newby: need help!

I took electronics back in college, but I'm quite rusty since its been well over 15yrs now. However I do have the knowledge to purchase whatever electronic components I need and to also solder them together. Here's what I need. I'm working on a "tool", to simplify a task. Basically I need a circuit that will activate a small DC motor when a push button is pressed. It would be best if the speed at which this motor operates, could be adjusted. Now, this motor will continue to operate until an "open" circuit closes.

I'm having a tough time explaning myself so lets use this example. Lets say we connected a DC motor to a continuity tester. I push a button, the motor starts to spin at a speed which I can regulate. I take my positive and negative leads on a continuity tester, join them, hence closing the circuit, and the device (motor) immediately shuts down. The procedure can be repeated again upon the push of the button.

I hope that makes sense. Hopefully someone can put something together for me that'll operate on say a 9V battery, making it portable. In case you are wondering, this motor should be something similar to a small remote controled car.

Thank you! I hope someone can help me out!

You need a windshield wiper motor self-completing circuit. A Windshield wiper always returns to the "park" position, regardless of when you turn off the switch...

Your signal circuit (The one you want to kill on close) could operate a relay that is wired Normally Closed. When the circuit is closed, the relay would then OPEN the motor power circuit stopping it.

When you open the signal circuit, the relay would return to a CLOSED state and the motor would start again.

Here's the thought. Lets visualize an eraser sitting on a metal plate. Now, you grab a screw and begin to drill through the eraser until you "feel" it bottoming out on the metal plate. This "tool" I need would essentially turn the screw and metal plate into a circuit which is open. At the push of a button the motor begins to turn the screw at a speed which I can adjust. Once the screw makes contact with the plate, the circuit is closed, and shuts the motor off. I can then move this "tool" unto another screw placed on another eraser, set the metal plate under it, push the button and allow it to once again rotate the screw inwards until it makes contact with the metal plate. So if I had 4 sets of erasers with screws in them, after using this tool, the depth of the screw would always be the same since once the screw makes contact with the metal plate (bottom of eraser) it would shut the motor off.

I need someone to actually draw me a schematic of what I'm trying to acomplish and I have very limited electrical knowledge. As mentioned, I am able to assemble it based on a schematic. RMMM, your idea sounds about right. Any chance you could draw me up a schematic? Thanks!

Does the 9V power the motor, or just the circuit? How much current/voltage does it take to make the motor run at the speed you would like?

MikeMl said:

Does the 9V power the motor, or just the circuit? How much current/voltage does it take to make the motor run at the speed you would like?

Click to expand...

Humm you've stumped me there.... I was thinking I could control the speed at which the motor operates with something like a "volume knob", for lack of the proper electronic term. A variable resistor, would that work well for this application? See the circuit below and let me know if I did it correctly. Pushing and holding the ON switch should activate the motor and keep it running until I release the button OR contact A and contact B close that circuit, which should cut power to the motor. I need to wire in a way to adjust the speed of the motor as mentioned above. How would I do that? I would need to convert this into portable battery power. 9V perhaps. Any 9V relays out there? lol Any thoughts?

**broken link removed**

If those are standard 12V automotive relays, a 9V battery would run this for about 3 minutes. Those relays have an 85 Ohm coil, which means each relay draws about 141mA at 12V (105mA at 9V). They will not pull-in very reliably at 9V, especially as the battery voltage drops off. I dont remember which pin does what on those relays?

I'm guessing that any motor worthy of the name will also draw ~100mA or more, so you will need a much more than a PP3 battery.

You are going to want to start from a different angle.

What it appears you want is an manufacturing automation tool.

You should not want to just kill the circuit when the screw contacts the metal plate, but a signal to trigger a "brain" (Micro controller, PLC, etc) to initiate another action.

Look up CNC machines. You can use this to run the motor, and plunge the motor/screw into the wood.

When the screw gets through, making contact, the CNC machine will:
1. Kill the motor
2. Reverse the Z-axis (pulling the driver motor back)
3. Move back to a "home" position to retrieve the next screw.
4. Move to the NEXT screws location
5. Drive screw while waiting for "STOP" trigger (screw touching the metal plate)
6. Repeat.

Is this what you are looking for?

That would perhaps be the next step in the evolution of this "tool" if indeed it does work. Currently this task is being performed manually. I just need to automate the turning, or shall we say "adjustment" of the screw, that's all. This device would be able to be portable, and hand held. I suppose you could say something like a small cordless screw driver, lol, but with the ability to shelf shut off upon the screw contacting a metal surface. Except this metal surface is obviously like a "probe" from the device inserted into the screw's path.

The person using this "tool", would simply turn it on while holding it in place, and it would shut itself off upon reaching the proper adjustment triggered by the metal plate or "probe" if you will. Once completed, the "tool" would be removed by the person using it, and then placed on the next screw.

