[Sanghera]

Hi,
Not sure if this is the right place to post this but.
Just wondering what kind of circuit is used to op/close cd trays on desktops. You push the button it opens and stops the motor at the end and then you push the same button and it closes and stops the motor at the limit. You can also switch direction pushing the same button again in the middle. Or even more complicated one of those car flip up screens where they come out stop, flip up stop move in a bit and stop. Then you can do the same thing in reverse.
Thanks.

[smarbaga]

mostly a mechanical cct , with opto's at the limits , or the mirro times the motor, with motor current scenseing.

[Sanghera]

What is mechanical cct? By current sensing, do you mean a pot connected to the device changing the current?
Thanks.

[smarbaga]

hello:
mechanical circuit, anything mechanical be it gear racks, coggs , worms, cams, leavwes, followers, etc. as to the current , it is more likely timming, opto detectors and voltage regulation, just a guess tho.

[abasir]

Most CD player has microcontroller of some sort (to read data, error correction, spindle speed control, etc). A switch is connected to the uC IO port. Once press, the uC will enable the H-bridge to switch the DC motor for the tray until a limit occur (either opto or current sense). If switch is press while tray in motion, the uC just reverse the current flow to the DC motor.

[Sanghera]

What kind of microcontroller would this be and what kind of H-bridge?
Thanks.

[smarbaga]

there are only a handfull of cd core manufacturers
the same as with laser print engines
the make there own chips and propriotary circuits

[Sanghera]

I was just wondering about the basis of these circuits to run these types of things. What it takes to get something to move in that way.
Thanks.

[j m]

hi Sanghera,
Are you trying to make one?

one of those car flip up screens where they come out stop, flip up stop move in a bit and stop.

I don’t know if this helps but on tv [pimp my ride] program they mounted a small tv screen in the back of a car seat and used a automatic aerial from a car to move it up and down.
Or if you wand some thing to go backward and forward you could use a 555 timer as a Bistable (flip-flop) - a memory circuit with a SPDT relay and read switches. I have done this for my brother’s model railway to get a train to travel a long a strait peace of track and the change direction when it get to the end, this I quite ease to do.
jm

[Sanghera]

Could you go into a bit further depth with that? Is there anyway to use an H-bridge with a single push button?
Thanks.

[j m]

Do you mean me?
With the aerial I think it was all inbuilt in the unit and they just connected a switch were the radio would be connected, but I don’t know.
And the 555 just sends it continually back and forth.
This mite help http://www.robotroom.com/HBridge.html
jm

[gar]

050903-1854 EST USA

Sanghera:

First, you need to define the logic of the circuit. This may need to be done interactively with your choice of components.

You need to define what the push button action is be at different points, what the position sensor will be, whether mechanical stops exist at each end, what to do when you get close to an end, what happens when the motor is overloaded ( you grab and stop the motion ), and is motor overload funtion the same at the ends as in the middle.

Suppose you make these assumptions ---
Limit switches exist at each end of travel, and they actuate 0.05" from the mechanical stop.
Mechanical stops also exist at each end.
The drive system has one overload level.
When overloaded the motor stops where it is, and does not restart when the overload is removed.
When a limit switch is in an asserted condition, then the pushbutton must be held until the assertion is lost.
If the motor stops for any reason, then the next operation of the pushbutton sends the motor in the opposite direction.
After a limit switch is asserted, then the motor runs until overloaded (meaning against the mechanical stop).

Maybe more information is needed, but with these assumptions or your own you can then at least start the design of a combinatorial logic circuit, or a microprocessor program.

Then you experiment with the system looking for bugs and figure out ways to solve them.

.
, .

[Sanghera]

What would it take to program a microprocessor? I have limited knowledge on this topic but I have tons of resources so I can learn. Actually all those assumptions above are similar to what I was thinking. Limit switches but also a mechanical stop. The only part is how would the circuit know when the motor is being overloaded?
Thanks.

[pmurray]

Sanghera,

Gar’s system description is good stuff. As far as the method of implementation of the electronics, I would probably us a microcontroller – probably one of the Microchip PIC’s with a built-in PWM that will make it easy to reverse direction and, if you want vary the speed.

