Wadkin/Bosch tacho generator for vertical Z?? servo

Hello, I have a friend with a 1992/3 vintage Bosch CNC marked as a Wadkin. I'm looking for a tacho generator that sits on the vertical axis servo. If it's any help in identifying the machine I understand it's one of the very first to have a switchmode supply instead of a genny-motor unit.

Thanks Mark Harriss

Sorry? clean up the tacho generator?: the windings have flung throughout the housing as it self destructed.

Thanks Mark

See if this will work for you

Micromo is a US distributor for Faulhaber. As you are in the UK(?), try this also

The tacho-generator is basically a PM DC motor that is designed to give very low ripple output. The output volts per thousand rpm is the main specification. Any device that you can mount mechanically to the motor can be re-scaled with either a voltage divider or amplifier to get the correct motor speed. As this is the feedback device, more voltage output gets lower motor rpm. Zero output or reversed feedback will drive the servo off at warp speed... particularly scary with large industrial servos.

Cheers - Oppie

Aussie, eh? The UK mail address threw me though the message ID was .au. I've never seen a multi-phase tacho generator before. That's a new one on me but like you wrote, it was a prototype. I thought at first that it may be a resolver. Those have a pair of stator windings (quadrature output) and a wound rotor (excitation). I have heard of brushless types that somehow transformer couple the excitation to the rotor. I suppose that in a pinch, a small brushless DC motor might work as a tacho-generator. It is as you wrote, a stationary multi-phase winding and a PM rotor.

(Read your message on news:alt.binaries.schematics.electronic )

Cheers, Robert Oppenheimer New York, USA

Your description sounds similar to the AC output tachgen I've seen on some small motors. They are using a (ceramic? ferrite?) magnetic rotor attached to the motor shaft, with a coil wound on a bobbin. There are 2 steel disks with fingers bent to surround the rotor (forming interlaced stator poles) which are probably what's inducing the AC into the coil (my guess). These are on Oriental Motor AC motors, where the output is 0 to about 100V for the input into their variable speed modules.

I've also seen a little (synchronous, I suppose) AC gearmotor (approx. a 2" cube) produced like this. It has two rotors and two coils on bobbins (2 leads per coil). It's labeled Hurst Mfg Corp, Princeton Indiana (USA). It's a 115VAC powered, and utilizes a 0.68uF capacitor when operated as a motor. I haven't actually spun it with a meter attached to see what the output would be (it's mounted on the back end of a motorized variac/autotransformer).

WB .................

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Thanks Oppie for the lead: I'm in Australia with a particular Wadkin that the owner was later told was a prototype unit that should never have left the UK let alone be sold to a client. The tacho is a multi- phase ac unit with stationary windings and a rotating iron magnet and no brushes. I'll have to get some part numbers next time I'm at the machine.

Thanks Mark

Is it a permanent-magnet generator, with 3-phase stator? Had you thought of getting the stator windings renewed?

I once took an unknown motor to a little rewind shop in the UK to get the armature rewound. They just copied and renewed what was there. Only took them 2 days, at a fraction of the cost of a new motor.

--
Tony Williams.

May not be the same system but small 400Hz aircraft servos sometimes use drag cup induction tachos. These use three phase stator windings.

A low inertia rotating copper drag cup surrounds a co-axial stationary iron "rotor". A set of output windings is located in the positions that give zero coupling to the undisturbed three phase field.

As the drag cup rotates it distorts the three phase pattern. This induces an output voltage, at supply frequency, accurately proportional to speed and with reversed phase indicating reverse speed.

Jim

Thanks Robert, I'll have to see if I can find a second hand part to repair the thing rather than modify it as I have very little info on it, but more or less it's a massive stepper motor about 5 inchs square and 24 inches long, after that is the tacho generator followed by a high resolution rotary encoder. The local agent wants a fair bit of money to get out of bed and drive his Ferrari over to even take a look and so this machine has been idle for two years now. I had been hoping somone would say "Yeah we have one out the back we cannibalise for the tacho". Ahh well I may be able to find one yet, thanks for your help.

Regards Mark Harriss

Hi Jim, It really looks like a small alternator with two centre tapped windings and six wires fed straight into an integrated circuit on a board.

Thanks Mark

I was able to do a detailed drawing of the windings that were left in the housing and then pull apart one of the laquered bundles and rewind the assembly, but there is some mistake as the machine brings up an error message corresponding to the tacho generator. I was hoping to obtain a known good unit as there may be another fault in the same area (connector etc) as the machine has been plagued with tacho errors from new.

Mark

You previously noted that the wires went straight into an integrated circuit. Does it have a part number printed on it? That might help to work what the tacho is and what is happening there.

A burnt out winding is probably the result of a short circuit, either from that ic or downstream of it. So it could be prudent to replace the ic anyway.

--
Tony Williams.

Could be - this type of tacho was commonly used in servo systems where the error signals were at supply frequency and the main drive motor was a two phase high resistance rotor induction motor.

Are you sure that your main drive motor is a stepper?

Jim

Thanks Tony, I'll head in tommorrow and actually get some more details: I misplaced my original notes on the project. The windings didn't burn up but were chewed at one end by a foreign object flying around inside.

Thanks Mark

Hi Jim: no I'm not, I'm hoping to find out over the next few days and get back to everyone with serial numbers etc.

Regards Mark