Episode #5: Distance vs Move Profiles vs Duty Cycles

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The Motion Control Show

We're continuing on with talking about some of the basic terminology and things you need to know in order to be able to size your own mechanics and gantry.  I'm Corey Foster with Valin Corporation.  Let's talk about this.

Let's talk about the distance or the length of the travel that your actuator needs to move.  There are a couple different terms.  There's the length of the actuator.  There's the travel length that you're going to move.  Now, some manufacturers will actually specify the actuator by the length of the actuator end-to-end.  But, the travel is something less because you lose some motion at the ends because of the mechanics.  Other manufacturers specify it by the travel and then the length of the actuator is longer than that travel.  There's also the thought that there needs to be some over-travel at the end.  This is particularly important for long travel and high-speed actuators like belt and pulley actuators where you have to have some safety travel at the end.  Your travel distance may be a meter, but perhaps you only need 800 millimeters and so you have 100 millimeters of safety travel at each end.  These are some things you need to ask.  If you tell me that you need a 100 meter, excuse me, a 100-millimeter stage, I'm going to ask you if that is the actual travel distance you need or do you need to have some safety distance and how much room.  I'm going to ask you some clarifying questions about that.

Let's talk about the move profile.  Here we have a standard chart, a standard graph, of velocity versus time.  The distance is the area under the curve.  Time is your X-axis.  Velocity is your Y-axis.  Now, the acceleration here as shown is instantaneous.  This would be like getting on to the freeway in the Porcshes that I know that you all have and going from zero to 60 instantaneously with no acceleration.  Not zero to 60 in two point two seconds, but 0-60 instantly.  That's not possible so there has to be some acceleration rate.  That looks like this.  It's the same distance here, same area under the curve, but we have to go a little bit higher of a velocity because we have to accelerate up and we don't get the full amount of distance here while we're accelerating.  This is a triangular profile.  This one here in the middle is a trapezoidal profile because it looks like a trapezoid.  This is more of a triangular profile.  It has a lower acceleration rate, but it has a higher velocity and gets the same area under the curve.  If we take a closer look at this trapezoidal move profile, in some applications the jerk, or the change caused by this constant acceleration here to a no acceleration here, this sharp edge right here can cause a problem, maybe if you're moving liquids or some sensitive load.  There's what is called an s-curve profile where you ramp up the acceleration rate to a higher rate and then slow it down to smooth out the curve.  What this does is it increases the peak acceleration right here up to twice the acceleration rate.  Now, acceleration times inertia gives you your torque so if you double your acceleration, your inertia is the same, if you double your acceleration, your torque is twice as much.  Your torque is twice the required amount of torque.  That's something that's important to know.  Then here's the trapezoidal motion profile and a triangular motion profile and some basic equations.

Duty cycle.  The distance here can be followed by a dwell time on that time versus velocity graph.  The duty cycle is the percentage of the overall time that you're actually moving.  If you're moving constantly, that's a hundred percent duty cycle.  If you're moving 50 percent of time versus not moving, that's 50 percent duty cycle.  That changes the life span of the actuator as well as the motor.  What I often times get is someone saying I need to do one cycle in one minute.  Well, what is one cycle?  Is that one-time moving out or is that one time moving out with a dwell time plus one time moving back?  And is the dwell time the same on both ends?  One cycle has to be defined.  Or is that one cycle at 60 times doing something and that's one cycle.  One cycle itself is not well defined unless someone figures that out.

LOSTPED.  This is a typical acronym for remembering all the things that we need to look at when sizing mechanics.  Load, orientation, speed, travel, precision, environment and duty cycle.  We haven't talked about environment here.  That's a big messy topic.  There's cleanroom, there's radiation, there's dirty, there's oil, there's water, there's all sorts of different things we can be talking about there.  But, this is just an easy reminder to help us to go through all that checklist of things we need to discuss.

I'm going to finish off with a little cheat sheet I put together to kind of be used as a reminder.  You can use this as just some of the basic terms and things to that we have to talk about.

We've talked about some more basic terminology and motion requirements and I've given you a summary, a cheat sheet.  I'm Corey Foster at Valin.  I hope that helps.

Contact Valin today for more information at (855) 737-4716, or fill out our online form.