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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.
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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,
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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.
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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.