produce sine waves of that amplitude and 0 degrees or 180 degrees to the rotor excitation. Not a real simple thing to do. Umm, one way to cheap out on this would be to rig a gear motor and pot or encoder to another of these motors, and use them like selsyns. Your computer drives the gearmotor to desired angle, the sending selsyn sends the 2-phase output to the instrument which displays the right position. Jon |
| Martinn:
Wouldn't it be possible to drive the coils with DC? Like connecting the reference coil to 5 V and the sin/cos coils to variable voltages (+- a few volts)? If you measure the inductance, you can calculate the DC level to get the same current as with 26 V 400 Hz (not burning the coils, but still delivering enough torque). |
| georges shed:
Thanks for the comment JMelson, As part of the mechanical arangement there is a potentiometer that is used to measure the position of the instrument. Ive got this working and can feed the diaplaed altitude within 10 ft. Is i can use the original motor that would be great but im now considering just replacing it with a normal dc motor. The next problem with that is making a suitable bracket and finding a correctly pitched gear. Either way im going to have to start thinking about a PID setup. |
| georges shed:
Thanks for the comment martinn, i will try to investigate that tommorow. |
| Navigation |
| Message Index |
| Next page |