Session 08 – Stepper Motors & Toothed Belts

The Concise RDWorks Learning Lab Series

Welcome to the new Concise RDWorks Learning Lab Series with Russ Sadler. In this session, Russ describes in detail the laser machine drive system, covering stepper motors, stepper drivers and toothed belts. He also goes on to describe the drawbacks and benefits associated with the drive systems used in the majority of laser machines.

Over the last 6 years, Russ has built up a formidable YouTube following for his RDWorks Learning Lab series which currently has over 200 videos.

The original RDWorks Learning Lab series on his “Sarbar Multimedia” YouTube Channel, follows Russ as he tries to make sense of his new Chinese laser machine and to sort out the truths, half truths and outright misleading information that is available on the web.

Six years later with over 4 million YouTube Views under his belt, Russ has become the go to resource for everything related to the Chinese CO2 laser machine user or wannabe user.

Stepper Motor & Stepper Driver
Stepper Motor & Stepper Driver

In this new series, Russ has condensed his knowledge and experience of the last 6 years to provide valuable information and insights into the purchasing, understanding, use, repair and maintenance of the Chinese CO2 laser machines and their key component parts.

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Rack & Pinion Drive System
Rack & Pinion Drive System

Transcript for Stepper Motors & Toothed Belts

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The Concise RDWorks Learning Lab with Russ Sadler. Session 8: Stepper Motors & Toothed Belts. Now in today’s session.

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We’re going to take a look at the way in which this head moves around the

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machine. Once the brain has decided where it wants this head to move to. It issues

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instructions to a stepperr motor, and then the stepper motor drives the head to a specific coordinate position.

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How does it get there? Well, that’s what we’re going to look at today, it’s the motors and the drive belt.

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Some of the very expensive machines use a servo motor to drive the head around.

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Now, a servo motor operates completely differently than the motors that we use on these machines, which are called stepper motors.

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If you haven’t seen a stepper motor before, I can assure you, you’ve actually seen it in operation.

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You’ve only got to look at one of the big analog clocks and you’ll see it going, tick, tick, tick, tick, tick.

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Those ticks are basically caused by a stepper motor as it jumped from one position to another in discrete steps.

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Now, what we’re going to do is we’re going to take a look at the stepper motor and this drive belt system,

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because it’s a very cheap way of achieving a CNC machine.

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But it’s got its problems and we’ll work our way through the various parts of this system and

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we’ll try and demonstrate to you what the strengths and weaknesses are of this drive system.

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OK, so what is a stepper motor? Well, a stepper motor has a shaft that runs right through the middle of it, running between bearings.

Transcript for Stepper Motors & Toothed Belts (Cont…)

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So this center section here is running completely freely and here on the shaft.

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We have two pieces of metal which look like gear teeth.

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Now, those two gear teeth are completely separate pieces of metal, and between these two lumps of metal,

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what we have in this section here is a permanent magnet, a very powerful magnet.

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Magnets have got a North Pole and a South Pole.

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So this set of teeth here have a permanent South Pole magnetization and these set of teeth here have a permanent North Pole magnetization.

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Now, these two sets of teeth here are not in line with each other.

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This set here is rotated around by half a tooth, now that’s for a very good reason.

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And that’s because what we have on this section here, as you can see,

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we’ve got sets of what they call stator poles round the outside here, which have got teeth on them as well.

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Now, these opposite poles are connected together.

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So when we connect an electrical DC current through these coils here, we get a certain type of magnetization.

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And that magnetization, for example, may well be north – north because these are electromagnets.

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And so that means the north – north will attract the south – south teeth on this stator and

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they will lock together in a very positive way because they’re magnetically attracted.

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Now, remember what I said. The back set of teeth here are magnetized in the opposite direction.

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And so consequently, when I put a north – north electromagnetization across this pair of poles.

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It will attract this set because these are the South Poles and it will repel this set because these are North poles,

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but the whole thing will remain stable and will be locked in one position.

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Now when we change the electricity.

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From one set of poles to another set of poles, the next set of poles would be magnetized in the opposite direction.

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So this was a north set of electromagnets. This is a south side of electromagnets.

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And so s a continual movement of the electrical field around these magnets will cause this rotor to gradually creep round in little steps.

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That’s what makes this a stepper motor. It does not move continuously.

