Morgan build in INDY – DRV8825

[Steve]

Hey Quentin

Still stuck. Tried several things after we spoke.
Pretty sure I’ve got the pairs identified on my 6 wire.
Identified R/Y and B/O. Does it matter the order? Is Y/R, O/B the same?

BUT I couldn’t get any voltage readings off the DRV8825s.
So, my replacement chips arrived.

I know for the A4988 chips the target for the flat side of the pot is supposedly 10 o’clock,
with the power supply on the left and the potentiometers to the right side.
I’ve heard the range is from 7 to 4, for min to max for the flat-side position on the A4988s.

For the DRV8825s – what is the range?

Neither of my chips have the stops for the pot. The DRV8825s are oriented differently, with the pots
on the left side, towards the power. That suggests the “sweet spot” for the flat spot on the pot is
either 10 o’clock or 4 o’clock.

Any help?
Where is the flat-side of your DRV8825s turned?
Do you know the range of pot movement for the DRV8825 ? with stops ?
Where should the flat-side of the pot be located to start on the DRV8825s ? I know it can be adjusted, but typically there is a starting point.

Any help appreciated.
Want to figure this out before I plea for help with motors that won’t turn.
Thanks

[Quentin Harley]

If you measure from the 0v input to your board to the plate on top of the pot, what do you get?

Current is two times the voltage:

0.4V = 0.8A
0.7V – 1.4A

Keep the amperage under the current rating of the motor for safety.

Also, be sure to insert the driver the right way around. the pot is on the other side with the DRV8825 boards.

The A1 A2 B1 B2 marked pins go towards the motor connection.

The regulation voltages are there even if the motors are not activated. Adjust before the first home command.

The pots do have stops, but they are weak stops, and will not stop the contentious rotation effectively. Try not to run over it too much, because you might eventually damage the wiper. This is why I measure while I adjust by connecting the positive lead of my multimeter to the screwdriver while I turn. You get immediate feedback while you adjust.

The midpoint is with the flat side towards the edge of the board (towards the two solder tabs next to each other)

Hope it helps a bit

[Steve]

Hi Quentin

Okay, thanks for response.
First, don’t know where “the 0v input to your board” is located. Is that VMOT ?
Been measuring from pot wiper to GND on corner of board.

I understand the DRV8825s are oriented with pot towards power and A4988s to the right away from the power, on the long side of the board.

I’m using a wooden post to turn instead of metal screw driver.

As to your mid-point, don’t understand about the “solder tabs”. Can’t find.
I see two black resistors? above, aligned on the short side, and two to the right, on the long side and pin tops below.

From my interpretation, the range you describe is either 6 o’clock til 12 -flat to the edge or 9 o’clock til 3 o’clock with flat to top long edge, min to max. ??

What about the wiring?
Is R/Y and B/O any different from Y/R and O/B or R/Y and O/B ?
Do the two coils need to be polarized, in sync?
Thanks.

[Quentin Harley]

If you look at the picture you posted, the pot has three solder connectors, two on the edge of the board at the top and another one to the inside.

When the flat side is towards those two it is the pot’s mid point.

I use a metal screwdriver on the pot, and touch the multimeter lead to the screwdriver. Easier to set.

Regarding the connection to the motor, you have to ensure that one of the coils connects to A1 A2, and the other to B1 B2.

To find the coils check for a low resistance between the two wires that form a coil. The others will have no electrical connection to those two.

[Steve]

Quentin

Fantastic. I understand what you’re saying.
I’m smarter. I now know what you meant by “solder tabs” – thanks.

From this, I deduce, the mid-point is at 9 o’clock.
So, the range would be approx 4 o’clock to 1 o’clock, based on my understanding it’s about a 270 degree range of motion for the pots.

Is this a good place, 9 o’clock, to start the pot setting when setting up the motors?

Where are your flat-sides setup on your DRV8825s ?

Yes, two pairs. “A” pair and “B” pair of wires.
How do you tell which wire in the “A” pair is 1 or 2 ? Does it matter ?

I assume my 2 pairs of wires are connected properly to the motor A1A2B1B2. I’m simply using the
4 pins that are next to the driver chips. Again, oriented as “A” pair – Red, Yellow,
“B” pair – Blue, Orange.

MY Ramps image:

What did you mean by “0v input” ? Is that the VMOT ?

Think that’s three questions to answer.
Thanks

[Quentin Harley]

Starting at mid is ok, but aiming for the rated motor current minus 0.2A is a better start point. If it is 1.7A rated, adjust for 0.75V (1.5A) and work down. You want the lowest current still delivering good movement. Then add 20%.

My flat sides? I don’t know. I cannot see my pots… I set it by voltage though a small hole in the cover.

Switching the A wires will make no difference, except that the direction will change. If the motor turns the wrong way, swap either the A or B ones. Swapping both pairs will have no net effect obviously…

Ov is the (-) on the power connector to the board.

[Steve]

Yeah, well that’s one of my problems. I don’t have the motor specs or datasheet, but I believe they are 1.2A. At least that was the guess from a Sanyo Denki rep I spoke with.

Guess I haven’t seen your setup recently, didn’t realize your ramps was under cover.

Which way is “the right way” ? Does + go to the left, and – go to right ?

So, with your approach you use multimeter against top of pot and the – (negative) input – interesting.
That may be easier than using the GND pin and possibly crossing pins or something bad shorting the board. I’ve already bought 4, two didn’t work to begin with.

Have you had or heard of any issues with 1/32 step with the unaltered DRV8825s? Without any mods, should I knock it back to 1/16 ?

If I can’t get the DRV8825s to work, can I go back to the A4988s for X and Y ? Besides the difference in steps, 200 v 400, and that A4988s can’t do 1/32 step, any other considerations ?

Thanks again – now I’m ready to finally get those .9s moving.

[Quentin Harley]

My first Morgan did its first prints on 4988. Not a problem – just have better results personally with the DRV8825.

If you look on the RAMPS board just behind the power connector, you will see the polarity.

If you are referring to which way to turn the pot… you will see when you have the multimeter attached. If it goes the wrong way – turn it the other way 😉

Q

[Steve]

Quentin
No, I was asking about the arm movements you mentioned regarding the “motor turns the wrong way”.
Which way is the right way ? Does + go to the left, and – go to right ?

Would you recommend changing the wires or using the INVERT_X_DIR to change direction?

For the pots, I understand counter-clockwise is less V and clockwise is more.

Any issues with 1/32 step with the unaltered DRV8825s?

Regarding polarity, I was wondering if the 2 coils of the motor needed to be “synced”.
Thanks

[Quentin Harley]

Looking from above, the arms turn anticlockwise when homing.

1/32 unaltered works fine, but has a granular feel to the movement. You can run it unaltered if you feel uneasy doing the mod. The effect is not critical.

The coils are exactly out of sync by design. The driver takes care of the movement.

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