Hi all-just joined this site.
I am a retired mechanical engineer with a little (and limited) electronics knowledge. I am attempting to design a safety stop device for a machine that I use on my smallholding. The idea is to disengage the drive shaft using a non-contact solution that would offer a more reliable result than a contact-type, mechanical method. The device is activated by the sensing of the sudden cessation of rotation of a driven pulley, which then instantly and automatically disengages the drive by pshing the on/off lever to the OFF position. I have posted the basic design here, but would appreciate any comments/suggestions on the design basis and particularly the electronic details regarding component type/values to help me achieve the best result - thanks!

 

To detect the cessation of rotation you could use a common proximity sensor and a watch-dog timer, that recognizes a minimal rotation (RPM).

 

MaxHeadRoom’s circuit suggestion is much simpler than your proposed design. But something to consider in such an electronic circuit is that it should be designed to be as ‘fail-safe’ as possible. Any circuit relays should be de-energised in the safe state, and a failure mode analysis conducted on the complete circuit to determine the potential for a loss of circuit protection and the hazard that would result. A decision could then be made as to whether the circuit provided an acceptable level of protection, given the hazard it is protecting against.

 

What is a "smallholding" ?

Is the "ON-OFF-Lever" a mechanical device, like a Dog-Clutch,
or is it an Electrical Switch for a Motor ?

You mention a "Solenoid" to operate the "Lever",
is this a device that You intend to fabricate, or is it part of an existing Safety-Stop-System ?
A Solenoid powerful enough to mechanically disengage a Gear-Box
will probably be huge and cumbersome.

Is fast response required ?
Should there be a Shut-Down response below a certain Pulley-RPM,
or should it be some specified time period "after" all movement has stopped.

Is this system intended to cut Power to a Motor ?
If so, how is the Motor currently controlled ? (Schematic diagram preferred)
Do You need "Motor-Braking" for an extremely abrupt stop ?

"" cessation of rotation of a driven pulley ""
Does this mean that there is an excessive Load placed on the Pulley, and the Belt is slipping ?
If the "Lever" mechanically disengages a Gear-Box, or a running Motor from a Gear-Box,
wouldn't it be a better solution to simply stop the Motor ?,
since you would not want to mechanically re-engage a
running Motor to a stopped Gear-Box in any case,
so the Motor will have to be stopped and re-started after the over-load has been removed.

Motor-Braking seems like the best option.
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MaxHeadRoom’s circuit suggestion is much simpler than your proposed design. But something to consider in such an electronic circuit is that it should be designed to be as ‘fail-safe’ as possible. Any circuit relays should be de-energised in the safe state, and a failure mode analysis conducted on the complete circuit to determine the potential for a loss of circuit protection and the hazard that would result. A decision could then be made as to whether the circuit provided an acceptable level of protection, given the hazard it is protecting against.

The essence of the 'fail safe' for e.g. if the sensor fails or a piece of metal becomes attached to the sensor, it will shut down, the rate of the F.S. circuit would be governed by the rate of sensing, or lack thereof

 

The most amazing mechanism I've seen for this is a table saw brake that stops the blade instantly when it comes in contact with human flesh, so fast that no damage whatsoever is done.
I've seen a video where they push a hot dog into the running blade and it stops without breaking the skin of the dog.
Would save a lot of missing digits that some table saw users seem to have.

 

Is this an application for DC-injection braking?

 

so fast that no damage whatsoever is done.

Other than the ~$100 brake cartridge and a new blade of around the same price. I know it's not a finger but still suffers damage all the same. And gives the user less of a reason to work safely in the first place.

 

A tad OT, that hot dog demo is cool. Are people becoming so moronic & devoid of common sense that manufacturers must remove every shred of personal accountablity on part of the end user?

 

The answer is to shoot all Lawyers, and let God sort'em out, like it's supposed to be.
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A tad OT, that hot dog demo is cool. Are people becoming so moronic & devoid of common sense that manufacturers must remove every shred of personal accountablity on part of the end user?

Tablesaws cut the fingers off even very careful users. There are many examples where accidents outside the control of the user leads to amputations. Tha Sawstop is a great piece of technology, and it saves limbs. It should be standard.

I use a tablesaw, and my current one is not a Sawstop but when I renovate my shop the new one will be. It’s just a statistical thing about which of us loses a finger. Accidents, real ones that don’t have to do with negligence, do happen and things like Sawstop prevent catastrophes.

It is not a free ride, and if you do manage to activate it you have to shell out (last I knew) about 60 bucks for a new brake which is sacrificial and possibly a new blade which might cost as much.

