heavy mirror for Galvo system - Page 1
Forum section: electronics / beginners

author: mr_byte31
author_rank: Regular Contributor
timestamp: February 10, 2025, 08:39:59 pm
content: Hi Folks,I am working on high power Laser projector that use Galvo system concept to direct laser in certain direction.Note: picture above is from google search.The system shall look like above picture but my requirements are more challenging. The mirrors I have to use are 250gm each (more silver inside and steel frame).The weight of the mirror is the bottle neck of the project since I have to move it in both direction with 100 Hz.I had some proposals to attempt but I am not sure if they will work fine:buy very expensive servobuild electomagnetic system using transformer core. concept similar tothisuse BLDC motor 2200KV+ gears + ODriveuse voice coil actuator...more?I highly appreciate if someone can direct me to which technique shall work better since I lack experience in such topics.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 10, 2025, 08:51:21 pm
content: Your application may allow a resonant design, one in which the mirror is spring mounted and twists back and forth at a frequency determined by spring rate and angular momentum of the mirror. Since energy is stored and returned in the springs the drive power requirements are greatly reduced. The aiming problem then becomes a matter of timing laser pulses so that the mirror is pointing in the right direction at firing time.

author: mr_byte31
author_rank: Regular Contributor
timestamp: February 10, 2025, 08:57:01 pm
content: Thx man but I am not resonating in the system. I get (X,Y) co-ordinates from the MCU and I have to follow it

author: benseno
author_rank: Regular Contributor
timestamp: February 10, 2025, 10:03:48 pm
content: Was the motor torque and speed it has to be maintained at estimated based on mechanical design (shaft, its frame coupling, bearer moving part)?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 11, 2025, 12:25:56 am
content: So how fast do you have to move it? Is the motion likely to follow a continuous curve or are you require to get to an arbitrary point in a fixed time interval? How accurate do you have to be? How physically large are your mirrors and what is their mass? All of those and more will be needed to recommend a "best" answer.You could had hydraulic and pneumatic actuators to your list. Also add possibility of segmenting the mirrors and scanning the smaller pieces individually.

author: mr_byte31
author_rank: Regular Contributor
timestamp: February 11, 2025, 07:11:12 am
content: I am trying to relax the requirement by making both mirrors to deflect +15 degree then -15degrees and then fire the laser at correct time. The same concept for old Tv ( scanning mechanism)This will require a precise angle measurement. I have then to use Absolute encoder for this task.Actually, I didn't like the idea of the spring since I have to maintain the energy in it. controlling it doesn't seem straight forward for me.

author: benseno
author_rank: Regular Contributor
timestamp: February 11, 2025, 07:27:12 am
content: Quote from: mr_byte31 on February 11, 2025, 07:11:12 amIt can be rotated too, like in laser printers - that will ease requirement down to the start/home position without absolute encoder.

author: mr_byte31
author_rank: Regular Contributor
timestamp: February 11, 2025, 08:01:52 am
content: Quote from: benseno on February 11, 2025, 07:27:12 amcan you please give more explanation?

author: benseno
author_rank: Regular Contributor
timestamp: February 11, 2025, 08:28:55 am
content: Quote from: mr_byte31 on February 11, 2025, 08:01:52 amat least for one axes it is possible:https://cdn.britannica.com/65/74065-050-F2C6800F/computer-laser-printer.jpg

author: mr_byte31
author_rank: Regular Contributor
timestamp: February 11, 2025, 08:37:24 am
content: Thx.The mirror I am using is looking similar to :it has holder from the side that shall be connected to the motor shaft.rotating the mirror is not an option !

author: manmanlin
author_rank: Newbie
timestamp: February 11, 2025, 09:58:26 am
content: This seems difficult.

