Key Questions to Ask When Ordering galvo controller

Galvanometer scanners are found wherever laser beams are steered: materials processing, laser light shows, manufacturing, packaging, cutting, marking, welding, and numerous other applications. The market has seen some significant developments in the past year.

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Originally, galvanometric devices measured the electric current flowing through a coil in a magnetic field–when current flows the coil experiences a torque proportional to the current. These devices were at the heart of all moving coil meters. In the laser world, the term "galvanometers," also known as galvoscanners, galvopositioners, or galvos, refers to a high-resolution rotary motor with a mirror mounted, instead of a pointer. And whereas nanopositioners (see Laser Focus World, May , p. 75) primarily move a stage with high precision but can also steer laser beams, galvopositioners are less precise and therefore less expensive solutions for beam steering. Very low-precision galvos may simply be called optical scanners, like those found in grocery store scanners. China manufactures mainly low-precision galvoscanners, one application of which is laser light show systems where speed is the primary performance factor (see Fig. 1).

Galva-what-now?

A galvo system consists of three main components: the motor or galvanometer, the mirror or mirrors, and the "servo"—the driver board that controls the system. These three parts and their tradeoffs drive the performance of the system. "System positioning performance used to be measured in milliseconds," says Red Aylward, president, Cambridge Technology Inc. (CTI, Lexington, MA). "As galvo systems have reached 100 μs step times and root-mean-square (rms) frequencies greater than 2 kHz, many of the design rules that used to apply are no longer adequate."

The galvo part, an actuator that manipulates the mirror, comes in two configurations for today's high-performance systems, says Aylward. The moving magnet configuration, in which the magnet is part of the rotor and the coil is part of the stator (the stationary part of the motor), tends to have higher system-resonant frequencies–a desirable trait in galvos that ranges from single to more than 20 KHz (see Fig. 2, left). The other is the moving coil configuration where the coil is integral to the rotor and the magnet is part of the stator, which optimizes torque-to-inertia ratio and torque efficiency. A third type of actuator, the original moving iron configuration, had an iron rotor that offered high torque but limited high-speed performance and positioning accuracy.

According to Monika Herzog, marketing manager at Raylase AG (Wessling, Germany), a two-axis (2D) galvanometer scanner moves two mirrors along the x and y directions to deflect a laser beam to any position within the two dimensions, known as the "marking field." A beam input unit accepts input only from compatible lasers. Depending on the type of laser, the beam output is either open or fitted with a protection window or an f-theta lens—a scanning lens that provides a flat field at the image plane of the scanning system (see Fig. 2, right).

In cases where a suitable flat-field lens is not available, the laser focus must be positioned in three dimensions (3D), or x, y, and z. In a 3D scanning unit, the laser beam first encounters a moving lens, from which it diverges to one or two focusing lenses. The beam is then directed by a set of x and y mirrors moved by the galvanometer scanners. The orthogonal arrangement of the x and y mirrors directs the beam down toward and over the length and width of the working field.

Positioning and servos

To measure the exact angular position of the mirrors mounted to the shaft, galvanometers are equipped with an extremely precise position detector, either an analog detector (which may be optical or capacitive), or a digital encoder. Explains Robert Milkowski, president, Nutfield Technology (Hudson, NH), "Capacitive detectors have a very good signal-to-noise ratio and good linearity, but the weight of the detector impacts its speed. Optical detection galvos are the inverse of capacitive; the upside is speed, while the downside is increased noise and less linearity."

Servo technologies can also be either analog or digital. Closed-loop galvos provide feedback to the servos, which can be analog PID-type driver boards or, within the past year, fully digital state-space driver boards. According to Aylward, "A closed-loop galvo is a limited-angle actuator (typically less than 40° of rotation) with an integral angular position detector, all designed for very fast (100 μs) and very accurate (single micro-radian repeatability) positioning when driven and controlled with a servo-control driver board. Open-loop galvos are generally lower accuracy and lower cost. And resonant scanners are also available for higher-speed applications up to 16 KHz. Complete closed-loop subsystems are the most common (also referred to as scan heads), with two mounted x-y galvos, two mirrors, and two servos packaged in a box and lens-ready for lots of applications like materials processing (the biggest app)."