Take a look at this development board: http://www.microchip.com/stellent/id...&part=DM163022. I think it’s about $100. This is a great learning tool if you have not worked with microcontrollers before and, you can use it for more than just one project. After you get the code and support hardware debugged, you can build up your own board with just the components you need.

You will need a way to program the PIC processor with the code you write. There are many cheap third party one’s built or kitted or, projects I’ve seen on the web for building your own. Here is a link to a complete development system (program & debug) from Microchip: http://www.microchip.com/stellent/id...&part=DV164005

Don’t know what your RPM and torque requirements are but would suspect low RPM and high torque. Look at some of these brushed DC motors at SDP: https://sdp-si.com/eStore/Direct.asp?GroupID=580. The nice thing about these motors is that they don’t draw much current – about 10-12 ma operating and I think they stall at about 20 ma. What this means is you can build an H-Bridge motor controller out of cheap common transistors.

Have a look at this App Note (893): AN893 - Low-Cost Bidirectional Brushed DC Motor Control Using the PIC16F684. It will show you basic brushed DC motor control and also show you how to hang a current sensing resister in your H-Bridge that you might be able to use to detect stall conditions.

The microcontroller will have plenty of spare I/O port pins to hook your control switch and limit switches to. You will be able to read with the firmware the state of these switches to determine what to do. (open tray, close tray, at the end of the travel – stop, etc.).

Mechanically you will need to determine the drive system: rack and pinion, worm, etc.

Just some food for thought. . .

Good Luck,

pmurray

[gar]

050904-1448 EST USA

Sanghera:

pmurray gave you a lot of useful information. Check out the references he provided.

You need to give us a better idea of what you want to do. Do you just want to play for a learning purpose? Do you have a product you want build? Or is this some sort one time machine?

In my outline of things to consider I did not say much about the pushbutton. In reality in many cases you would not want to hold the button until you were off the limit switch. On the other hand if it is for machine control, then you would want a two button system with an anti-tie-down function.

At least you would want a debnounce function. You would probably want positive edge triggering when at a limit that would not require holding until you were off the limit.

There are many little pieces to your problem that you may want to solve individually.

For example make a dc motor control circuit and current sense detector that can differentiate between running current and an overload current. You can use a DPDT relay (double pole double throw) to control direction, or for that matter a DPDT switch.

For a current sensor a resistor in series with the motor that produces maybe 0.2 volts at running current, and a threshold detector at maybe 0.3 v to detect overload. You might inhibit the over current detection for a short time to allow for motor starting inrush.

When you enter a limit zone it may be important to slow down the motor to reduce impact at the mechanical stop. But when leaving and while still in the limit zone you would want to go fast.

There is much for you to learn here. A summary of your background would be useful.

.

[Sanghera]

I don't have much electronic circuit knowledge, but I am learning slowly from your replies. This is something just to fool around with. To learn how to automate something with dc motors to get them to do what you want. The links are all great and I have a lot of reading to do.
Thanks for the help.
I really appreciate it.

[Sanghera]

I figured out how to reverse the motor with the relay. Now I need to figure out how to make the relay hold until the limit is it with a single push button. For example. The device is closed, I push the momentary push button, and this switches the relay over and make the motor run, but, the contact is broken from the switch as soon as I let go, so how is the relay going to hold until the next limit is hit, and I push the motor again to reverse it? Do any of you Electrical Engineers know this one? I guess it needs to latch or something. I'm not sure.
Thanks.

[Al_The_Man]

You need to use a 'retaining' contact on the relay, this is a normally open contact that is in parallel with the push button.
In series with this 'string' could be an NC limit switch that would open when the end of travel is reached.
There is obviously more needed for the same reverse action.
Al.

[Sanghera]

Would this have the same effect as a Flip flop. Would you be able to draw the retaining contact circuit in paint shop so that I can get a clearer understanding? Thanks.

[Al_The_Man]

Would this have the same effect as a Flip flop. Would you be able to draw the retaining contact circuit in paint shop so that I can get a clearer understanding? Thanks.

I could draw it up but you may find it quicker on the web as it is a standard type of circuit (starting/retaining), you would need a spare N.open contact on the relay and just wire it across (parallel) to the Push.b. It has the effect of a latch.
Al.