As mentioned, this task is being performed manually. Since the person doing this task has to ensure the screw reaches a pre-determined depth, he/she requires time to visually see the screw being adjusted, and stop based on his/her discretion. That depth has a certain tolerance which varies from screw to screw due to human operation. This "automation" is my atempt to guarantee acuracy in the adjustment of this screw.

I'm certain that my diagram above using 12v automotive relays would work. I still need to incorporate a speed adjustment dial for the motor to insure acuracy of the screw adjustment. Too fast may drive it in deeper than desired, thus the need to adjust, and determine the correct speed at which the motor turns.

Since its going to be a portable "tool", I thought that 9V would be a good place to start, but obviously my lack of knowledge in this area is showing. There is always the option of using electricity, ie pluging the tool in, and using a step down transformer to 12V.

It does not require a large amount of force, since the screw that needs to be "adjusted" moves quite freely. It is already hand temped in, if you will. It just needs to be driven in to a certain depth, but it does not bottom, or torque out. It just sits there.

So as originally requested, I just need it to operate a motor, which will turn the screwdriver "bit", at a speed I can adjust. Once the screw contacts a metal "probe" from the automated device, it will stop the motor from turning. It will then repeat its function when the person using it, places it on the next screw and again turns the device on.

There will still be some issues to work out, but for now I'm trying to determine a basic working schematic, that I can trial and evolve.

Look at it from another angle. Use a stepper motor because you said the torque is small. There may be 400 steps/revolution and it will stop on a dime. The # of pulses/min applied to the controller represents RPM.

Any DC motor will overrun unless dynamic braking or a brake is employed.

Typically, I do this with a cordless drill and an adjustable clutch on the drill.

I've used this item: B0013813M2 from **broken link removed**

You can buy torque screwdrivers where the torque is very precise.

I agree.

DeWalt is the only drill motor I have worked with that STOPS within a revolution or so.

The stepper driver would be nice.

You COULD attach a brush, like those used in an electric motor, to "touch" the drill bit. That would allow it to be a conductor.
If you had a motor brush against the bit with the (+) lead attached, and the metal plate had the (-) lead attached, you could use the relay idea to kill the drill.

You would have to feed the trigger wire out of the case of the drill and connect it through the NC relay. When the bit and screw, which are now "energized" by the brush, hits the metal plate(-) it would open the relay and the motor would stop.

From having done some research, indeed a stepper motor seems like it would be the perfect candidate for this task. It could operate relatively quickly and be able to stop pretty much right away. This would ensure the accuracy that I am looking for. I only require this motor to rotate in a single direction. Is there a simple controller circuit that I can put together, or purchase already assembled, that would not require any PC input/programming?

I COULD use an existing cordless tool, but it would be rather bulky and heavy. The tool that we currently use operates much like a torque wrench. The operator turns the screwdriver handle, and when it senses a certain "load", caused by the tightening of the screw, it "clicks" and you can still continue to turn the handle, but it will not actually move the screwdriver bit. I'm trying to convert this tool into an electrical device for a more accurate setting. I'm trying to narrow the accuracy down to 20 microns, give or take! Right now the manual tool, combined with human input is accurate to a 30 micron range. Not bad at all, but we see many "resets" or re-adjustments with this manual tool.

I would like to use the stepper motor as suggested, and simple have it come ON in a clock-wise direction, at an adjustable speed until the "kill" signal is received. You guys have been great thus far in guiding me in the right direction!!!! Any suggestions as to where I can buy a small stepper motor controller for a low angle motor, for more accurate results?

I like the idea of the "brush" up against the screwdriver bit. Now I just have to figure out how to "kill" the motor upon it making contact with my metal plate. Can I somehow incorporate that into the stepper motor circuit? Thanks!

I used stuff from this company before: 2 phase bipolar Motor Drivers, stepper motor driver, motion control drivers, step motor driver to automate a monochometer-based scientific instrument.

The system consists of a driver, a couple of resistors, a 5V supply and a stepper supply if I remember at a minimum. You can select a microstepping controller if you needed it. Microstepping in simplistic terms means the motor ends up having finer resolution by allowing one to use a number of discrete currents between each steps. I think you could select CW, CCW or Direction and not enable type inputs if you liked.

There is some wierd stuff when selecting a motor when it comes to unipolar/bipolar and the voltages required. What you learn is that there is at least one thing you need to be aware of: Two 12 volt center tapped windings can be considered two different types of motors: I think 12V unipolar and a 24 V bipolar motor.

This might be enough for you. All you would have to do is provide a gated pulse source.