With the recent advances in digital control, galvanometer systems are often divided by whether their position sensing technology and the driver-board technology are analog or digital. "The world of laser scanning is now moving to fully digital systems," says Dominik Brunner, sales manager at Scanlab AG (Puchheim, Germany; see Fig. 3, left).

Yet analog systems will remain on the market because they offer price for performance, says Milkowski at Nutfield. "For the average user, analog servos do the job at a lower cost. But they typically need to be hand-tuned by a technician. Digital servos do offer auto-tuning but offer little performance benefit. Digital position detectors offer more precision but are more expensive" (see Fig. 3, right).

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Two servo configurations commonly compete to balance speed and accuracy requirements, says Aylward. An integrating servo, or Class 1, uses integrated position error to settle to the highest level of positioning accuracy with the least angular error. Applications that value precision over speed often rely on integrating Class 1 servo controllers. A nonintegrating servo, or Class 0, can provide higher system speeds because it avoids the integration time. This configuration is used when some precision (up to approximately 100 μrad) is sacrificed to increase the speed, often by 10% or more. Many of the highest-speed applications rely on nonintegrating Class 0 servos.

The categorization of galvanometers among manufacturers can be confusing. Different manufacturers categorize galvos differently, such as by application, closed-loop versus open loop, or technology type. At CTI, galvos are broken down by actuator type (moving magnet or moving coil) and type of position detector (capacitive, optical, or digital). "The most popular today is the moving magnet with an optical position detector. Digital position detection is the newest advance for applications requiring extreme accuracy but it comes at a price premium for that added accuracy," says Aylward. These digital systems are ideal for applications that require a particularly high level of thermal and positional stability, or dither.

Nutfield Technology divides the field into three categories by type: position detector, servo, and suspension. Suspension technologies are divided into bearing- or flexure-based systems. In bearing-based suspension systems, adequate for 90% of applications, the rotor is mounted using a classic round bearing technology. In flexure suspension systems, the rotor is mounted using steel flat springs, improving precision in imaging applications.

Where to start?

The Raylase web site helps narrow down a customers' needs with some key questions. "We start by asking what industry focus a customer has: automobile, semiconductor, packaging, medical, and so on," says Herzog. "Then, what application: marking, cutting, perforating, drilling, welding, engraving, prototyping, tooling, and so on. We ask what nanometer of wavelength and wattage of power you are working with (which depends on your laser), what field size, and whether or not the target is moving. Are you more concerned with speed, low drift, or both? Then we find out whether you need a package including electronic board and software."

Working fields range from 100 × 100 mm on the small end to 1.5 × 1.5 m on the large end. Spot sizes down to 300 μm are possible with CO2 lasers in a 500 × 500 mm working field for fast processing of different kinds of material. Another application is processing of objects with an uneven surface, addressed by the new Focus-Shifter family of products at Raylase, which allows the focus of the laser beam to move along the z-axis of the target.

Manufacturers agree that the top buying criteria are speed, accuracy, cost, and size. Speed or step response time is inversely related to the galvo/mirror inertia and resonant frequency, and is typically specified as the time to move to 99% of 0.1° (mechanical) positioning move. Maximum rms frequency or repetition rate is related to both the bandwidth and the capability of the galvo to dissipate the heat of high currents, rate, and duty cycle. Accuracy is given in terms of short-term repeatability, or the range of error that occurs when the galvo is repetitively commanded to the same position. For users doing alpha-numeric character marking, speed can be measured in characters per second. Generally, the higher the accuracy of a galvo system, the higher the cost. A larger beam and galvo reduce the speed and increase the cost.

Nonlinearity is another accuracy consideration involving the error difference from a perfectly linear slope of the command voltage to scan-angle position over the scan range (for example, when the command signal for -10 V gives -20°, 0 V gives 0°, and 10 V gives +20°). Size of the system is another important concern. Generally, the bigger the beam and the mirror to be deflected, the larger the galvo must be to deliver higher torque fast enough. Lifetime is also a consideration, given in terms of cycles (perhaps in the billions). Another factor to consider in very high-precision applications is thermal drift, with both offset and scale components—the position error related to changes in environmental temperature and over time.

Editor's note: The "Product Focus" series is intended to provide a broad overview of the product types discussed. Laser Focus World does not endorse or recommend any of the products mentioned in this article.

