Amazing old school electromechanical ramp generator - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 18, 2020, 06:33:06 pm
content: I've pulled this out of the scrap heap at my university! It's an "electromechanical sweep generator" no manufacturer. Might be Oxford.It's a rather sleek 2U rack box that delivered the slow ramp for a set of coils to sweep the field of a 1970s superconducting magnet! Look how simple it's built (electrically)A forward/reverse switch delivers power to a motor that through a huge gear reduction turns a 10 turn pot set up as a voltage divider, giving 0-5 V out over 1 min to 1000 min!AstonishingUnfortunately I cant get it working.. the voltage out is spot on( preset by 10 vernier knob) but no motor movement. I can't quite recognise the motor type either, it has 4 wires out.The motor gets 24VAC when the thing is switched to forward or reverse so I'm afraid its the motor..Taking apart the gear stacks are gonna be hell but I pray that it's just seized up with grease, and it can be fixed by exercising it.Any ideas what else to try? I'd love to have this wierd thing functioning!edit: I think this is a synchronous motor

author: Refrigerator
author_rank: Super Contributor
timestamp: August 18, 2020, 06:52:00 pm
content: For starters you could measure the resistance of the motor windings to check whether they're ok.Also for a single phase motor to run it either needs a shaded pole or a start winding with a capacitor.This looks like the latter so you might want to look at that as well.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 18, 2020, 07:30:18 pm
content: Both windings measure ~1.3K, so the motor is probably good. It doesn't hum or buzz enough to feel or hear when it's in "run" position.The limit switches (seen by the potentiometer) are ok too.There is a cap between the two windings, perhaps it's fried.

author: Renate
author_rank: Super Contributor
timestamp: August 18, 2020, 09:18:17 pm
content: Quote from: ChristofferB on August 18, 2020, 07:30:18 pmMore likely dried?That's a pretty standard bidirectional AC motor with two windings and a run capacitor forming a triangle.Connect common and "A" to power it runs forward, connect common and "B" to power it runs backwards.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 18, 2020, 11:29:12 pm
content: The motor kinda looks like a stepper motor to me.Some stepper motors have their coils in that orientation.A (2 phase) Stepper motor ideally runs on sinusoidal voltages with a 90 degree phase difference, so mains voltage (frequency) can easily be used as a time reference with one or 2 capacitors to get your phase shifts right.If your motor windings are 1.3kOhm, then your motor likely runs directly from mains voltage.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 19, 2020, 08:04:09 am
content: Motor definitely runs on 24V. It's connected to the 24v tap of a transformer. Whether or not 1.4K winding resistance is good or bad I dont know, but the two windings atleast measure the same.I'm going to swap the motor capacitor tomorrow just to rule that out, and if that doesnt work I'll have to dig the motor out I'm afraidThanks for the suggestions!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 19, 2020, 05:10:00 pm
content: Problem solved!Oil in the synchro had completely gummed up, it could only be turned with pliers.Repeated turning, and fresh oil to dilute the gunk made it run fine again! Apparantly this is quite common for synchros.

author: jbb
author_rank: Super Contributor
timestamp: August 19, 2020, 08:28:04 pm
content: Nice!Out of curiosity, does that ramp time selection knob connect to some drive electronics or a cunning variable ratio gearbox?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 19, 2020, 09:03:46 pm
content: Amazingly enough the latter! It's difficult to make out through the orange perspex, but it looks to be switching in and out some planets on a sun/planet reduction stack - 2nd picture, you can sorta see it rightmost above the big black potentiometer.I've since learned that using synchro motors for timing is pretty common - their RPM is tied to the mains freq. which is very spot on 50 Hz. You can get synchro motor clocks and timers too, and it sets the time base in record players.This is really a work of art, more clockwork than electronics. What confuses me is that this isn't THAT old - mid 1970s. Why not include more features? You can only start and stop the sweep with a switch. No external logic/relay to trigger it. The output isn't buffered, and the 5V reference is just a bridge rectifier, 2 electrolytics and 2 zeners. It doesn't have any makers mark or company logo or even model number, so originally I thought it had been manufactured in-house. This was quite common still in the 1970s, but it has a serial number in the multi-hundreds!I'm pretty sure it's made by Oxford instruments, but only because the shade of beige is EXACTLY the same as an Oxford unit sitting above it in the rack - and various articles mentions an oxford "electromechanical sweep generator".Strange beast indeed. Too good to be tossed in the bin, that's for sure.

author: DaJMasta
author_rank: Super Contributor
timestamp: August 19, 2020, 09:06:10 pm
content: What's the application for a ~15uHz ramp, anyways?While I can think of things that need a very slow, even sweep, I don't usually think of them as needing to be repetitive.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 19, 2020, 09:12:13 pm
content: It's not repetitive, making the lack of a trigger input even weirder.It's been driving a current source for some magnetic coils in a superconducting magnet setup. Sweeping the magnetic field allows for stuff like MRI, NMR and electron paramagnetic resonance.Today you would scan many times and average out the noise, back then youd do one slow scan to avoid artifacts. The sweep out would also be the X axis of a 2D pen plotted.