Stepper Motor Questions for PRT

[Mayo]

In trying to learn about stepper motors, and I'm wondering if there's a "Dummies Guide" type of book that puts motion control with stepper motors into terms a non engineer can understand?

What would the real world difference be between these two motors as related to running a PRT Shopbot:
PK296A1A SG7.2
704 oz in.
Gear Ratio 7.2:1
Step angle 0.25

PK296A1a SG3.6
352 oz in.
Gear Ratio 3.6:1
Step angle 0.5

From these specs, the only thing I think I understand is the 3.6 is half as powerful as the 7.2?
Are these really the same motor but one is just geared differently?

Is one of these capable of producing smoother curves than the other?

Any recommendations for simple to understand books or web sites explaining motion control for CNC would be greatly appreciated too.
Most web sites I've seen with CNC parts and supplies assume you already know what you're looking at - which is not the case for me!

[blackhawk]

Mayo - Yes, both of these models use the exact same motor. The gearheads are the only difference. The 7.2 will give you twice the resolution and torque as the 3.6. The trade off is that you lose speed the higher that you go with the gear ratio. If you look at the speed-torque graphs on the Oriental website, you can see what this looks like. The 3.6 can go up to 500 rpm, although it will have very little torque at that speed. The 7.2 tops out at 250 rpm.

You just have to match up the motor/gearbox combo with the speed, torque, and resolution that you need.

[Brady Watson]

Mayo,
A 1/2hp motor is capable of doing 1/2hp worth of work. It would be similar to your drill press with step pulleys. You either get RPM and less torque, or torque at less RPM - or like riding your bicycle in 1st or 5th gear. If you wanted higher resolution and low end torque, you would run a 7.2:1 motor; and if you wanted more speed on the top end, you'd run the 3.6:1 motor.

There is some math involved to quantify the differences between the motors and what they will do on a particular system. On newer tools (4G & Alpha) the controller is capable of a conservative rating of 35000 pulses per second, although I have seen systems go over 50000 reliably. This means that the communication speed to send pulses out to the motors is limited to 35000 pulses per second.

Assuming 7.2:1 motors & only one axis moving (not coordinated movement like a diagonal move) and a 25T pinion, the calculation would be as follows:

7.2 * 200 (full steps per rev) * (10 microsteps) = 14400 pulses per revolution

1.25 (dia of 25T pinion) * pi = 3.927" circumferential/linear distance per revolution

Max speed possible per second = (35000 / 14400) * 3.927 = 9.54 inches per second

A 3.6:1 motor would be theoretically twice the max speed.

This does NOT take into consideration the 'real world' - the weight of the gantry, cutting forces OR the torque curve of the motor. This number would be considered max JOG speed with no load - so you would have to analyze the torque curve of that motor and interpolate where the torque curve fell off to calculate max reliable cutting speed. It also does not consider the motor driver power supply voltage, since this can influence the torque curve with higher voltage pushing the curve up higher in the RPM range.

-B

[danhamm]

Mayo, I have a couple of 296 a1a 7.2 left they were on the machine for less than a month. bought 4 and tried them.
Also tried 299 F4.5 with 3/1 belt drives, they cost 207. delivered the a1a cost 257. delivered.

[Mayo]

Thank you Brad and Brady for the explanations.
Is a pulse the same as a step? Or is it X number of pulses equal a step?

What is a step angle? The 7.2 motors have a 0.25 degree and the 3.6 motors have a 0.5 degree step angle.

Based on the explanations, the 7.2 motors are better for my purposes than the 3.6 motors, so I won't be buying 3.6 motors.

Dan - I currently have the 7.2 motors and was contemplating replacing them with something else.

Anyone with the PK296A1A SG7.2 hooked up - can you tell me how much rotational play there is when the motor is powered up? If you clamp or vice grip the smooth part of the pinion, how far can you move it manually when it's powered up? I checked mine with the pinions away from the track.

[danhamm]

Mayo I had 5 to 7 thou. movement on all 4 motors with 30 tooth gears on, this was measured dial caliper. They didn't stay on long.

[Brady Watson]

Mayo,
If your goal is better precision, then I would not buy the SG series motors. Instead, buy a direct drive motor & gear it down with timing pulleys to minimize backlash & slop. A 3 to 5:1 ratio is in the right range.

The 'pulse' depends on your hardware. The .25 deg step angle is at the output shaft. The motor still only puts out 200 discrete full steps. Microstepping breaks this down to 2000 steps per rev (1:1) times whatever gear/belt reduction you have. Pulses are pegged to microsteps, not full steps.

What are you trying to do exactly with your setup?

-B

[Mayo]

I'm wanting to get smooth looking cuts on curves...

I think with the right motion control system and motors, even on my PRT gantry and rails I should get smoother looking cuts than the ones shown in the photo.

[richards]

I agree with Brady that the best drive system would be to use belt-drives. I converted my PRT-Alpha to use 3:1 belt drives (20-tooth pulley on the motor, 60-tooth pulley on the drive shaft). There are probably still some photos on the forum showing cutting results. After installing the belt-drives the "chatter" was almost completely gone.

Another factor to consider is motors. The PK296A1A motors have limited top speed because of their high inductance. The PK296-F4.5A motor has much more speed. It can be wired either bipolar parallel or bipolar-half-coil. Parallel gives full torque. Half-coil gives 70% of full torque. Holding torque is 450 oz*in when wired parallel and 300 oz*in when wired half-coil. With a 3:1 belt drive, torque is multiplied 3X. Either type of wiring should give all the torque that a (modified, i.e. strengthened) Shopbot can use without too much flexing. Using that motor will require that you use Geckodrive (or similar) stepper motor drives and that you use a heat sink attached to the stepper drivers. For best results, you should use a 35VDC linear power supply that has enough capacitance (20,000 uF or thereabouts) to keep ripple to 5% or less. The fly in the ointment is the pulse stream. With 25-tooth pinion gears and a 3:1 belt-drive (and the Geckodrive's 2,000 steps per shaft rotation), each step will move an axis 0.0006545 inches. That means that it will take 1,528.88 steps per inch of axis movement. To run at 10-inches per second, the pulse stream will have to handle at over 15,000 pulses per second. You'll need to check with Shopbot to verify that the controller that you use can supply at least that many pulses per second. I've run that motor very successfully at 750 RPM (12.5 revolutions per second). That would jog an axis at a little over 16 inches per second, but that would require at least 25,000 steps per second.

I've worked with several Shopbotters who "hot-rodded" their machines. It's an intensive process, but it can turn an older machine into a modern miracle. Keep in mind that at higher speeds, your machine may also need substantial bracing to keep it from flexing.

[Mayo]

Thanks for the response Mike.
I sent you an email through this forum but my log in timed out so I don't know if it went through.

I'm not so concerned with more speed as I am with getting a more smooth cut on curves especially.