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As I described in my welcom thread, I recently built a very simple XYscanner with some speakers. I liked it quite a bit, but wasn't happy with the quality. (LOTS of noise, it worked only kind of properly near the speakers resonance, only stereo audio input, etc)
I'd like to play with this (a lot) more, so I'm considering to buy a simple XY-scanner set in order to be able to play more / better with it, without having to worry the whole setup generally breaking apart from vibrations and other hardware related issues.

The set I was looking at can be found here:
"Click me, Click me!"

I'll admit that I'm not too sure about what I should be looking for. This looks pretty neat, and has everything I need - as far as I can tell. (I do have a simple 5mW laser to start with, if I like it enough I'll buy a better one, but not yet)

My main question though is about signal input. For my own rig I just used speakers and audio, but that'll be more complicated in this set. Will I need (an)other component(s) in order to be able to make simple images? Think about lissajous-figures, with multiple waves of different amplitude, frequency and phase (might write the program for that myself) and just some simple animations. (So simple that it's (almost) possible to make them by hand by just writing down X, Y and timestamp or so.)

Any nudge in the right direction would be greatly appreciated! I'd love to buy this set and have some fun, but I don't want to spend ~€100 only to discover I need €500 more of equipment to get the most basic stuff to work. (Yes, I know, lasers can be expensive, but I just want to 'taste' first)

Thanks a lot!

EDIT: I also have an Arduino, which I'm sure will come in handy somewhere, if that makes any difference.

They take analog signals, meaning you'll need a DAC in there somewhere to interface it with an Arduino. They take a 0V to 5V signal.

This guy would probably work, one for x and one for y.
http://www.adafruit.com/datasheets/mcp.pdf
it's already got an arduino library here:
https://learn.adafruit.com/mcp-12-bit-dac-tutorial

That galvo set is a good starter set, but keep in mind they may be a little fiddly to get set up and tuned since it's a cheap set.

You guys are awesome. Posted a similar question to photonlexicon (saw a link to there somewhere on here) and basically got a hocus-pocus answer no-where near what I (thought) I needed

Seems like the Showcard in that set doesn't do anything special and that the +-5V signal is the real input, as I originally suspected. Thanks for verifying. The DAC-info definitely is useful as well! Didn't realize that wasn't on the arduino by default.

I've got two follow-up questions, and if they're answered as well as the previous one I think I'll be ordering some things this week

1) As far as my circuitry / driving knowledge goes, I could still use the audio channel from my laptop as the signal, right? In that case, that might even be easier for me, since the sound card is one huge DAC after all, and together with my (sufficient) knowledge in programming I guess I could fiddle with producing sound files containing "galvo coordinates". (I can't see why this wouldn't work, but since it's a new area to me I figured I should verify)

2) Related to 1). I don't mind fiddling a lot with 'writing' animation stuff from scratch (and probably learning a lot in the meantime), but I was wondering whether there are any open source / free programs you know of that don't use the 'exotic' connectors and plugs I don't have Well, I've read through some more threads and will be ordering the galvo set tonight. I'm convinced I'll have enough fun for my money at this stage

In the meantime, I've come up with some additional questions.

- Do I understand correctly that using my laptops soundcard as-is for e.g. lissajous figures, but not for anything that requires a DC-signal? (I read that some capacitors need to be bypassed on the card, which is why people mod external cards?

- Correction amps are just DC-DC converters that convert the soundcard output to a (higher) voltage that's accepted by the galvo set?

- If my galvo set only accepts -5 to +5 V, a correction amp wouldn't really be necessary, right? (I think that's standard audio output?)

Cheers! Audio outputs have capacitors on their outputs that block DC signals. You need to connect to the output prior to those capacitors. The sound card DAC tutorial thread, once you wade through all the shit-posting, actually has a lot of good information about sound card DACs, correction amps, and even how to create analog laser drivers.

You can't use the output of the sound card directly to drive your galvos. The sound cards depend on those output capacitors to filter out the DC, and without them, they have minimum voltage levels that are above ground reference, usually from ~1.25V for the negative peak of the waveform, to about 4.5V for the positive peak. Galvos work with a -5V to +5V differential input voltage. The correction amp allows you to take the voltage output range of the sound card (or any input you may choose to use) and adjust the output range and offset of the output for the galvo set.