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It moves under the control of these magnets here.

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So this effectively, when you switch these on and off digitally, makes this into a digital motor.

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It’s got discrete steps. It’s not continuous. Purely by switching these fields, you can make the motor run in a very, very controllable manner.

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You can make it run slowly, you can make it run accurately, you can make it stop.

Transcript for Stepper Motors & Toothed Belts (Cont…)

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You can make it start in exactly the right position. Let’s just say 200 teeth on here.

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So if you want this motor to rotate one revolution,

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you put 200 pulses into the coils and that will drive this motor around 200 steps for one revolution.

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So that’s how you get digital control of this motor.

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This is a very cheap way of achieving positional control on our CNC machine.

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It is something called an open loop process.

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The brain, the controller that we’ve already spoken about, says I want you to move one step.

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And in moving one step, it moves ahead a fixed amount.

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So the controller sends out a signal and one pulse would be sent to a piece of intermediate software,

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which we’ll talk about in a minute, called a stepper driver, which controls this motor and it tells this motor to move one step.

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So the controller says one pulse, the stepper driver says one pulse, the pulse coil has moved from here to here,

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and driven it by one step. The problem is, if I happen to be holding this shaft, jamming it, stopping it from rotating. The field will rotate,

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but the stepper motor rotor will not rotate. But the controller has no idea,

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that the stepper motor has not done what it’s asked it to do.

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This is the problem with an open loop system. It’s a message in a bottle.

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And there’s no feedback from here to the controller that says, yes, I’ve done what you’ve asked me to do.

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Now, the stepper motors on one of my machines are slightly different.

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They are still stepper motors. But what they have attached to the shaft, they have one of these things, which basically is a rotary encoder.

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And what that does is completely separately counts steps.

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So when the controller says, I want you to move one step, the driving current rotates, but this is not following the drive current magnetism.

Transcript for Stepper Motors & Toothed Belts (Cont…)

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This is locked up. This part here will not move either, and because this part doesn’t move, it will not count

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the steps. So it’s actually monitoring what this is doing and it will tell tales on this stepper motor back to the brain,

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the controller, and say, yeah, I know you’ve asked me to move one step, but, hey, this stepper motor is stupid.

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It hasn’t moved one step. So keep issuing pulses. So the controller will issue another pulse.

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And still, the motor won’t move. Eventually, the controller will give up.

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So if the signals to accelerate this motor move faster than the motor can respond, effectively, it’s a bit like wheelspin spin on your car.

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You put your foot down and the wheels spin, but you get nowhere.

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The wheels are spinning round and probably you may well find that your speedometer showing you doing 40 miles an hour, but in fact, you’re doing nothing.

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This will not be rotating at the speed of the rotating field.

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This may eventually start picking up speed and moving, and it then moves to the correct position.

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I want you to move 40 steps. So it then moves it’s 40 steps, because this is counting whether or not it has moved

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it’s 40 steps. And when it’s moved, it’s 40 steps. The controller then stops asking for this stepper motor to move.

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This is what they call a closed loop feedback system.

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So you cannot ever get any errors in your position.

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Your position would always be totally wrong, in which case the system will crash or it will be totally right,

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because this system will continue to try to correct it to put you into the right position.

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So this is a very good system.

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It’s still a digital stepper motor system with all the problems associated with a stepper motor. Now stepper motors come in two basic specifications.

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One of them is 200 steps per revolution and one of them is 400 steps per revolution.

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There is an intermediary box of electronics between the brain (the controller) and the stepper motor.

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And it’s this thing here called a stepper driver.

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Now, the stepper driver allows the signals that come from the controller, which says, I want you to move one millimetre.

Transcript for Stepper Motors & Toothed Belts (Cont…)

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But it will interpret that and decide that, hey, I know this stepper motor has only got 200 steps per revolution,

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but I can break those steps down into much smaller increments and make this stepper motor more accurate.

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And so it has the opportunity to change the step pulses per revolution here.

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By messing around with these switches, you can change the resolution, if you like, of your stepper motor.

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You have the opportunity to make the stepper motors more precise, but that more precise comes with a penalty of less torque.

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Even with these very fine steps per revolution, it’s still not continuous motion.