But, the neat part is that you could combine the driver with a motion controller with an optional program if you bought the optional programming software. The controller also contains some digital inputs and outputs. The controller also had the ability to use CW and CCW limits and had an ORG sensor input that could be used.

To find the origin, the controller would go to the CCW limit and then increment slowly until ORG was found.

Details are a little sketchy because it was some time ago and I didn't look at the current datasheets.

Auto Driver

Here is a circuit using a brushed gear motor like this one. **broken link removed**

A switch to set a latch to start the driver, and the brush idea to reset the latch.
An inverter to get the polarity right for the motor driver. A motor driver.
I think this configuration will brake the motor when it hits the stop so it should stop quickly. If your screw is like a 4-40 you should be able to get withing a couple of hundred micro inches.

Ok I've read right thru this thread and something just doesn't click, the op wants to turn a screw and get 20 micron or better tolerance ???????? Thats 0.02mm which is a pretty tight tolerance to keep. Now if the OP can be a bit more specific on the application rather than just saying I want a screw set within 0.02mm every time using a hand held device. Like I say something just doesn't click here......

bryan1 said:

Ok I've read right thru this thread and something just doesn't click, the op wants to turn a screw and get 20 micron or better tolerance ???????? Thats 0.02mm which is a pretty tight tolerance to keep. Now if the OP can be a bit more specific on the application rather than just saying I want a screw set within 0.02mm every time using a hand held device. Like I say something just doesn't click here......

Click to expand...

I might as well be a little more informative. The tool we currently use is what we call a "clutch ball". The application is automotive valve adjustment. Currently as mentioned, we use a manual tool that works much like a torque wrench. Something very similar to the pic below.

**broken link removed**


As you can see in the photo, a shim is inserted between the valve stem and the set screw. The tool we use is slightly different. The "handle" is shaped like a ball, and it connected to the screw driver bit via a "clutch" which is preset at a certain tension. As the operator spins the ball, it adjusts the screw down towards the shim. Once a preset tension is reached between the set screw and shim, you can continue to spin the ball (handle), but it will not move the screwdriver. Its fairly accurate, and "resets", as we call them, or re-adjustments are minimal. But there are some, and we are finding that its rather difficult to maintain a fairly constant setting. My goal was to automate this tool. Operator would still have to manually place the tool on the set screw, but now he/she would turn the device on, and it would turn the set screw until it made contact with the shim. It would need to stop almost immediately to guarantee accuracy.

Of course, there are other issues to work out. I would have to determine which shim size which yield the required results, but would just be a trial and error operation, until we would obtain the clearance we are looking for. So maybe this can shed some light on what I'm trying to accomplish.

If a stepper motor is indeed the most accurate motor for my application, I would require just a stand-alone driver, with speed control. It would also only need to rotate in a clock-wise direction. And of course a way to stop the spinning of the motor as soon as it makes contact with the shim.

Using your electrical sense method, how would you over come the problem of, as soon as you put the driver on the screw it would complete the circuit? Unless your rocker arms are made from a non-conductive material as soon as the shim and driver are both in contact with the engine the circuit is completed, and the motor won't turn.

I thought about that, and I wondered if it would be possible to coat the underside of the shim with a non-conductive material. But then again, that would most certainly wear out after only a short little while, and render the entire project a failure. I could change my point of view slightly and try to keep things simple. The "clutch ball" works as it should in its current design. I could keep it as simple as possible and use a motor to spin the clutch assembly, and allow the clutch to disengage as it currently does. That would certainly reduce the wrist strain demonstrated by the operator who has to constantly spin it to adjust the tappets.

We utilize a certain sensor on our torque wrenches, that "trigger" everytime they detect a "click" of the torque wrench. I could certainly utilize one of these mounted on the clutch assy, and as it detects the clutch disengaging and "clicking", it could send a shut off signal to the motor. That would probably be the simplest, I think.

There may be other variables involved, but I'm rather anxious to get a working model built. One that I could test on a cylinder head and take smoe measurements and evaluate its operation. From there, I can make design changes, etc, etc. I just want to test a working model to determine if its even worth the time, effort, and money to build. My main goal is to provide better ergonomic design for the operator. Full automation could be a possibility, but that is remotely in the plan right now.

How about we limit the current (thus toque) to the motor and just let it stall when the whole thing comes together. This would keep the torque constant on the screw while the nut is tightened. 20 um! Things have changed since I adjusted lifters.

Perhaps I may have confused some of you. The spec for these particular lifters is 0.24mm, if I'm not mistaken. The range is plus or minus 10 microns, thus the 20 micron range. The clutch ball does an excellent job of keeping it in that range. It does however vary, and its quite inconsistent. The goal is to make the job ergonomically more friendly, and also hopefully try to illiminate certain factors that would cause those inconsistencies.