 

Hi all again. Made some progress. Have an ac tachogenerator (ex-washing machine motor) that outputs 1 Vac/100rpm. All I need now is to have a simple circuit that will switch a relay to operate a push type solenoid. When machine is operating correctly, tachogenerator voltage is in range 5-10 Vac . I need the circuit to switch on a supply to a standard relay using appropriate contacts (The relay then operates a solenoid). An opamp or comparator circuit seems the way to go. The concept is fail-safe ie device failure or supply loss results in drive to machine being removed. Any suggestions for an opamp or comparator circuit/components?

 

Oh-boy,
not what I expected in the way of answers.
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"Tacho-generator using ex washing m/c motor"
1v/ 100rpm
What technology does this motor consist of?

 

The tachogenerator was originally driven by a washing machine motor. It uses Hall effect to generate a signal that is processed to control motor speed during wash cycles. Now it is to be used to signal when a shaft rotation ceases ie its output is zero. When the shaft is rotating, the signal has voltage, but negligible current. So, needs boosting to operate a 12v relay. This is the bit that I need help with. I want to keep it as simple as possible, so don't need anything super fancy.

 

I think you will find the tacho from a washing machine motor is just a coil. A Hall effect device would not generate a voltage that varied with speed.

Les.

 

What You are asking for sounds simple enough,
but You keep leaving out all the details that make all the difference.

What is a "Tacogenerator", where did this word come from ?
Now it appears that You simply have a Washing-Machine-Motor with
a Hall-Effect Speed sensor built-in.
Totally standard stuff.

Next, is what are You talking about when You say ..........
Operating a Relay, to operate a Solenoid, to push a Lever ?
It almost sounds like You are trying to create a "Rube-Goldberg-Machine".
You may be adding a lot of unnecessary complexity and/or expense.

Why do You need to "push a Lever" ?
What does the Lever actuate ?
Why do You need this actuation to occur ?

The item #1 in your drawing shows a Hall-Sensor switching Power for this whole arrangement,
that's not likely to be feasible.

The item #3 shows what You call an "Inductive Coupler", (whatever that is),
providing a Power-Ground for an Op-Amp.
I don't know of any part that will work in this arrangement.

#4 is labeled "Relay".
Is there any particular reason that you want to use an actual mechanical-Relay ?

#5 is labeled "Solenoid-Actuator",
I'm reasonably sure that this Solenoid will require
more Current than the Hall-Sensor (#1) can handle.
How much force must the Solenoid generate ?
How much travel must the Solenoid have for complete, and reliable, actuation of the "Lever".

It would appear that You don't actually know how any of these devices function.
If You will explain in detail, what kind of a machine You are building,
and exactly what it is expected to do, (or not do),
then maybe we can help You to get a reliable system working.
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I think you will find the tacho from a washing machine motor is just a coil. A Hall effect device would not generate a voltage that varied with speed.

Les.

Please read this before you doubt my intelligence
https://all-errors.com/tacho-sensor...r-in-bosch-washing-machine-check-and-replace/

 

What You are asking for sounds simple enough,
but You keep leaving out all the details that make all the difference.

What is a "Tacogenerator", where did this word come from ?
Now it appears that You simply have a Washing-Machine-Motor with
a Hall-Effect Speed sensor built-in.
Totally standard stuff.

Next, is what are You talking about when You say ..........
Operating a Relay, to operate a Solenoid, to push a Lever ?
It almost sounds like You are trying to create a "Rube-Goldberg-Machine".
You may be adding a lot of unnecessary complexity and/or expense.

Why do You need to "push a Lever" ?
What does the Lever actuate ?
Why do You need this actuation to occur ?

The item #1 in your drawing shows a Hall-Sensor switching Power for this whole arrangement,
that's not likely to be feasible.

The item #3 shows what You call an "Inductive Coupler", (whatever that is),
providing a Power-Ground for an Op-Amp.
I don't know of any part that will work in this arrangement.

#4 is labeled "Relay".
Is there any particular reason that you want to use an actual mechanical-Relay ?

#5 is labeled "Solenoid-Actuator",
I'm reasonably sure that this Solenoid will require
more Current than the Hall-Sensor (#1) can handle.
How much force must the Solenoid generate ?
How much travel must the Solenoid have for complete, and reliable, actuation of the "Lever".

It would appear that You don't actually know how any of these devices function.
If You will explain in detail, what kind of a machine You are building,
and exactly what it is expected to do, (or not do),
then maybe we can help You to get a reliable system working.
.
.
.

Unbelievable!

 

Your link is wrong. It says the sensor is a Hall sensor but then goes on to describe an inductive sensor. A Hall sensor is a semiconductor device.

Les.