# Why does delta robots have high precision?

Posted by Glentved
 Why does delta robots have high precision? August 07, 2014 10:00AM Registered: 6 years ago Posts: 4
I am building my own delta 3D printer, now I just ordered the pulley wheels. I use GT2-20teeth
But when I calculate the pulley wheels circumference with the steppers 200steps, I only get about 0,2mm mechanical precision, + the "gearing" from the arms i get a precision on about 0,15mm.

So how can these delta printers have layer hights on 0,1mm, I have even seen down to 0,05mm?

Do they have a gear on the steppers before the pulleys? or should i have bought a stepper with 400 steps, 200 is more standard (cheaper)?

Dont say micro stepping, a stepper can not position its rotor between 2 poles.
 Re: Why does delta robots have high precision? August 07, 2014 02:02PM Admin Registered: 13 years ago Posts: 13,166
Quote
Glentved
...
Dont say micro stepping, a stepper can not position its rotor between 2 poles.

... as long the motor's torque is high enough and the friction and holding forces are negligible compared to the torque, microstepping IS the gearing

Viktor
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 Re: Why does delta robots have high precision? August 07, 2014 06:59PM Registered: 7 years ago Posts: 240
Quote
Glentved
Dont say micro stepping, a stepper can not position its rotor between 2 poles.
That's known as half-stepping, and yes they can do it. You lose half your torque when doing it.

As for accuracy, you have to account for the diagonal, not just the pulley.
The original Rostock was estimated to do 30 steps per mm in the center and higher at the edges, that's .03333333 mm. That's without half stepping and it was using a large diameter belt pulley. We can do much higher with smaller pulleys. While we can go higher, there is slop in parts you have to account for, so even though your drive mechanism can go down further, that doesn't mean you will actually get it.
 Re: Why does delta robots have high precision? August 08, 2014 01:26AM Registered: 6 years ago Posts: 4
Okay, thank you for your input. I did not believe that a stepper could actually position between 2 poles.
Now I can continue my printer design with more peace in mind.

It has a nice size. Build volume of Ø280x750mm, cabinet made of steel to make everything stiff enough.
 Re: Why does delta robots have high precision? August 08, 2014 04:45AM Registered: 11 years ago Posts: 7,615
Quote
sheepdog43
You lose half your torque when doing it.

Motor torque is exactly the same, no matter which microstepping you use. Only half-stepping can be an exception.

What you don't get is less deviation from the target position when applying a momentum on the motor shaft ( = a force to the belts). That's why you should prefer smaller pulleys over higher microstepping.

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 Re: Why does delta robots have high precision? August 09, 2014 07:27PM Registered: 7 years ago Posts: 240
Sorry, but that's simply not true.
Minebea (a major Nema 17 manufacturer) claims a 15% loss for half stepping and 30% loss for microstepping.
Wikipedia claims up to 70% for half stepping, but makes no mention of torque regarding micro-setpping.

As per:
[www.nmbtc.com]
[en.wikipedia.org]
 Re: Why does delta robots have high precision? August 10, 2014 04:39AM Registered: 11 years ago Posts: 7,615
From the Mineba page: "Microstepping produces roughly 30% less torque than dual phase full stepping."

"dual phase", that's half stepping and the exeption I mentioned. People should stop mentioning dual phase, because that's an ineffective way of driving a stepper motor and confuses people a lot more than it helps. Dual phase works with half-stepping, only. Not with full stepping and not with microstepping.

Looking at the situation with some simplification it's simple: At full step position, full current goes through one coil and no current through the other coil. Doing one full step switches the current from one coil to the other coil, so you have 100% current on the other coil, no current on the first coil (and the motor advanced by 1.8 deg). This difference of currents going through the coils makes the torque.

If you do 1/16 microstepping and you're at full step position, you have the same situation: 100% current through one coil, no current through the other coil. After 16 microsteps, you have also exactly the same situation as after doing one step at full stepping: full current through the other coil, no current through the first coil, and accordingly exactly the same torque.

In reality the situation is a bit more complex, because currents flow not only in one, but, dependent on the step position, in both directions through the coils, still currents as well as torque are precisely the same, regardless wether you do one full step, 8 times a 1/8 microstep or 32 times a 1/32 microstep. You can read up this in stepper driver data sheets, they usually come with current/microstep diagrams, which look similar to sine waves at higher microsteppings.

The difference with microstepping is, you have these 7 or 15 or 31 intermediate steps. No more, no less.

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 Re: Why does delta robots have high precision? August 10, 2014 07:35PM Registered: 7 years ago Posts: 240
Okay, that makes more sense.
 Re: Why does delta robots have high precision? August 11, 2014 02:45PM Registered: 6 years ago Posts: 26
I looked at DRV8825 and A4988 stepper motors controllers datasheets and both of them at Full step mode uses 2-phase excitation. I do not see what is confusing about it. 1-phase mode uses single coil, while 2-phase uses both coils to at least in theory to provide more torque and it's step positions are shifted by half step compared to 1-phase drive.

Also I do not see a point in "Dual phase works with half-stepping, only. Not with full stepping and not with microstepping.", half stepping does not differ from microstepping. It is the same thing.

About torque it is not that microstepping has less torque, but more that each step can have different torque, because at each position trough each coil runs different amount of current. It can be clearly seen if you look at steps and coils tables in datasheets.
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