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It’s still click, click, click, click. Even at high speed, it’s an intermittent motion that this stepper motor is providing.

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So here is how we connect the stepper motor to the laser head, it’s with a toothed belt.

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Now, there are some great advantages to this toothed belt configuration, because it’s got teeth on it

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it must be accurate. It has these tension strands built into the outside part of the belt.

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The belt does not stretch. It’s absolutely perfect.

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So you put one step into here, and because it’s linked through these teeth and via this very,

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very stiff, drive system here, you get one step of movement of the head.

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In simple precision terms, this is a great system, one step, one step, perfect alignment, but in practice it’s not quite as amazing as it sounds.

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It’s a very cheap CNC drive system with its problems.

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It’s a, it’s a virtual certainty that your drive belt will look like this with rounded teeth on it.

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And that’s something called an HTD high torque drive profile.

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These toothed belts were never designed for CNC machine control.

Transcript for Stepper Motors & Toothed Belts (Cont…)

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These belts were designed originally and why they’re called timing belts, because they’re designed for controlling the timing of valves.

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Now, these replaced noisy chain drives. These drive in one direction only.

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This petrol engine does not continuously keeps going backwards and forwards and changing its direction.

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It’s a one direction drive. And so consequently, the teeth have been designed to allow a certain amount of compliance.

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Remember, around the outside here in this section, we’ve got lots and lots of steel cords.

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So we’ve got no movement at all in this outer stretched cable.

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It’s a completely stiff drive system. But these teeth are completely unsupported pieces of rubber.

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Just like a bouncy ball, which you bounce on the ground and if you hit it at the right frequency, it continues to bounce.

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That’s one of the fundamental problems with using a rubber drive belt with teeth in, on a CNC machine.

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We’re not driving in one direction only. We’re changing the direction continuously.

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But the other problem is we’re not driving at a continuous speed.

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We’re driving with a pulsed speed, remember, stepper motor, click, click, click, click, click, click. This tooth is continuously just like a rubber ball.

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It’s absorbing and releasing energy back into the system.

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And that has got a tendency in mechanical terms to produce something called resonance.

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Now, that all depends upon the stiffness of this, the speed at which it’s being driven from the stepper motor,

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the pulses that arehitting it, and the mass of the head that it’s trying to move.

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I did a very short video that tries to describe the problems that we have with our machine.

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So, I’m just going to cut to that video now and demonstrate to you what I mean about Resonance. Designed a little piece of acrylic,

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which is 25 millimetres square, and it’s sitting in a little fixture here,

Transcript for Stepper Motors & Toothed Belts (Cont…)

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to hold upright. My piece of material is twenty five millimeters long and eight millimeters wide.

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And you see that the rectangle is the black layer and the black layer is set to scan and it’s set to scan at five millimeter pitching.

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So this is 10 millimeters wide and I’ve got a piece of eight millimeter thick material.

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So what I’m hoping to get is one scanline on my material and one Scanline out into fresh air.

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And then in addition to that, the scanline, I’m hoping is going to be somewhere around about the middle of the material.

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And then we’ve got this blue line here, which is a cut line.

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And what I’ve done, I’ve set the cut line to exactly the same parameters as the Scanline. At the moment

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they’re set to 50 percent power and 200 millimeters a second.

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And we’re just going to just generate two, hopefully, grooves on the edge of this material and we’ll see what the results are.

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Now we’re going to view these two traces against a red background.

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And what you can see here is the cut trace, which starts just here.

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And obviously because the speed is zero, as soon as the power switches on, we get a deep cut.

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As the heat starts to move, the beam does less damage because it’s gradually getting faster and faster and faster.

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Compare that with this top trace here, which is this one that you can see.

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And this is already running at 200 millimeters a second because it is a scan trace and not a cut.

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The cut away starts from zero and accelerates up to 200 millimeters a second, whereas the trace,

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the top trace always starts at zero way off the screen here, accelerate up to 200 millimeters a second,

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and that’s calculated by the software.

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So by the time we get to this point here, we’re already doing a two hundred millimeters a second and then they switch the power on.

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And as you can see, the power is moderately stable.

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And as I move across the cut, you’ll see that the top traces, it’s remaining pretty stable, but there’s a little bit of waviness there.

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But at no point does the bottom trace ever get as shallow as the top trace.

