[MECHUP]

Servos vs. steppers. Nice list, I wrote and I still like it.:-)

Let me add a little to it that may be otherwise practical:

If you are designing a machine and you get to motors, the first thing you should do is calculate the power you need. Never buy a motor (stepper or sevro) first and then figure out if it will fit what you need. That is the sign of an amateur or hack.

Motors are motors. They couple power to your mechanism and power is what makes things happen. The choice of a motor comes after you know what's needed.

Power is velocity times force or torque times RPM. It doesn't matter if the motors are steppers, servos or a gerbil in a spinning squirrel cage at the start.

To seperate what motor need (neglect the gerbil), is the power your mechanism needs.

Rule #1: If you need 100 Watts or less, use a step motor. If you need 200 Watts or more, you must use a servo. In between, either will do.

So, how do you figure the power you need?

Method 1: You have a plasma table, wood router or some other low work-load mechanism. You have a clear idea of how many IPM you want but your'e not sure of what force you want at that speed.

Pick the weight of the heaviest item you are pushing around. If it weighs 40lbs, use 40lbs. Multiply it by the IPM you want. Say that's 1,000 IPM. Divide the result by the magic number "531". The answer is 75.3 Watts so use a step motor.

Eq: Watts = IPM * Lbs / 531

Method 2: You have a Bridgeport CNC conversion you are doing. The machine has a 5 TPI screw and you need a work feedrate of 120 IPM. 120 IPM on a 5TPI screw 5 * 120 or 600 RPM.

How about force? Not a clue? Use your machinist's experience on a manual machine. The handcrank is about 6" inches in diameter. How much force would you place on the handcranck before you figure you're not doing something right? I hear about 10 Lbs.

!0 Lbs is 160 oz, 160 oz on the end of a 3" moment-arm (6" diameter, remember?) is 480 in-oz (3 times 160) ot torque on the leadscrew.

The equation for rotary power is: Watts = in-oz * RPM / 1351

For this example, Watts = 480 in-oz * 600 RPM / 1351 or 213 Watts.

213 Watts is servo territory. You have to use a servo motor to get that, about a NEMA-34 one.

OK. Long post, late night. If anyone cares, let me know. Proper application of servo motors is an entirely different topic, it's involved but not particularly difficult. Servos are not steppers and they are not interchanchable. Let me know if I should continue.

Mariss

Hi Mariss, forgive my ignorance but where do we get the magic number 531 or the number 1351 when calculating rotary power above?

[Mariss Freimanis]

MECHUP,

It took about 5 minutes of quality-time with my calculator.

I started with the basic definition of a horsepower which I memorized in high school as 1HP = 550 lbs lifted one foot in 1 second and 1HP = 746 Watts.


LINEAR POWER (Watts = Lbs * IPM / 531)------------------------------

Since most CNC math prefers IPM (inches per minute) instead of feet per second, rephrase the HP definition as lifting 550 lbs at 720 IPM (12" times 60 seconds). Now, let's get rid of that pesky '550' number; make 1 HP = lifting 1lb at 396,000 IPM (550 times 720).

OK, but that's for 746 Watts; how much is it per Watt? Take 396,000 IPM and divide it by 746 and you get 530.831 as an answer. I just round it off to 531.

This means 1 Watt of power can lift (or push with) 1lb at 530 IPM. So just multiply pounds of 'push' times IPM and divide the result by 531 to get your answer in Watts mechanical.


ROTARY POWER (Watts = in-oz * RPM / 1351)------------------------

Start with the same HP definition but imagine you have a 1/16" radius pulley on a motor. On this pulley you have wound a string attached to a 1lb weight that you will be lifting. 1 oz-in of torque will do the job (1 oz-in with a 1/16" radius will exert a 1lb force on the string).

The pulley circumference is 2 * pi * radius or 0.3927". If you turn the shaft at 1RPM, you will be lifting the 1lb weight at a speed of 0.3927 IPM. We have a linear force and a velocity now.

Since we already calculated 1 Watt = 1lb * 530.831 IPM, all we have to do is divide 530.831 IPM by 0.3927 IPM to get 1351.747 as the answer.

1 Watt = 1 in-oz * 1 RPM / 1351.747 which means I should have rounded the denominator off as 1352 instead of 1351. Doesn't really matter since the result is off by only 0.06%.

This took 5 minutes to write which is about as long as it took to generate the equations in the first place using just two memorized definitions for power: 1HP = 746 Watts and 1 HP = 550 ft-lbs / sec.

I'm glad I paid attention in Mr. Long's physics class my senior year at Northwestern High School 45 years ago; you never know when you might need stuff later on in life. They certainly didn't teach this stuff to EE's in graduate school at Ohio State or UCLA.

Mariss

[MECHUP]

excellent explanation, that clears up a lot of questions i had. examples like this just can't be beaten. thanks!
i'm a wiser man now!!!