How to Select a Stepper Motor - SureStep Stepper Motors from AutomationDirect

AutomationDirect has a wide assortment of stepper motors in both single and dual output shaft versions and different levels of output torque that will usually cover pretty much any need you have. And I have to admit, that for simple quick projects I’ll just grab a motor that I know is more than I need to do the job and it usually works just fine. But, for those times when you really need to know exactly which motor to choose to reduce size, or to reduce weight or cost of your system, how DO you figure out which motor is the best choice? In the Stepper Motor User Manual, Chapter 5 has all kinds of information about the motors, but it doesn’t really tell you how to select one. That’s because it’s not really a motor decision, it’s a system design issue – you have to take into account all parts of the system to select the right motor. And the user manual shows you how to do that in Appendix C. That’s the good news. The bad news is, if you go to Appendix C, it’s kind of intimidating. There’s lots of equations and weird symbols and caveats and different things you need to take into account depending on what kind of system you are building. Yuck. So, in this video we’re going to help you de-mystify all of that and show you it’s really pretty easy to figure out exactly which motor you need. There are five things we need to figure out: How many pulses do we need to make the move? What is the position resolution we need? How fast does the motor need to turn? How much torque do we need? And to figure out the torque, we’ll need to figure out the inertia of the system. Given all of those, we will now be able to select the appropriate motor for our application. We’ll use this 18 inch, 0.2 pitch lead screw setup for this video, then at the end talk about how to modify that to do the exact same thing for a belt drive systems and a direct drive indexing tables. Here we go .. Step 1 – How many pulses do we need? Let’s assume one of the moves we need to make on this 18 inch slide is to move a load 3 inches in 1.5 seconds. The pitch of the slide tells us each rotation moves the screw .2 inches, and the drive is setup to issue 2000 pulses per revolution, so we will need a total of 30,000 pulses to travel the 3 inches. Is that a reasonable number? Well, we need to issue those 30,000 pulses in 1.5 seconds so our controller needs to output 20,000 pulses per second on average. Can our controller handle that? Well, I’m using a Productivity 2000 controller and it looks like its high speed output module can handle up to a million pulses per second using the RS422 outputs or 500,000 pulses per second if using the open drain outputs. Either way we are in great shape since we only need around 20,000 pulses per second. So Step 1 was: Calculate the number of pulses needed to make the move and do a quick check to see if our controller can handle it. Step 2: How accurately can we control the position of the carriage? We know our lead screw moves the load .2 inches for each revolution, and the drive is sending 2000 pulses per revolution … units cancel and we get a ten thousandth of an inch per pulse. That’s pretty good! If we don’t need that much resolution we could switch to a .5 pitch lead screw or we could simply reduce the pulses per revolution out of the drive. If we need more resolution, we could increase the pulses per revolution out of the drive. We’ll use this ten thousandth of an inch and move on to step 3. How fast does the motor need to turn to do this? Well, we need to move 3 inches, and the .2 pitch screw says 1 revolution moves us .2 inches, so we need 15 revolutions to move 3 inches. We want to do that in 1.5 seconds, so we need to average 10 revolutions per second which is 600 RPM. Is that a reasonable number? Well, rf we go back to those stepper motor curves we see that steppers tend to operate up to around 1000 RPM and even up to 2000 RPM, so if we are around 600 RPM we are in good shape on average. That is we need to average 600 RPM to get to the 3 inch mark in 1.5 seconds. The problem is motors don’t instantly go to max speed and they don’t instantly stop, do they? No, they typically accelerate to speed and decelerate back down. And because it takes time to speed up and time to slow down, the peak speed needs to be greater to make up for that lost time so we can still get the carriage where it needs to be in 1.5 seconds. How much greater does the speed need to be during this time? The easiest way to handle this is to think in terms of pulses. Back in step one, we found that this was 20,000 pulses per second and that we have to issue a total of 30,000 pulses to complete the move. Which is total area under this graph because pulses per second times seconds is pulses - right? And if we subdivide it into equal triangles like this we see there are 10 equal triangles so each triangle would get one tenth of the 30,000 pulses or 3000 pulses each. This rectangle has 8 of those triangles so it has 24,000 of the 30,000 pulses. And since this is just the area of a rectangle and we know this time is 1 second, then the frequency must be 24,000 pulses per second. The drive issues 2000 pulses per rotation so we need 12 revolutions per second or about 720 RPM. And that is still under the 1000 RPM or so we typically think of for stepper motors, so we are still in great shape from a speed standpoint. So when you break it down to simple geometries like this, you can easily figure out how many of the total pulses are just in the rectangle portion, and since you know the duration - which is one side of this rectangle - you can figure out this max RPM which is the other side of the rectangle. If we look at this speed on this motors torque cure, it looks like we need to be under about 65 ounce inches of torque. And most folks will tell you to stay way under that – even 50% - just to make absolutely sure you have plenty of margin. So, how to de calculate torque? Well, that’s a little more involved, so we broke it out into a separate video. So, take a break, let what just did sink in, and then click here to go directly to part II where we calculate the torque required and finally select the motor we need. If you need any help with selecting an AutomationDirect Stepper Motor please contact AutomationDirect’s free, award winning support team during regular business hours – they will be happy to help. And don’t forget the forums. There are lots of folks there that love to share their years of experience. Just don’t post and questions directed at AutomationDirect’s support staff there, they don’t monitor the forums on a regular basis.