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# Reed contact for disk position

#### Dr.Dree

##### New Member
Hi,

What would be the most basic circuit for controlling 1 revolution of a disk at the push of a button? As per below schematic, the disk turns CW until the marker meets a sensor. Then, with a single push/release of the button, the disk makes the next revolution. Etc.
Disk speed 60 rpm. Position accuracy is not required. Circuit power ±10W. Mechanical, optical, magnetic, etc. solutions are allowed.

Thank you

Solution
Have you decided on the sensor, or is that part of the question?

This can be as simple as a SPST switch with NC (normally-closed) contacts. When a protrusion/extension/whatever on the wheel hits the switch, the contacts open and the motor stops. To start things, connect a SPST switch with NO (normally-open) contacts, like most simple pushbutton switches, in parallel with the sensor switch. Hold down the pushbutton long enough for the motor to move the disk off of the sensor switch.

ak
Welcome to ETO!
Methinks a lot will depend on the mechanical inertia involved, as a disk (or the motor) with non-zero mass takes a finite time to accelerate and decelerate.
What braking system is allowed?
Is 60rpm the mean speed or the peak speed?
What length of time is allowed to accelerate to 60rpm?
Is this a school/college assignment?

Have you decided on the sensor, or is that part of the question?

This can be as simple as a SPST switch with NC (normally-closed) contacts. When a protrusion/extension/whatever on the wheel hits the switch, the contacts open and the motor stops. To start things, connect a SPST switch with NO (normally-open) contacts, like most simple pushbutton switches, in parallel with the sensor switch. Hold down the pushbutton long enough for the motor to move the disk off of the sensor switch.

ak

Welcome to ETO!
Methinks a lot will depend on the mechanical inertia involved, as a disk (or the motor) with non-zero mass takes a finite time to accelerate and decelerate.
What braking system is allowed?
Is 60rpm the mean speed or the peak speed?
What length of time is allowed to accelerate to 60rpm?
Is this a school/college assignment?
Thank you. The disk is the final stage of a 1:120 gear ratio so disk inertia is not expected to be an issue. As soon as the current is cut, the disk will stop. 60 rpm is peak. No requirement for acceleration. It is a trigger mechanism to be used in a bigger school project.

Have you decided on the sensor, or is that part of the question?

This can be as simple as a SPST switch with NC (normally-closed) contacts. When a protrusion/extension/whatever on the wheel hits the switch, the contacts open and the motor stops. To start things, connect a SPST switch with NO (normally-open) contacts, like most simple pushbutton switches, in parallel with the sensor switch. Hold down the pushbutton long enough for the motor to move the disk off of the sensor switch.

ak
Sensor type was open for selection as asked. I understand your set up as per sketch below? With one NO and one NC switch. I think it is brilliant, exactly what we need! I can do this with some micro switches. Thank you!

Yur wekkum.

The upper switch is shown as closed. This is correct only when the button is pressed. Unless you are showing the restart operation, it probably should be shown in the normally open position.

Please post photos of the final project.

ak

Yur wekkum.

The upper switch is shown as closed. This is correct only when the button is pressed. Unless you are showing the restart operation, it probably should be shown in the normally open position.

Please post photos of the final project.

ak
Yes, the sketch is the restart position. Does this set-up have a name? As it seems quite common and would work with many type of sensors. Yes, i will add a video when it's running.

This and other forums have many threads regarding linear actuators, limit switches, direction reversing, etc. It's a thing, but I don't know if it has a name other than something generic like motion control.

ak

Floppy disks and early hard disks have used simple magnetic sensors. Optical sensing for reflective and transmissive are also trivial with the right target for this INDEX pulse. Optical reflection can use a black stripe using a "Sharpie Pen" on the shaft of disk surface which I have tested as the best IR blocking ink or paint.

The start logic could be SET from a momentary pulsed 1-shot to an RS latch using 2 gates with crossed feedback. RESET would be created the Index pulse which would be removed as the disk coasts past.

