Hello all!

Back in 2007 in the Kuwait Science Club, I worked as volunteer on upgrading the control system of an ailing observatory (2 50 cm scopes on an equatorial fork mount) and developed a closed loop control system via a custom PCB and NI 6509 that sits between the existing telescope control system and a PC. This worked quite well until we had a few electrical issues in the primary control cabin and the telescope stopped working. The club's electrician couldn't diagnose the issue and no one cared to follow up

The observatory was built by a defunct company and there is absolutely no documentation, so everything had to be reversed engineered.

Unfortunately, there was no interest to invest in refurbishing or fixing the observatory which requires a complete overhaul electrically, mechanically, and optically. At any rate, since the old control system is pretty complex and beyond me (numerous relays and timers and tons of wires with no documentation), I thought it might be better to work backward and try to control the two motors controlling the telescope's axis (RA = Right Ascension, DEC = Declination). If the motor leads are identified, I thought it would be relatively easy to build/buy a solid-state control board and get rid of the old control cabin. But when I opened the cover of the declination axis motor shaft to investigate the motor, I was at a loss.

Back in 2007, I installed an 8 bit absolute optical encoder on the declination shaft, but I didn't alter any of the motor wiring.

I did a bit of reading and even bought a book on motors, and while it was pretty useful, it emphasized heavily on the theory and math and left little for practical understanding of AC/DC motors and drivers. Therefore, it would be great if anyone can help me identify the components of the systems and the leads if possible. I _think_ it's composed of a brake (translated French words on it for fail-safe brake) and a motor (I think AC but I'm not sure). I put a '????' on the wires/components which I have no idea what function they serve.

The following are photos I snapped today of the scope dec axis:

Any insight is highly appreciated! Also, if anyone knows any _practical_ guides to learn these stuff, it would be great

 

wild guess, but what you've marked as AC motor may well be a clutched drive (spring wound), possibly with reduction. What you've marked as Brake appears to be a motor with brake, with possibly some type of feedback. The handwritten delta symbol suggests 3 phase AC. Your best bet is to get a clear image of the nameplates on both those items and post.

 

Just based on the pictures, I would guess that the component you've labeled "AC motor?" is actually a reduction-gear unit, maybe a harmonic drive. Are there any wires going into it? I would expect the big unit with a ribbed outer case to be the motor (with big wires going into it) and the brake is as you've shown it, on the back. I see a metal data plate on the part that I think is the motor--it's just below and to the right of the ??? that you've placed by the thin black cable (which I think is the wire bringing power to the brake). So what does that data plate say? Your third picture also shows a data plate on the component I think is the reduction gear unit. What does that say?

If you want anyone to help, you need to provide all the information that's available. Read those data plates!

 

knro,

The device labeled brake is indeed the brake. Connected directly to the brake is the motor and connected directly to the motor is a gear reduction, probably planetary. The device at the end of the gear reduction appears to be a fexable shaft coupling. Given the application of the equipment I'm guessing that the device labeled "encoder" is most likely a resolver, for controlling movement in arcs of a circle. I could be wrong but that's my guess.

The motor looks to be three phase, one wire per phase and one wire for earth ground (multi conductor #2). The wires from the brake (multi conductor #3) eventually go back the the control panel and are connected to a motor brake contactor or auxilory contacts on the motor control contactor. When the contactor is energized the brake is released and when de-energized the brake is engaged. The wires coming from the motor (multi conductor #1) are most likey for thermal overload protection.

The black device in the last picture, appears to be, either a mechanism or sensor for maintaining or monitoring precision engagement of the driving and driven positioning gears.

Well, that's my two cents worth at any rate.

Good Luck,
williamj

 

Your best bet is to get a clear image of the nameplates on both those items and post.

I got the nameplates today. The first for the motor+brake, the 2nd for the "gear reduction".

I would guess that the component you've labeled "AC motor?" is actually a reduction-gear unit, maybe a harmonic drive. Are there any wires going into it?

No, no wires going there.

The wires from the brake (multi conductor #3) eventually go back the the control panel and are connected to a motor brake contactor or auxilory contacts on the motor control contactor. When the contactor is energized the brake is released and when de-energized the brake is engaged.

I think that's it! Actually, there was a "kill-switch" on the other side of the declination axis and unless we pull the handle outward, the motor wouldn't start. I bet this is the same wire as #3. Photo below.

The wires coming from the motor (multi conductor #1) are most likey for thermal overload protection.

