Stepper Motor Hat compatability

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seanpo
 
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Stepper Motor Hat compatability

Post by seanpo »

Hey there,

I'm looking to get an Adafruit Stepper Motor Hat for raspberry pi (https://www.adafruit.com/products/2348) but wanted to make sure it would be compatible with my stepper motor first. I'm looking to get a 17pm-k054-g7ws (http://www.aliexpress.com/store/product ... 58607.html) - and I believe it will work but I wanted to make sure.

Also a couple other questions:
First, on the stepper motor page (https://www.adafruit.com/products/324) it says the holding torque is 28 oz*in or 2 Kg*cm. Shouldn't this be oz/in or kg/cm? If it is just a different notation I'm a little confused because google says 2 kg/cm equates to 179 oz/in. I was hoping to get my stepper motor from Adafruit, but I need at least 100 oz/in holding torque. How often do those stepper motors get re-stocked, since they are out of stock?

Lastly, if you are stacking several stepper motor hats, do the stepper motor hats need their own power supply or can one power supply give power to all the hats?

Thanks!

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adafruit_support_mike
 
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Re: Stepper Motor Hat compatability

Post by adafruit_support_mike »

The ounce*inch or kilogram*meter units are right. It means there's a tradeoff between weight and distance.

If you have a motor with 10 oz*in of torque, it can lift 10 oz with a lever arm 1" long, or it can lift 1 oz with a lever arm 10" long. The product of ounces and inches is the same either way.

A force stated as oz/in gets stronger the farther it goes. A motor whose force was described as 1 oz/in would be able to lift a ton if you gave it a lever arm half a mile long.

The motor you linked has a current rating of 1.2A per phase, and the Motor Hat can do that.

If you stack Motor Hats, you can connect them all to the same power rails, but will need to connect those rails to each Hat's terminal blocks. Each hat has a pair of TB6612 H-bridges that take power from the terminal blocks and deliver it to the motor coils.

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adafruit_support_bill
 
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Re: Stepper Motor Hat compatability

Post by adafruit_support_bill »

The Ali Express motor is not a good match for the HAT. You have to be careful when interpreting stepper motor specs:
Current / Phase: 1.2A
This is how much the motor is rated to handle. NOT the maximum the motor will draw.
Drive voltage:: 5V
This is just a recommendation. It is pretty meaningless. Especially in light of the resistance spec:
Resistance: 2.5 Ω
This is the most important parameter for determining compatibility between a motor and the HAT. Since the HAT is a constant-voltage driver, the maximum motor current draw will obey Ohms Law at slow step rates. If you try to drive this motor with the suggested 5v, you will get a current of 5v/2.5 ohms = 2A. This exceed both the motor and HAT current ratings.

https://learn.adafruit.com/all-about-st ... he-stepper

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seanpo
 
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Re: Stepper Motor Hat compatability

Post by seanpo »

Thanks for the responses! That makes sense - so would something like this be better (running at 5V)? https://www.pololu.com/product/1472/specs
Running at 5V / 7.4 ohm = .68 A

I don't see anything about the motor's RPM on the specs, is there a way to calculate that? Also is there a calculation to find what the holding torque would be running at 5V as opposed to the 7.4V in the spec?

It's hard to find the right stepper motor in the sea of them on the internet.

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adafruit_support_bill
 
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Re: Stepper Motor Hat compatability

Post by adafruit_support_bill »

That would probably work. But I don't think the performance would be very good. There is a pull-out torque vs step-rate curve, but it is with a drive voltage of 30v (using a current-limiting driver). https://www.pololu.com/file/0J672/SY57STH56-1006A.pdf

Using a constant voltage driver like the HAT, the pullout torque will be somewhat lower than the holding torque. This is because the inductance and back EMF from the coils limits the current that can be applied at the start of each step. And since torque is roughly proportional to current, the torque will fall off rapidly at higher step-rates.
https://learn.adafruit.com/all-about-st ... ve-the-law

For more detail, these are informative links:
http://www.applied-motion.com/news/2015 ... out-torque
http://www.BANNED.com/step-motors/engine ... ationship/
I don't see anything about the motor's RPM on the specs,
RPM is a controlled by the step-rate. With the HAT, the I2C and Python overhead tend to limit the top step-rate. I haven't done any formal tests on it, but ~50 RPM is what most people report using a 200-step motor.

And due to the reasons discussed above, torque falls off as the step-rate increases, so top speed will also be limited by available torque.

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seanpo
 
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Re: Stepper Motor Hat compatability

Post by seanpo »

Ah, I see. Thanks for the explanation, it is dense but helpful. Unfortunately I'm still unsure of what exactly I should be looking for in my stepper motor to maximize performance with the HAT while maintaining at least 100 oz/in holding torque - nonetheless finding where on the internet to purchase said motor. I imagine one with high enough resistance to be able to run at a higher voltage (12V?)? Or is the running voltage not so important, so much as the best combination of voltage and resistance to get closest to 1.2A without going over? Do you have any suggestions in terms of what to look for and how to find that motor?