Likewise, you can make correction amps for tuning analog laser outputs too. There are correction amp designs and analog laser driver designs in the sound card DAC tutorial by Benm that I use all the time.

Audio outputs have capacitors on their outputs that block DC signals. You need to connect to the output prior to those capacitors. The sound card DAC tutorial thread, once you wade through all the shit-posting, actually has a lot of good information about sound card DACs, correction amps, and even how to create analog laser drivers.

You can't use the output of the sound card directly to drive your galvos. The sound cards depend on those output capacitors to filter out the DC, and without them, they have minimum voltage levels that are above ground reference, usually from ~1.25V for the negative peak of the waveform, to about 4.5V for the positive peak. Galvos work with a -5V to +5V differential input voltage. The correction amp allows you to take the voltage output range of the sound card (or any input you may choose to use) and adjust the output range and offset of the output for the galvo set.

Likewise, you can make correction amps for tuning analog laser outputs too. There are correction amp designs and analog laser driver designs in the sound card DAC tutorial by Benm that I use all the time.

Thank you so much! Didn't know regular soundcards had this problem (or rather, property), this clears up a lot of things. And you're right, while the info is there (in the DAC tutorial thread as well as some others) , it seems to be rather dispersed.

I've ordered the scanner set, so now I'll just have to wait 2-3 weeks for it to arrive, and learn more in the meantime. It comes with 2 galvo drivers, and the description says
"Driver Adjustments: Gain,Size Position,Linearity,Damping,Servo gain"
I'd say this includes the 'rescaling the input/output voltage' that the correction amp does, but I'm not sure whether it also deals with the DC offset? As in, I'll need a soundcard DAC anyway, but will I need the correction amps as well or is everything I need included in the drivers?

If you don't know, I'll see whether I can buy all the correction amps components at the electronics store at uni The soundcard DAC tutorial, as with all threads on this forum, used to be a lot nicer before Coldshadow reduced the number of posts per page. Now you have to wade through multiple pages to find what you want. It used to be that those correction amp diagrams were on the 3rd page, but now it's a bit further in.

For the driver tuning, no, those settings affects the scale/offset for the galvo's PID feedback driver that performs the positioning. The driver itself expects to have a +/-5V differential signal inputs and you'll need to provide that regardless of what settings you change on the driver itself. You might be able to get away with just tuning the driver itself (usually it works okay out of the box) if you had a signal source that provided the +/- 5V differential voltage, but a soundcard DAC will not do that, so you need the correction amp regardless. Thanks. The small amount of posts/page was one of the first thing I noticed here, and have been looking for a setting to change that, but it seems like it's a forum setting after all then.

The link in the soundcard DAC tutorial to the actual soundcard is broken. I found a post some pages in linking to its ebay page - which is broken too - but the search terms gave me an idea of what to look for. I think this card is very similar, but there's one thing that's bothering me:

"Only R L channel PCM audio in 5.1 channel can be conveyed in digital playback mode. In analog playback mode, it supports 6 channels codec for analog playback"

Is the 'digital playback mode' something fancy we don't need? Because 3 channels at the least (X, Y and some sort of TTL on the laser) would be nice. I'd say analog playback is the standard?

Aaaand more. I should be able to produce the correction amp by BenM without problem, but would there by any chance be a similar schematic / help on how to turn the laser diode on/off with the sound card? I'm pretty sure it's just with a transistor, but I don't have a clue on whether you can get all the required power through the single transistor?

I do realize I'm asking quite a lot of questions by now. Please bear with me, and I promise to merge everything into an (updated) "Step by step laserscanner" thread for those interested! Here is the direct link to Benm's correction amp schematic, and here is the link to the current controller for analog modulation of lasers. The laser current controller linked above functions as a correction amp, so you just need to connect it to your soundcard and make the adjustments.

I also use this circuit for regulating current to my laser diodes. It can also be analog-controlled by varying the voltage to the gate of the MOSFET.

Note that with all these transistor-based analog laser drivers, the transistors are in linear mode, so they act like resistors. This means that they will also get really hot and may need to be heatsinked depending on the current.