And such wires are usually connected to what at the other end?

Looks like I can use a V/Hz inverter then to control these two motors then. The brake wires are energized by a AC signal or DC? From the nameplate, looks like 24v DC, but I'm not sure.

At any rate, thanks a lot all for the extremely usefull information! I'm grateful!

 

"Frein electromagnetique" means electromagnetic brake, and "Manque de courant" means "Power off". I think that means that you apply power to release the brake.

On the SETAM unit, I think it's saying "Ref: 1/300" or maybe 1/309. Either way, that sounds like a reduction gear unit.

I don't think you've got any information on the motor yet.

 

knro,

The thermal overload is in series with the contactor coil wires so when the overload heats up enough the overload contact (in the motor itsself) opens and interupts current flow to the motor contactor coil thus returning the contactor to it's de-energized state (open) turning off the motor.

As for the brake, the nomeclature plate indicates a DC voltage...

24 Vcc 20 W
POLARITE + rouge - bleu


24 V @ 20 Watts
Polarity + red - blue


The power train (brake/motor/gear reduction/flex coupling) shown in the picture is a pretty standard package for machine control/operation. Here in the state 3 phase power is generally 240/480 volts, globally I'm not familiar with the standards. If you follow the motor wires in the control cabinet you can trace them down to find motor voltage. Generally the motor is fused or connected to a circuit breaker, you can learn a lot from reading either of these.

Keep at it and you'll get there,
williamj

 

The thermal overload is in series with the contactor coil wires so when the overload heats up enough the overload contact (in the motor itsself) opens and interupts current flow to the motor contactor coil thus returning the contactor to it's de-energized state (open) turning off the motor.

Great. I'm going in this Saturday to check where all these wires are exactly terminated. Give that the nameplate for the motor is completely washed out, is it safe to assume it's a 3-phase induction motor? Or do I need to test the winding..etc to find out?

I have to control 4 totals in motors. 2 for the telescope, one for the dome, and another for the dome door shutter. Each can have its own V/Hz inverter and may be controlled via Modbus over Ethernet. The name plates for the motors door are there, so it's easy to pick an inverter knowing the power requirements of each, however, I might have to estimate/guess for the telescope motors which are already moving a counter-weight balanced telescope so it shouldn't take a big motor to get it running.

Thanks Williamj and everyone else for your contrition on this! I'm really glad there is finally a way to restore the observatory!

 

Ok, so finally after a _year_, we got some budget to work on restoring the observatory to a working condition. We had the motors checked by a couple of of technicians, and we found out that they are 110 VDC motors rated 600 watts @ 7 amps. Since we plan to disconnect the motor from the old defunct control cabin in order to directly control the speed & direction of the motors, I wonder how can you control a monster 110 DC motor?

I googled a bit and it seems most DC controllers are rated at a much lower voltage. The idea is to use PWM at the rated voltage to control the motor, but I'm not sure where to exactly look for this?

 

Well, more surprises today! The motor is now determined by be a 1982 Brown Beveri (aquired by ABB) permanent magnet DC brushed servo motor. When we opened the motor enclosure, we found the motor + brake + tachometer. So at least this part of the "mystery" is resolved. What is the best way to control it?

There is already a 16bit absolute optical encoder installed on the telescope axis, so I get position feedback from there. All I need now is a way to control direction and speed, and from software I will accelerate and decelerate & stop based on the encoder position. Would I just require a servo drive since a servo controller is not exactly required because the software is acting as the controller in this case?

 

I think what you'd need to control this would be dependent on the characteristics of the overall system. How much mass and friction is there, is the load counterbalanced i.e. does it change much at different positions, and how much precision do you need anyway? And is there an intention to use this device dynamically to track stars as the earth moves, or do you just go to a certain point and stop?

In the past I've dealt with a company called Advanced Motion Controls, which makes a range of units to drive various kinds of motor and provide interfaces to a computer. (Disclaimer: My only relationship with that company is as a customer.) I suggest getting in touch with them, or a similar company, and describe what you need to control. They'd most likely be able to suggest a complete system.

http://www.a-m-c.com/

 

Thanks John for the reply. I contacted a few companies yesterday, including AMC to inquire about driving the DC servo motor via a control interface (RS232/RS485) with a PC. The weight is already counter-balanced, and there is requirement for both "slewing" (i.e. moving to point A to point B), and "tracking" (i.e. moving at sidereal speed to track the stars).