Thanks so much for helping me with this so far, I've learned a lot in the process.

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adafruit_support_bill
 
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Re: Stepper Motor Hat compatability

Post by adafruit_support_bill »

at least 100 oz/in holding torque
So that is your primary requirement? Constant voltage drivers like the HAT do fine with holding torque. But their performance falls off faster with increased step-rate when compared to a constant-current driver.
..the best combination of voltage and resistance to get closest to 1.2A without going over?
Using a constant voltage driver, this is what you are looking for. Watts is volts times amps and your holding torque (as well as heat) will be proportional to the watts. Given that our #324 motor is rated for 28 oz/in at 12v/350mA, 100 oz/in with 4 times the current sounds achievable - provided that you can find a motor with 10 ohm windings and a 1.2A or higher current rating.

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seanpo
 
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Re: Stepper Motor Hat compatability

Post by seanpo »

Upon further searching, I found this motor - http://www.alibaba.com/product-detail/5 ... 05954.html

It seems to fit the bill? Though the holding torque is 155 oz-in, so I'm once again not sure if that satisfies my oz/in requirement. It says the current / phase is 1.0 - 4.0, is that the current rating or is the "1.1 A phase current" the current rating? If it is the latter, is 1.1A close enough to 1.2A to make it okay to run at 12V?

Thanks,
Sean

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adafruit_support_bill
 
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Re: Stepper Motor Hat compatability

Post by adafruit_support_bill »

You need an alibaba account to view that link.

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seanpo
 
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Re: Stepper Motor Hat compatability

Post by seanpo »

Really? I don't have an account and it loads up fine. I've attached a screenshot of the item description.
Attachments
motor.png
motor.png (127.05 KiB) Viewed 318 times

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adafruit_support_bill
 
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Re: Stepper Motor Hat compatability

Post by adafruit_support_bill »

OK. I eventually got through to see the specs. Unfortunately, the are somewhat contradictory. According to the first table, the model is 57HJB455-30 and the phase resistance is 10 ohms. Sounds good so far. But further down the page there is this table which tells a different story (sorry for the formatting). The numbers for the 57HJB455-30 model don't agree with what is posted at the top of the page (for example, the phase resistance is listed as 1.8 ohms).

I would ask for some clarification before purchasing.

Model Step angle Number of phase
Current
/phase
Resistance
/phase
Inductance
/phase
Holding torque Rotor inertia Wiring diagram Motor mass Motor length
A Ω mH mN.m g.cm2 kg mm

57HJB451-21 1.8° 2 2.0 0.9 3.0 750 275 a 0.65 51
57HJB455-30 1.8° 2 2.0 1.8 6.3 800 310 a 0.71 55
57HJB478-26 1.8° 2 4.0 0.6 2.2 2100 470 a 1.10 78
57HJB4100-28 1.8° 2 4.0 0.7 2.3 3500 620 a 1.25 100
57HJB641-21 1.8° 4 2.0 1.5 1.5 400 120 b 0.46 41
57HJB655-25 1.8° 4 3.0 0.7 1.2 900 310 b 0.71 55
57HJB663-25 1.8° 4 3.0 4.0 6.4 1200 350 b 0.80 63
57HJB678-30 1.8° 2 3.0 1.2 2.2 1500 470 b 1.10 78

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seanpo
 
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Re: Stepper Motor Hat compatability

Post by seanpo »

I tried reaching out to the manufacturer, but I don't have a lot of hope for them getting back to me with a useful reply. There is also this motor - https://www.linengineeringstore.com/pro ... -5718X-05E - but it looks like it would have to be run at 5V.

All this investigation has led me to the potential conclusion that I probably shouldn't be using the Pi HAT for what I'm trying to do. I'm attempting to build a sort of modified CNC machine - which in viewtopic.php?f=24&t=78493 you suggest using an Arduino and gShield. I'd still like the Pi to be the main brains of the operation, but it seems there are options out there for sending gcode from a Pi to an Arduino. Presumably switching to an Arduino would open up my options in terms of which motor to buy. What would be the main specs to look for in a stepper motor if I were to go with that approach? Do you think that is the best option?

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adafruit_support_bill
 
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Re: Stepper Motor Hat compatability

Post by adafruit_support_bill »

I think that the GShield or theTinyG CNC controllers are probably better suited to your application. These both have current limiting, so they can drive a much wider selection of motors and deliver higher performance than either our HAT or motor shield.
https://www.adafruit.com/search?q=synthetos&b=1

With one of those boards, your original motor selection would be fine.

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