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That sounds like a lot of work for something so simple. No matter how far the disk coasts after the index switch is tripped, that distance will be a constant for all cycles. Given the 120:1 reduction from the motor to the disk, I'll be surprised if it is more than a few degrees. The only important thing is that the index switch remains pressed continually while the motor and disk come to a complete stop. The width of the index bump on the disk covers this.

ak

Does this set-up have a name?
It's the same principle as the simplest windscreen wiper "Auto park", where the motor can only stop at a specific position after the on/off switch is turned off.

Photos or details of the mechanics are needed to choose an interface ;
The details are a disk with a bump on the edge (see post #5). What is wrong with a SPDT limit switch?

ak

going to build this design as a POP model...

I overlooked the 60 RPM. Whatever method is sensed, the trigger must be leading edge sensitive as it is likely stopped while the sensor is active.

Photos or details of the mechanics are needed to choose an interface ;
- edge transmissive optical https://www.digikey.ca/en/products/...output/548?s=N4IgTCBcDaIPYAcAuBLAxgQwDYgLoF8g

or discrete reflective optical.
I think this concept is the epitome of "over-engineered" - especially when presented AFTER the OP says this about a simple dimple...
I think it is brilliant, exactly what we need! I can do this with some micro switches. Thank you!

going to build this design as a POP model...

View attachment 146238
Just be careful with the shape of the leading edge of your dimple - too steep and you may have unnecessary wear on the dimple on the disk or the nub on the micro switch. I would have kept part of the actuator arm from the micro switch since the steel is nice and smooth for the nub on the disk and it presses flat onto the microswitch. High sheer plastic-plastic glide surfaces can cause melting and "stiction".

I think this concept is the epitome of "over-engineered" - especially when presented AFTER the OP says this about a simple dimple...
Which detail is over-engineered? The comment about leading-edge triggered. Will the mechanics be stable enough over time? Will the inertia of the wheel coast past the dimple as it wears down or switch ages, close the switch and keep going since there is no brake in this solution.
Then the dimple needs to be wider. __/ ---\__ Is it smooth enough? Will the switch arc when opened slowly? Is there going to be an RCD snubber? What are the motor Elec. specs.

It's more like under-appreciated. This isn't rock-science, but there still needs to be a trivial timing-state machine.

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going to build this design as a POP model...

View attachment 146238
Is POP a teaching term? Then it should have more detailed specs in the "purpose" for start/stop in terms of time or rotational degrees. The motor current and load interia will have consequences on the "bump and switch design"

This thread has gone on quite a ramble

It's pretty trivial requirement, due to the slow rotational speed.

It's been used for decades in all kinds of electrical/electronic devices, such as VCR's, Microwave Ovens, Record Players, a VERY simple mechanical and electrical arrangement.

Usually the reverse of post #14, instead of a projection on the wheel, they generally used a depression instead, with the micro-switch normally depressed, and going open when it hit the depression in the wheel.

In later VCR days they used multi-contact system switches, basically a row of contact brushed rubbing along a rotating PCB, this allowed multiple position sensing.

Which detail is over-engineered? The comment about leading-edge triggered. Will the mechanics be stable enough over time? Will the inertia of the wheel coast past the dimple as it wears down or switch ages, close the switch and keep going since there is no brake in this solution.
Then the dimple needs to be wider. __/ ---\__ Is it smooth enough? Will the switch arc when opened slowly? Is there going to be an RCD snubber? What are the motor Elec. specs.

It's more like under-appreciated. This isn't rock-science, but there still needs to be a trivial timing-state machine.
Who cares! The customer is happy and not paying for more billable hours. Once the customer completes the minimal build to prove the concept works or fails (and how) they can iteratively improve or move on to a new concept. They'll ring us up if they need help. For now, the concepts offered were described as "brilliant" and our marketing department is already using it in a publicity & promotion campaign. Move on to the next project where the customer is paying for your time - I'm not running a charity here. If you view your time as worthless, then it just might be. Do you understand me?

Oh, sorry, I was just repeating a speech I gave to an intern about 30-years ago who wouldn't stop thinking about a closed project. He listened the first time he was told he recently retired as an Engineering Director from a major consulting company.

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