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And that tells me that I’ve never actually got to 200 millimetre is a second during that cut. Nearly 200 millimetres a second,

Transcript for Stepper Motors & Toothed Belts (Cont…)

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but if it was 200 millimetres a second, that bottom trace would be the same as roughly the top trace. And as you see it all the time,

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if I scan across there quickly. You’ll see that it comes up to speed nearly and then back down and decelerates to zero.

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OK, now that’s what 200 millimeters a second looks like on the Lightblade machine for both traces, a cut and a scan.

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And it is interesting to note that as we decelerate, we do not get the stepper motor pulses.

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OK, so we now run at 50 millimeters a second. There we go, we can see the two traces again, the one in the background is the cutline,

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and because we’re running slower, it’s getting up to speed quicker.

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So the acceleration is much, it looks to be faster, but it’s reaching speed quicker and once it’s at speed,

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look at all those little knobs and blobs and the same is happening on the scan.

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So we’re getting something which is driving the head in an oscillatory way, it’s accelerating and decelerating as it scans along.

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Here’s what, twenty five millimeters a second looks like on the Lightblade.

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Now, there will be virtually no difference between the two traces now,

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because the acceleration required to get up to speed on the cutline in the background

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is just about the same as that required to start on the scanline on the top.

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You can see they’ve both got the same sort of frequency of pulses on them.

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I’m going to give you an example of what resonance is. I’ve got a rubber belt here, and my hand is the stepper motor.

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And look, if I move my hand very slightly at the right frequency, the head, which is this thing here, look, I can make it go into really violent oscillation.

Transcript for Stepper Motors & Toothed Belts (Cont…)

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But as I get faster, in other words, as the stepper motor steps get faster.

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Look what happens. All the energy is being absorbed in here, and it’s not being transmitted through to the mass.

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It’s only when I happened to run at slow speed at a certain frequency that I get the mass to bounce around, OK?

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But as I run faster, things slow down.

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And this energy absorber against that mass is storing the energy and giving it back at the same rate that I’m getting it out.

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There are several possibilities for resonance on this machine. One is the teeth.

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I can feel this is not a normal rubber. This is a urethane teeth.

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There are urethane teeth on this belt, which means the teeth are pretty stiff. So the chances of energy storage in the teeth are low on this machine.

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The most likely place that I’m getting movement is actually down here, look.

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I’m getting transmitted energy from here, which is causing this to wobble. These two gears here and here are the same gear.

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So we’re going from one to three, probably three times as many teeth, back to one again.

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So the relationship between this one and this one is about three to one when this one here runs at one revolution a second.

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It’s making this one run at three revolutions a second.

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So working backwards, it means that we’re running this stepper motor faster when we’re running at slow speeds on here.

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So running at slow speeds, we should get more energy absorption through this gear

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Train here. To stop the frequency of the stepper motor getting transmitted as much to the belt.

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This is what twenty five millimeters a second on the tangerine tiger.

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Looks like. Actually, it’s not as bad.

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Yeah, we got some pulses and they are definitely stepper motor pulses. And you can see there lovely synchronized sets of pulses.

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Here’s what 50 millimeters a second looks like on the tangerine tiger.

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So there’s a scanline on the top and the cut line on the bottom.

Transcript for Stepper Motors & Toothed Belts (Cont…)

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So you can see now, we’ve got quite a rapid acceleration on the cutline because I’ve got this acceleration on this machine.

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this machine set very high.

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But we’ve got lovely synchronized pulses there, which are obviously coming from the stepper motor in some way, shape or form. But

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I’m going to do nothing else now, other than one change. You can see that I’ve been rather brutal with this machine, now.

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I’ve stuck a very large G clamp onto the machine.

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This weighs about a kilogram. We’re going to run exactly the same test at 50 millimeters a second again.

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I’ll tip them up, so that we can see them both together. Can you see immediately the effect of the mass?

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Look at the cutline in the background. It’s accelerated up to speed.

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But where are the pulses? You’ve seen how mass can affect the pulsing effect on the head.

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We can damp the frequency out by increasing the mass in the same way that I showed you those balls jumping up and down.

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We can keep the balls still if we increase the frequency high enough or make the balls heavy enough.

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That proves beyond any doubt that we’ve got a resonance effect happening on the motion of the head when we cut at slow speed.