author: indeterminate
author_rank: Regular Contributor
timestamp: February 11, 2025, 10:06:40 am
content: Open up a mechanical hard disk and take a look at its voice coil like driveimagine your mirror conceited to the pivot arm of the head assembly.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 11, 2025, 10:08:42 am
content: Most galvos use closed-loop optical feedback to measure the mirror position in real time.Seems like you are trying to re-invent the wheel when there are plenty of off-the-shelf solutions for this available - if you look at teh complexity of drive circuits for commercial galvos you will see that this is a rather nontrivial problem.

author: Infraviolet
author_rank: Super Contributor
timestamp: February 11, 2025, 08:33:05 pm
content: "use BLDC motor 2200KV+ gears + ODrive"If you go with the BLDC and O-drive route you definitely wouldn't want high KV like that. 2200KV would be a very low torque-per-amp motor, perhaps the sort used in racing drones. You'd want a KV down in the several hundred, which gets you much better torque-per-amp, look for 5065 sized motors with a KV rating between 100 and 400. With those larger motors you can get a lot of power through one of them, many tens of amps for fairly sustained time periods, and to my understanding you probably aren't too worried about motor weight. Afterall your motors are in the fixed part of the frame, and your mirror is already heavy so my guess is that this laser projecting system is not going to be mounted on something like a drone or RC plane where keeping overall weight low is vital. O-drive's website will tell you a lot about the different ways you can position-track fast BLDC motors, and do so accurately even when they're at full speed.If you want to move the mirror that fast back and forth you might also consider a counterweight, so the power needed is only that required for accelerating the mirror and not any extra power needed to overcome lifting the weight of it.But with a different mechanical setup you could consider having mirrors continuosly rotating rather than going back and forth, this would take a lot less power and generate a lot less mechanical vibrations than having a mirror going back an forth at 100Hz.

author: Smokey
author_rank: Super Contributor
timestamp: February 11, 2025, 08:44:19 pm
content: Optical galvos exist for a reason. They were engineered to solve the type of problem you are talking about. If there was a "better" solution for that problem, then they would have done that instead of designing the optical galvo. So if you want optical galvo performance, then get an optical galvo. If you are operating a system outside the spec of the optical galvo (like mirrors that are significantly bigger than is typically used with an optical galvo system) then maybe take a step back and ask why no one else uses mirrors that big, and solve that problem first.

author: mr_byte31
author_rank: Regular Contributor
timestamp: February 11, 2025, 09:46:35 pm
content: Quote from: Infraviolet on February 11, 2025, 08:33:05 pmThe Gears shall reduce the speed and provide torque. I was thinking of gear ration of 20 that's why I was planning to use cheap high KV motor.

author: Smokey
author_rank: Super Contributor
timestamp: February 11, 2025, 09:50:04 pm
content: do you have any spec for angle accuracy? any geartrain will result in some backlash and position errors.

author: mr_byte31
author_rank: Regular Contributor
timestamp: February 11, 2025, 09:56:12 pm
content: I didn't think about the angle accuracy. what worries me is the weight of the mirror and the required speed !

author: Smokey
author_rank: Super Contributor
timestamp: February 11, 2025, 11:01:36 pm
content: Quote from: mr_byte31 on February 11, 2025, 09:56:12 pm?? your thing is probably expected to do something. If you have a +/-30deg backlash, will your thing still do what it's expected to do? If you don't have feedback and it drifts a ton and ends up pointing at the ground, is that OK? If it has a dead zone at each end of the rotation where is stops and does nothing for a portion of the cycle, is that ok?

author: LaserSteve
author_rank: Super Contributor
timestamp: February 11, 2025, 11:26:00 pm
content: So ask yourself why you are not contactingScannermax, Scanlab, Cambridge-Novantia, or Dragon-Tiger via Goldenstar?All the above make fast Galvos with large mirrors.Increasing Coil Size and Coil Current does not significantly increase speed. Your goal is to lower system inertia, not raise it. Your mirror is not a model airplane propeller.For laser show projectors the theoretical limits of galvos have nearly been reached. Marking Galvos are probably already there.Position Sensors are optical or capacitive and are built into the Galvos.We even have lenses for boosting scan angle with marginal increase in spot size.Fast scanning does not use gears.What are you really trying to do? What is your desired scan angle? What is your beam diameter? Two of the companies mentioned above make some models that are not listed on their web site for very large mirrors.Physics dictates that optical angle off a galvo mirrorIs twice the mechanical deflection.Steve

