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Automatic gearbox with 20 speeds
« on: October 12, 2022, 02:13:57 AM »
Hello guys,

i am trying to retrofit an old Mikron-Wf3  mill with CSMIO-A.

I have almost finished the project except for the automatic gearbox.

The gearbox shifting mechanism consists of 3 small dc motors, which turn in both directions, and each one has 2 hall sensors to give
feedback of reached position. So a given combination of moves selects the desired speed.

So I would like your input of how should I draw a plan of attack.

Also is it possible to change the speeds using an S command from G-code?

Thanks in advance for your input.

Tasos

Offline Tweakie.CNC

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Re: Automatic gearbox with 20 speeds
« Reply #1 on: October 13, 2022, 01:58:17 AM »
Yes it would be possible to change your speeds from the S command within your GCode.

Obviously everyone would look at this task differently but my take on the solution would be as follows…

Firstly do plenty of research to see if a ‘ready made’ controller is available to do the job. It could save you a lot of work.

Because I have them and use them in various applications I would use four Microchip PIC microprocessors (with suitable drivers) to control the gearbox operation. They are cheap and easy to use and have outstanding and long term reliability.
I would use one PIC chip to control and position each of the 3 dc motors and a fourth PIC chip to disseminate the PWM signal (generated from the S command within the GCode) and decide the exact position of each of the 3 motors based on the mark/space ratio of the PWM. There may be some learning to do (using and programming PIC microprocessors) but it is not rocket science and there is plenty of online help and forums detailing PIC chip usage.

Alternatives could be the Arduino (It would be a bit slower in operation than using a number of dedicated microprocessors operating in harmony but it would still be a good solution and plenty fast enough for this application) or one of the many available Programmable Logic Controllers (PLC) which are often used with CNC tool changers.

Whatever you choose it can be done and it would certainly be a fun project.

Tweakie.
PEACE
Re: Automatic gearbox with 20 speeds
« Reply #2 on: October 13, 2022, 04:04:00 PM »
Thanks for the answer Tweaky.

You have certainly given me a lot of good ideas. The idea of using PIC microprocessors is very appealing.
And I have already though of using a plc since I have been working with the damned things for over 15 years.

My first though, was to use the current layout and utilize 2 relays per motor (since we want the small dc motor to turn both ways)
and connect them to CSMIO, and try to use LUA or PMC Editor for programming.

In my screen I could have a 'list control object' and select gearbox speeds from that.

Anyway, this will be a fun project.

Thank you,

Tasos
Re: Automatic gearbox with 20 speeds
« Reply #3 on: October 13, 2022, 09:55:13 PM »
Hi,
I have some ideas but it rather depends on the DC motors, and the characteristics of the gear movement.

Do the motors run until the gear is engaged and then stop?  Does the motor when stalled like this draw a great deal of current?

My idea is that using one of Machs outputs a motor would be driven CW say, until the gear engages and then stalls as its up against a stop. After a short period I
would assume the Hall sensor would detect that the gear has engaged and that signal would turn the current off to the motor. Thus the motor might have to withstand
say a second or two of stall current. Is this possible?

If that is the case then using six Mach outputs could control all three DC motors. Your controller would require six spare inputs to monitor all six Hall sensors.
Then the control of the gear select is all Lua code...easy.

Craig
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'
Re: Automatic gearbox with 20 speeds
« Reply #4 on: October 14, 2022, 01:18:29 PM »
Thanks for the reply Craig.

When I bought the mill, I was fortunate enough to resurrected and use it for quit some time before the heidenhain control started acting
up.

During that time I logged the behavior of everything in case of a breakdown.

The gear change mechanism works as described bellow:

Once you select a given speed the dc motors run in the required direction sequentialy (e.g. motor 1, 2 turns cw and motor 3 ,1ccw)
As soon they touch the hall sensors the required times, they stop immediately, so there is no loading of the motors.

Now there is catch in the above configuration. What happens when the gears bind up and the motor cannot finalize their move?

The solution to that problem is an integrated circuit, which to my understanding, by measuring possibly the current draw of the 3 dc
motors does the following:
1. A speed change command is given
2. The dc motors do their thing
3. If gears bind up then the circuit would sense that, and activate a 24V to give feedback to the plc to disable the motion of the dc
    motor.
4. Then it would return that motor to it's original position.
5. Activate the spindle motor momenterally
6.Then go to step 2 and rinse and repeat until gear change is complete.