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This machine was never really planned as a cutting machine, this has always been, in my mind, a high speed engraving machine.

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I’m going to tip the sample up, and what we can see here is the start of the engraving line.

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And you can see that the engraving line is absolutely rock solid, steady all the way along.

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Not a hint of a ripple on it at all. Because we’re running so fast with the stepper motor now, that we’re not generating any pulses.

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The stepper motor is, the inertia of the stepper motor is now killing the pulses.

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Again, on The Cutline, which is this line at the bottom here, we’ve got virtually no pulses at all.

Transcript for Stepper Motors & Toothed Belts (Cont…)

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Just as we accelerate up here, we’ve got a hint of the pulses and then when we get to speed.

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Pretty good. We’re going to see what 20 millimeters a second at full power looks like.

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Now here’s the top surface of the material across here,

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and look at these huge spikes of apparent energy that have dug into the material, that must be pulsing power.

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We’re getting spikes of power which are causing that. No, totally wrong!

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What you’ve got is a constant power that keeps stopping and starting.

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So when you stop, the power has a chance to dig in deeper.

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And when you start and accelerate, it comes high and then it goes low as it stops again.

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This is the stop start motion of the stepper motor.

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And if we take a look at the end of this track, and that’s where I stopped and left the pulse on. Look how deep the pulse has gone.

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Something else I want to show you, which is quite a serious problem with toothed belts, and that is a feature that I call belt climb.

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If you watch the belt just here, you will see that when it travels in this direction,

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the belt comes up in the air and when it travels in this direction, it drops back down again.

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Now It’s doing exactly the same thing on the other side here, climbing and falling.

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Which means that the belt is actually changing the position of the head every time it changes direction. By by pulling the belt down like I’m doing here,

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I’m sort of exaggerating that belt climb effect. I’ll just put a marker on there, that piece of acrylic so that you can see what’s happening when I

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push the belt down. So there is one set of coordinates when the head is traveling in a positive direction

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and another set of coordinates that the head lands up at when it travels in the other direction.

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So much for the accuracy of this CNC system.

Transcript for Stepper Motors & Toothed Belts (Cont…)

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In addition to the stepper motor problem, there are times when you can get this problem as well, which is a problem with the belt.

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As the belt goes on and off of the teeth, it causes the head to change speed as well.

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Very, very small increments of speed change when you’ve got high power cutting causes, change of depth of cut.

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And you can see here the depth of cut has changed and it’s totally replicated the three millimeter pitch of the timing belt.

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This is what I call curtains. I have found a solution for this problem on my machines,

225
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but I haven’t done it on this machine because this is this is not suitable for the modification. On this machine.

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You might see something strange about the timing belt.

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Apart from the fact that it’s white and I’ve changed it away from rubber, to a very hard urethane. And I’ve changed the tooth shape as well.

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So it’s got a very low profile tooth. So I’m trying to remove as much of the flexibility in the system as I can.

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So that’s trying to work on the resonance problem. But on the other hand, the belt has turned over.

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But at this end, where the stepper motor is, you can’t actually see what’s going on.

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So let me give you one of my superduper pictures. This machine has got a drive pully at this end and it’s got a smooth pulley on the other end.

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That’s how the machine was supplied with a drive belt around those pullys, exactly as you saw on the other machine.

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The teeth jumping on and off of the pullys were causing the head to accelerate and decelerate and cause these curtains.

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I’ve added a wheel there and a wheel there, and the belt has been turned over so that the teeth are on the outside

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of the belt. These pullys are driving that belt into the teeth, the tension on this belt is always basically round these three smooth pullys.

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So we’ve got a smooth belt surface on smooth pulleys and we’re driving it on the outside.

Transcript for Stepper Motors & Toothed Belts (Cont…)

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And that flexible rack and pinion system cures the curtains issue. After this session,

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where I’m afraid I’ve told you that we’ve got a pretty poor motor drive system, which doesn’t

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work with a belt that’s been hijacked from the car industry. Between the pair of them,

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they don’t actually make a very satisfactory CNC drive for this machine.

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It works and it works OK. And it’s certainly very cheap.

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And that’s the key thing. You’ve got a lot for very little.

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So don’t complain about it.

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Last updated August 26, 2021

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