[Mark Harriss]

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

 

[PrecisionMachinisT]

..au...

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.

Suggest clean up the tach outout from the motor you already got and then
get back to us.

 

[Mark Harriss]

Suggest clean up the tach outout from the motor you already got and then
get back to us.

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


Thanks
Mark

 

[Oppie]

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
http://www.micromo.com/Products/productdisplay.asp?CPTID=46
Micromo is a US distributor for Faulhaber. As you are in the UK(?), try this
also
http://www.faulhaber-group.com/

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

 

[Mark Harriss]

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

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

 

[Oppie]

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

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 )

Cheers,
Robert Oppenheimer
New York, USA

 

[Wild Bill]

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
..................

 

[Mark Harriss]

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 )

Cheers,
Robert Oppenheimer
New York, USA

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

 

[Tony Williams]

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,

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.

 

[[email protected]]

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 )
New York, USA

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

 

[Mark Harriss]

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

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

 

[Mark Harriss]

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.

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

 

[Rich Grise]

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.

If, as per your previous post, you have a stepper motor _AND_ a
position encoder, what do you need a tach for anyway? You
should probably be looking for someone who can hack your
firmware, and fix the broken code.

If you _really_ need a speed indication (what, to tell if
your stepper is skipping steps? Isn't that what the position
encoder is for?), just get any old PM toy motor - they make
dandy little generators.

Good Luck!
Rich

 

[Fred Abse]

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.

I doubt very much it's a stepper. Serious machine tools don't use them -
'cos they.. er.. step. No good for interpolating slopes and curves - crap
finish.

Much more likely it's an AC or DC servo motor with separate velocity and
position control loops. Tacho or encoder for velocity, encoder or resolver
for position.

 

[Mark Harriss]

If, as per your previous post, you have a stepper motor _AND_ a
position encoder, what do you need a tach for anyway? You
should probably be looking for someone who can hack your
firmware, and fix the broken code.

If you _really_ need a speed indication (what, to tell if
your stepper is skipping steps? Isn't that what the position
encoder is for?), just get any old PM toy motor - they make
dandy little generators.

Good Luck!
Rich

I wonder myself how these people have implemented things,
but at $10K for a manual with schematics , I'm not that curious
heh heh. The whole thing is fairly unorthodox but it seems to
tend toward electronic complexity to allow for a simpler
controller computer. I'm hoping to keep the machine original
but if it has to be modified, then it has to be done to get
it going.

All in all if I had some detailed schematics and diagrams
then fixing this monster would be a whole lot easier.

 

[Tony Williams]

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.

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.

 

[Mark Harriss]

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.

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

 

[[email protected]]

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

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

 

[Rich Grise]

I wonder myself how these people have implemented things,
but at $10K for a manual with schematics , I'm not that curious
heh heh. The whole thing is fairly unorthodox but it seems to
tend toward electronic complexity to allow for a simpler
controller computer. I'm hoping to keep the machine original
but if it has to be modified, then it has to be done to get
it going.

All in all if I had some detailed schematics and diagrams
then fixing this monster would be a whole lot easier.

Holy yikes. I've got a similar search in process, but doing it
on the cheap is somewhat of a challenge. And, of course, if I had
ten thousand bucks cash in hand, I'd be on my way to the Cayman
Islands. Or Bangkok. [makes my eyes red just thinkin' about it!
;-} ]

Good Luck!
Rich

 

[Rich Grise]

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.

Please accept my apology for bluntness bordering upon
rudeness, but if you have foreign objects flying around
inside of heavy machinery, my recommendation is:

*** RUN FOR YOUR LIFE!!!!!! ***

Or offer them an offer - you'll fix it for, say, fifty thousand
dollars, [Or half the cost of a new machine, or so] plus you
require the shop to implememt proper safety procedures, and
write them into the manual. They _do_ have a manual, don't
they?

By the way, I'm available to write the manual from scratch,
if that's what's needed.

Good Luck, but Stay Safe!
Rich
[this post needs a smiley, somewhere... probably inside of
heavy machinery. =:-O ]