So all in all, I need 6 inputs for the hall sensors + 1 input for jammed gears sensor
and of course 6 outputs for the relays driving the 3 dc motors both direction (I have an expansion board with just enough spares).


So all should be left to be synchronized by LUA code...(not that easy for a lua newbie like me)

One thing I don't know, if I will be able to use S commands for speed selection.

Anyway, thank you guys for your time and effort you put in your answers and one thing is certain, that this is a fan project.

Tasos
Re: Automatic gearbox with 20 speeds
« Reply #5 on: October 14, 2022, 04:52:35 PM »
Hi,

Quote
Once you select a given speed the dc motors run in the required direction sequentialy (e.g. motor 1, 2 turns cw and motor 3 ,1ccw)
As soon they touch the hall sensors the required times, they stop immediately, so there is no loading of the m

This is the critical point.

If a DC motor is powered....what stops it?

Does for instance, once the gear is selected, does it have a mechanical stop that prevents the motor from turning or otherwise overdriving?
If the motor stalls because its traveled as far as it can does it draw excessive current? How long could that situation be tolerated?

There is little point in trying to devise a control strategy until you know EXCATLY how the controlled parts, namely the DC motors operate.

Just to flesh out an idea......which may or may not be applicable depending on the answers to the questions:
Imagine the power supply to each motor being current limited, such that should the motor stall by virtue of it coming up against a mechanical stop
OR the gears not lining up internally such that the motor cannot complete the gear change without the spindle rotating a small amount to allow the gears to mesh,
then the current would be low enough that no damage be done, at least for 20 seconds or so.

Electric windows in cars work that way. If the window stalls, ie it has reached the end of travel OR some unfortunate has his arm stuck in the window the motor will
stall thereby drawing more current, which is measured by the driver circuit and turns off the supply to the motor. This saves the motor and wiring from damage
but also provides a measure of safety should someones arm be caught.

I imagine a control setup something like pictured. I have shown only one of the gear select motors. The selection/synchronisation can be handled programmatically
by Mach or by an Arduino.....whichever seems easiest.

Craig
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'

Offline MN300

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Re: Automatic gearbox with 20 speeds
« Reply #6 on: October 15, 2022, 07:58:33 AM »
It may well be worth the effort to build a manual control panel with 6 pushbuttons for the motors and 6 LEDs for the hall sensors. A small bit of LUA code could display the desired speed and wait for an operator response to continue. This would allow you to learn learn how often the planned sequence fails, either due to gears not meshing or some other motor stall.
Information gathered by manual testing will help you design the automatic control. If there are failures an automatic sequence won't be able to handle you will have to plan for manual intervention.
If the motors can tolerate a stall that condition could be detected by timing the cycle.

I'm curious about how 20 speeds can be set by 3 motors going to 2 positions. That's only 8 combinations.

John
Re: Automatic gearbox with 20 speeds
« Reply #7 on: October 15, 2022, 11:28:41 AM »
Hello John,

thanks for the reply.

When I had the original controller operational I had tested the operation of the gears.
The board of the gears has some kind of ''fail safe"  so when the gears bind it outputs a signal of 24V, so the controller stops the motors
and backs up the motor. Then it starts the spindle motor momentaraly, (the gears now stop at a different position) and tries again the last sequence.

With the heidenhain controller it was working perfect.

The motors aren't going in only 2 positions. They can go 2 times past every sensor, and it has a 2 speed spindle
In fact it has 17 speeds + neutral (spindle low speed setting)
and another 3 speeds (spindle high speed).

Thanks Tasos
Re: Automatic gearbox with 20 speeds
« Reply #8 on: October 15, 2022, 11:46:53 AM »
Hello Craig,

the dc motors are very small  (24V, 35W, 1,45A).

I don't know if sth stops the motor, but I assume that the gear train is heavy enough, that if the power to the motor is cut it should
stop instantly.

In the original configuration I had noticed that when the gears did bind, it instantly reversed the motor so I must assume the motors can handle some overcurent for  a couple of seconds.

So i will try to implement that operation into LUA and see where this path will get me. Just to play it safe, I will remove the motors
with the sensors as an assembly so as to test the operation fist.

Thank you all for your answers,

Tasos