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 12, 2025, 01:44:02 am
content: Quote from: LaserSteve on February 11, 2025, 11:26:00 pmThe OP doesn't seem to have thought about this enough to understand the critical questions. My questions were directed at the same issues.Perhaps the first order of business is to get and read a book on control systems. Or as you suggest, farm this out to someone who already knows these things.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 14, 2025, 02:31:35 pm
content: Torque is cheap.

author: mr_byte31
author_rank: Regular Contributor
timestamp: February 17, 2025, 04:39:19 pm
content: I will measure the angle of the mirror via AS5047 and the maximum error shall be in range of 1 degree.I tried some research paper and it seem my issue is a classical problem in optical field. I was not aware of this.I am trying to make some experiments using voice coil.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 17, 2025, 04:52:16 pm
content: Please check your requirements. It would be an unusual optical system that only requires one degree of angle accuracy. The good news? If that is really all that is required your problem is relatively easy.This smells like a student team project. If so you need to be doubly suspicious of the requirement. Having the system fail because requirements are wrong will be very unsatisfying and may negatively impact your grade.

author: tggzzz
author_rank: Super Contributor
timestamp: February 17, 2025, 05:24:20 pm
content: Quote from: CatalinaWOW on February 17, 2025, 04:52:16 pmI haven't seen any requirement for eye safety - and it is a "high power" laser system. The physics is unforgiving.Blinding random people in the vicinity is regarded as "severely suboptimal". (For the avoidance of doubt, I don't care if the OP blinds himself!)Remember to avoid specular reflections, and not to look into the beam with your remaining eye.

author: LaserSteve
author_rank: Super Contributor
timestamp: February 18, 2025, 09:32:13 pm
content: Sounds more like a job for a Cambridge Technology 6830,6840, or 6860, with a 6850 series amplifier board, which are on Ebay for peanuts.These have optical feedback consisting of one or two Leds and two or four photodiodes depending on options, producing current proportional to angle referred to as Ia and Ib.Something is wrong here, it must be a student project, and the details are very strange.One degree of error is huge, in any industry with scanned optics.Nobody builds this sort of thing, we just buy it used.By the time you figure out that decoupling the Torquer's magnetic fields from the AS5047P is non-trival, and that mirror to rotor inertia ratio is critical, you can buy a used galvo. Next your going to tell us you'll write the control law program in a DSP, which is a Doctoral Project by itself.I've tacked on a 6850 Schematic.Steve

author: Infraviolet
author_rank: Super Contributor
timestamp: February 19, 2025, 05:45:46 pm
content: My guess is still that the simplest answer is to have two mirrors spinning at a constant speed, and turn the laser on and off (or vary it's power in a more continuous manner) depending on the measurement of exactly where the two mirrors are at any given time. I got the feeling from this thread that the OP was trying to project or otherwise raster some sort of image or text, having mirrors constantly spinning so that once every time period the laser spot traced out a pattern of parallel lines covering the area of interest would be a lot simple that trying to make mirrors move back and forth so fast. It is hard to make mechanical things happen quickly, but trivial to get electronic things like a laser diode to switch on and off at frequencies right up in the many megahertz (with more expertise getting up in to the gigahertz can be done). The laser beam's position would spent quite a lot of each cycle pointed in useless directions (one or other mirror not facing it at all, or reflected spot going "backwards" in to some part of the mechanism's housing), but during the proportion of the cycle when it was pointed the right way to be in the grid it would be easy to very quickly (plenty of rotation sensors can give you readings at many kHz) turn the laser on and of as needed so that the spot was only illuminated where desired within the pattern.