Closed loop stepper motor for anti mosquito laser turret
Thongvan.alexis wrote 10/25/2017 at 08:22 • 1 pointHi,
I never used motors before, I need a lower than 0.092° step on my motors, at first I was looking at stepper motor but then I learned that you can loose steps and that closed loop stepper motor are better.
To get that accuracy I will maybe need to use a gearbox.
So I found this motor https://www.teknic.com/model-info/CPM-SDSK-2310P-RLN/
Closed loop, 0.057° per step, PERFECT.
But it's 260$ and I need 2 of them.
Do anyone know where I could find dirt cheap closed loop stepper motor? I looked everywhere on the internet and the minimum seems to be 100$
Thanks.
ps : check my projects page to see the current turret.
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A cheap method would be a gear reduction, for instance a 1,8° motor with a X reduction ratio to get to 0,092°. No microstepping. That X is around 20 times, so you could perform two or three gears in order to get that (1 / 20) angular speed reduction. That will increase torque too, but I don't know what speed of reaciton you need for it to be useful.
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Take a look at this open source closed loop stepper motor. It fits your budget. It has a 14 bit encoder it it meets you resolution as well.
http://tropical-labs.com/index.php/product/mechaduino-0-2-servo
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For new motors do a search on globalspec.com username: "demo", pass: "go"
http://www.globalspec.com/search/products?page=mi&srchItem=2&srchWebHitId=881997997#sqid=21906286&comp=5488&vid=90562&RegEvent=login
If you're interested, open your desktop optical mouse, there is probably an optical encoder for the mousewheel. You could get a sufficiently accurate optical absolute encoder by 3d-printing a sufficiently large wheel.
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If you want something like the Teknic ClearPath but cheaper, then this is a good option: https://hackaday.io/project/11583-odrive-high-performance-motor-control
But honestly to aim lasers the standard way to do it is with galvanometers or voice coils; and mirrors.
I once used the voice coil actuator from an old hard-drive and an audio amplifier to aim laser mirrors, worked really well, it cost exactly $0 as the harddrives were scrap anyway.
Good luck!
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PointyOintment? In theory the micro-stepping is accurate (refer to the datasheet if in doubt), in practice you are right. My CNC machine was set up with 1/32 micro-stepping which in my mind is a joke. I reset it up for 1/4 stepping which is still a bit of a joke.
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I would go with a voice coil motor of some kind. It will need a slightly more complicated driver, but it's analog, so the accuracy/precision can be great once calibrated. A friend of mine has built a couple of devices like this, one a laser squiggle generator that responds to music using hard drive arm VCMs, and the other a laser projector that uses commercial laser galvanometers (specifically made for this kind of application). Both work great. The hard drive VCMs do act as low-pass filters, so the squiggle generator only responds to bass, but that probably won't be a problem in your application.
I have been thinking of building a couple of things like this. One would be a stationary turret with only one axis, acting as a barrier, to be placed e.g. across the top of a sink to shoot the tiny flies that live in the drain. The other would a pistol with two axes but only a few degrees of motion each. That one will require lots of work for laser safety purposes, to ensure that it doesn't fire when aimed at something reflective, for example.
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Sounds like a standard stepper (1.8 degee per step) and a standard DRV8825 stepper driver set up for 1/32 microsteps (=0.056 degrees). If it was my project I would go a 0.9 degree stepper motor. No need for an encoder disk and closed loop. A geared stepper has backlash and is slow.
AlanX
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Keep in mind that microsteps are nonlinear in angle vs. PWM value. I believe some stepper drivers compensate for that, but not all do.
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I agree that you can use open loop and home the position on start up. And don't forget that you can use micro-stepping to get more resolution at the motor.
You could probably just move a first surface mirror and have the laser fixed to keep the moving mass to a minimum. I have used this guy for optics before https://www.surplusshed.com/
At first I was going to suggest buying this https://www.monoprice.com/product?p_id=15365 to make your own gears, but to hit a mosquito I think at least the final gear/pulley/drive with the laser/mirror attached will have to be very stable so it doesn't wobble.
I believe Stock Drive Products are a reputable source for gears, etc. http://www.sdp-si.com/products/Gears/Index.php
I have also used gears for from old appliances, printers, toys, etc.
I hope your project is wildly successful, down with mosquitoes!
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Well, possible solutions of your gear box problem depends a lot on things e.g. the tools available to you and the angle you need to be able to turn. As you only need a 1:20 reduction you might be able to build your own reduction. One way could be using string wrapped around the stepper axis turning a bigger wheel. You could even wind the string around the axis and tension the whole thing with a rubber band for zero backlash.
For small angles you could use a lever instead of a wheel.
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Do you really need closed loop steppers? The closed loop only prevents the stepper to loose its position when skipping steps. As you are no not moving anything that is likely to have any resistance I don't get how you are so hung up on this. Many machines and projects - including most 3D printers and even laser cutters - are just fine with open loop steppers (and end stops for homing).
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I never used motor before, I thought step skipping was something common, I will try the open loop then.
One question tho, if there is power loss during operation, how do I bring back the turret to it's "home" position.
Or if I accidentaly move it while it's not powered.
The turret has to start in a known position for me to then be able to know how many steps are needed to target something.
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It is called "homing". You move all axis until they hit the end of travel. Typically micro switches, light barriers or hall sensors are used to detect the end of travel. If you use some gcode interpreter like grbl they will be able to do he homing procedure for you. But it is not that difficult to do on your own. To increase speed and precision of the homing procedure the axis is backed off after hitting the switch and then is driven against the switch again very slowly.
Just have a look how 3D printers work.
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Oh I see, well thanks, I'll get to it then.
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Skipping may occur if the load is too great. Increasing the current to the motor windings helps prevent skipping. Occasional skips may happen depending on your setup.
Hit one of the hard limits (if a hard limit exists) and all the motor can do is skip. I imagine on your project you'd want to have two adjustable hard limits (with sensors) so that the user can configure the turret to only target within those two limits (e.g. look at how a lawn watering rainbird does hard limits).
If you have an optical sensor on the turret, I don't imagine occasional skipping would be a big deal anyway (if move commanded, and mosquito not centered, then move again by delta). Further, work more on your detection algorithm so that it is real-time. Consider using a simpler algorithm against a constant color background.
Given bats use acoustic sensors to find mosquitoes, maybe that would be a route to consider. I did see at least one echo-location project (for blind people) here on hackaday. Then use your optical sensor to make sure no humans are downrange.
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Allright I saw this, but I still have some questions, is it possible to find gearboxes alone? I bought some regular 200 steps nema 17 stepper. But on the net I can only find motors with gearboxes, not gearboxes alone.
Second : the encoder I saw make 300 pulse per revolution, shouldn't I put it betwen the motor and the gearbox? after the gearbox the steps will be too low to be tracked no?
Thanks.
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