Mc-70 Control “coasting”

Thread Starter

Upnorth4

Joined Nov 21, 2020
10
I have an MC-70 harvested from a treadmill. I am hoping to use it to power a milling machine.
I have it set up on the bench and it runs as expected when I put a 10k pot as a voltage divider on the “W-H-L” terminals. Two of the features I want to incorporate are a “pause switch” and a “jog” button. I have tried to do this by putting in a switch in the “W” terminal and a button to momentarily bypass that switch.
Issue is:
When the voltage is cut off from the “W” terminal the motor keeps running, at the last “commanded speed”
I would have thought the voltage going to zero on the wiper would cuts the power off. It doesn’t....
Is this part of a soft Start circuit? A bit of logic built in to prevent a sudden stoppage of a treadmill? If so can it be defeated?
 

MaxHeadRoom

Joined Jul 18, 2013
22,036
When you open up the W input from the pot slider, the op-amp input is now floating and it is also influenced by feedback from the motor current monitor circuit.
This is why it has to be taken to L to truly zero it.
Try momentarily shorting the W to the L to stop instead of opening it.
Also on the MC-60 there is the way to defeat the requirement to take the pot to zero before the motor will start, not sure if it is the same for the MC-70?
Max.
 

Thread Starter

Upnorth4

Joined Nov 21, 2020
10
Thank you for the reply,
I would very much like to have the jog and pause features running. Would there be any harm in setting up the “pause switch” to short the “w” to the “L” on the pot?

the Mc-70 does have the “R19” that seems to be commonly cut to defeat the “belt stopped on treadmill feature”
Some user have posted that cutting it didn’t have the desired effect. Seems a bit odd to me that R19 is through hole, is it odd? Most other components are SMD.
 

MaxHeadRoom

Joined Jul 18, 2013
22,036
IIRC R19 had the outline around it, designating some kind of selection, option or function.
It should not harm anything, shorting the W to L as the circuit is a fairly high impedance input.
Edit: actually it can cause a problem if the pot is at the high end, short the P.S.!
You would need to open the high side at the same time.
Max.
 

Thread Starter

Upnorth4

Joined Nov 21, 2020
10
I’m stuck at work now, very eager to get home and try this out.
Given that I am defeating the circuit I’ll try it while leaving all else alone, hopefully it works. If the “zero before start” gets annoying I’ll pull the resistor.
I have also noticed what I suspect is the current limit seems low, if I crank up the pot quickly it cuts out. The only “load” on the motor is the massive stock flywheel.

Max, I have read many of your posts and want to the say a whole hearted THANK YOU. You knowledge and patience have helped this novice, and is suspect many others, along.

Cheers
 

MaxHeadRoom

Joined Jul 18, 2013
22,036
Do you really need the flywheel, for most non-TM applications, it can be removed.
Especially a mill.
The later MC2100's were a little better for this, PWM control.
Max.
 

Thread Starter

Upnorth4

Joined Nov 21, 2020
10
Do you really need the flywheel, for most non-TM applications, it can be removed.
Especially a mill.
The later MC2100's were a little better for this, PWM control.
Max.
The flywheel will be removed. In fact I won’t be using this motor if I can avoid it. The milling machine’s stock motor is 90vlt DC brushed, same as this dreadmill motor. It is doing fine, it’s the cheap controller that failed.
In time I may upgrade to a MC2100, I’ve already been reading up on the PWM input possibility with the CNC controller, Mach3 software.
But.... for my needs the ability to tap holes without buying a taping head far outweighs controlling the speed from the Gcode.
This mill has a quill so I can control/allow the “feed” through that and reverse the “speed” at the end of the hole.
it is possible to “rigid” tap (no float in the spindle or quill or tapping head) IF you can index the spindle to the proper threads per inch. I have no desire to delve that far into spindle control....

I would like to tap a couple hundred holes in a fixture plate, so I am taking the opportunity to set up a jog, pause and reverse switch now that the stock motor controller failed.
 

MaxHeadRoom

Joined Jul 18, 2013
22,036
I also made up a small qty of 50mm x 50mm PWM boards for the MC2100 series,
The problem with reverse on a T.M. controller is that you generally have to use a reversing relay on the output.
For synchronized tapping you need to gear the spindle off of the Z axis motor.
Max.
 

Thread Starter

Upnorth4

Joined Nov 21, 2020
10
I also made up a small qty of 50mm x 50mm PWM boards for the MC2100 series,
The problem with reverse on a T.M. controller is that you generally have to use a reversing relay on the output.
For synchronized tapping you need to gear the spindle off of the Z axis motor.
Max.
I think I have reverse working okay with a hefty DPDT switch. I will use restraint and not throw it while the spindle is turning, I suspect that would cause an overload.

I have tapped on CNC mills where the spindle isn’t geared to the Z, if it has an encoder the speed is know accurately enough to allow the code to feed z at the right feed. It is best to use a tapping head that has “float”, this gives the tap room if there is a small difference in the speed and feed.
I have done it without the head and I works, but it isn’t comfortable.
with a quill however it’s no problem you just need to be able to stop and reverse the spindle manually. Proper taps are also crucial, spiral machine taps that clear the chip up and out.
the only time I have seen the spindle geared to feed is on a lathe, and they often have a large gear box with many combinations.
 

Thread Starter

Upnorth4

Joined Nov 21, 2020
10
This is the circuit I have right now, how would I modify it to connect the “w” lead to the “l” lead when the “pause switch “ is thrown, and then disconnect it when the “jog” momentary switch is pushed?

I’m thinking some sort of arrangement of DPDT switches but I can’t seem to get my head around it.....
 

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MaxHeadRoom

Joined Jul 18, 2013
22,036
For the switch, you could use a SPDT that would connect W to L and open the H side. (change over switch)
For different combo's there is the panel mount lever switch with spring return from L&R to centre and can be made up by add on switch sections of N.C. and N.O
Telemecanique used to make a line of these which were very expensive, but now the Chinese are selling a knock-off of them.
You have to be careful on the reverse that the motor has stopped.
Max.
 

Thread Starter

Upnorth4

Joined Nov 21, 2020
10
Edit: actually it can cause a problem if the pot is at the high end, short the P.S.!
You would need to open the high side at the same time.
Max.
Just saw this edit, after I sketched out the above.
I missed that, indeed if the wiper was close to the “H” when it is directly connected to the the “L” it would bypass most, or all, of the resistor, a short circuit.
back to the sketch pad.....
 

Thread Starter

Upnorth4

Joined Nov 21, 2020
10
Final revision to the drawing...
It works great, ran the mill for a few hours over the weekend and the switches are exactly what I wanted.
the trick was the jumper around the first switch when in the “pause” position.
The controller does have the “zero volt before start” feature but that is easily defeated by cycling the pause switch.
It also has a “soft start/stop” which isn’t a big issue, may even extend motor and controller life. It may create a problem for tapping but I think the solution will be to use a steep ratio on the belt drive, so that the few revs on the ramp don’t add up to much on the spindle....
Appropriate drive ratio may be key to the life of the controller. Avoiding low rpms may take the load off the 10watt ceramic resistor, it gets hot enough to heat the room.... would it be reasonable to try to add a heat sink to help it out?

For a total novice this was a very fun project with some big rewards.
 

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