Aimless exploration with power ultrasonic applications
I have previously ruled out the Apex PA314 as not having the power desired for this project. It turns out that they have even more crazy solutions, including the PA03 (among others). https://www.apexanalog.com/products/pa03.html
Look at the Package on this thing
It dissipates 500W thermal, and can supply ±75V at 30A Pk. This is designed for, among other things, driving sonar. The only downside here is price tag, clocking in at nearly $600 dollars on Digikey.
it can be had cheaper on eBay, and they do make some more "down market" offerings that should also work.
Part of me wants to just shell out ~$150 for one of these premo solutions and be done with this. Buys power, voltage, low harmonic distortion, etc...
On the other hand there's replicating some of the work that's been done by others in the hobbyist community with half bridge FET solutions. The main downside to these is the high harmonic content (I.E. square wave generation), which makes tuning more difficult.
So I've been stalled on selecting the power amplifier for this project for a while. because of the high frequencies (40kHz, possibly higher), class-D amplifiers (i.e. PWM) aren't really viable. Many other projects have just used power MOSFETs that produce square waves and either filter that or just eat the harmonic distortion.
I may yet go with the power FETs due to their expense, but I really wanted to have reasonably sinusoidal outputs.
Enter the Apex PA341, which purpose built for power ultrasonic devices. Here's the suggested implementation from the datasheet.
This balanced driver pushes 660V at 60 mA continuous! This means I can just take an eBay AD9851 DDS source and just push that out. Sadly that doesn't get me to the 400+ W I was hoping for.
That said, this is a really cool part for higher precision power ultrasonic devices such as motors or similar.
Finally got around to ordering an SHV cable, and it finally arrived from HK.
Just ran the eBay ultrasonic cleaning supply with the Branson welding converter. They are not in tune, but it does make a noise and make some weird currents when immersed in water.
Next up: tuning and alternative power supplies.
I bought two different transducers: an el-cheapo langevian and a proper Branson converter. Both are 40kHz units.
Not much to see here. The piezo is 4.4nF. Here is a first draft of the schematic of the single layer driver board, with as many components marked as I could get with a bit of googling.
On to the real prize: Made in 'Murica! Branson 4TP RF converter. my google fu has largely failed to turn up a datasheet for the specific model number, but a 4TP unit is listed as compatible with the Branson 2000BDC 40:.8 power supply, which is a 450W continuous, 800W peak output unit. The input connector shows 5.7 nF, which leads me to believe there are be 3 or more piezo elements inside.
She's a beauty to be sure. The green booster is 1:1.5, and the output horn appears to be a high amplitude gain step horn. Of course, the used equipment purchasing experience wouldn't be complete without weird incompatibilities, so while it looks like it takes BNC, the connector is actually a reverse polarity SHV connector.
the output horn is threaded with 1/4" fine (28 tpi) threads, and the hole is about 5/8" deep, so it takes hardware store bolts as viable working tips. I'll be using these once I get a tuning rig in place. I'm hoping that a longer bolt will work as a 1/2 wavelength element and can be used as the working tip for sonifying liquids.
Here is an electrical model of a power piezo transducer. I pulled the component values from the following video: https://www.youtube.com/watch?v=EAFNjyx3uX0. This matches behaviorally with what I’ve read. C3 is representative of the electrical capacitance of the piezo element and electrodes, and the series LCR circuit composed of L1, C4 and R3 is representative of the mechanical domain inertia and compressive spring forces.
An AC sweep reveals the following current response:
Note the resonance at 28kHz where current is maximized, and the anti-resonance at 34kHz where current is minimized. The goal for most ultrasonic drivers is to drive at either resonance or antiresonance, depending on application.
I am starting with the power driver from: http://www.imajeenyus.com/electronics/20110514_power_ultrasonic_driver/index.shtml
To increase output power I’m turning the half bridge to an H bridge, which will approximately double the drive voltage. Here is a spice model of an h-bridge driver, piezo transducer, and matching network. The H-bridge will approximately generate a square wave output voltage.
You can see the issue that is addressed at the top of the above article: namely lack of matching network. This causes huge current spikes in the mosfets on switching and poor drive voltage, as shown below:
So clearly I need some sort of matching network, or a sinusoidal driver. The major advantage of an H-bridge is that the FETs are nearly always in saturation, which greatly reduces the heat they produce. That said, I’m fairly confused about how to go about matching this. The fundamental frequency is already tuned to match the fundamental of the transducer. From playing around with spice a bit, I now suspect it has to do with matching higher-order harmonics of the drive waveform. The impedance of the transducer is inductive for anything much higher than the anti-resonant frequency. I added an LC matching network that puts a resonance at the 3rd harmonic (87kHz). Here is the full model with matching network and the current response with and without the network.
This barely shifts the primary resonance, but does substantially change the impedance to the 3rd harmonic. I would expect this to improve my current spikes, but instead, it does nothing:
Here is the network added to the total model, and the resulting current through M1:
I’m not really sure what to do from here. I’m having trouble finding useful results on google as most searches for “impedance matching” and variations thereof turn up RF matching systems. These mostly discuss matching resistive loads to the intrinsic impedance of a transmission line, not inductive loads in a lumped element system.
I’ll verify these model results with some real world devices when I have all the parts necessary. If i come up with anything, I’ll post it here. In the meantime, any ideas would be great.
If you are looking to build an at-home power ultrasonics system, there are a lot of options available to you, and deciding what you need can be confusing.Power ultrasonics are heavy industrial tools with heavy industrial price tags, especially if you look to do anything with high power greater than about 50 watts. Here is a quick breakdown of what you need, and what’s available.
A power ultrasonic system has 3 main components. These often come bundled together as either completed or partial systems. The components are:
These three components generate electrical power, convert electrical power to mechanical vibrations, and direct the vibrations into your target.
A power supply needs to provide high voltage, high power AC at the resonant or anti-resonant frequency of the transducer system. The main things to consider are: drive frequency, drive power, user friendliness, and safety of the design. Your transducer must match the resonant frequency of the transducer for best results and longest tool life. High quality industrial systems will provide features that automatically detect the resonant frequency and adjust their output accordingly. This will control for drift due to tool wear, cleanliness, loosening of fasteners, temperature shifts, component aging, and the like. This can also provide a warning to the operator if the system has gone out of tune and needs maintenance, repair, or replacement. High quality systems also permit reliable adjustment and measurement of output power to permit process control, especially for sonochemical or cell disruption processes. Safety consists of fuses, grounding, isolation, and other common high power safety systems. When I get my hands on some physical units, I will determine how safe these various options are.
You have three main options for power supplies:
The transducer is a large piezoelectric element. They come as “raw” ceramic components, or completed assemblies that stack several ceramic elements, electrodes, and possibly mechanical preload screws, mounting enclosures, cooling elements, and mounting fasteners.
Power systems require mechanical preload. Almost all piezo elements are made from a ceramic material called lead zirconate titanate, or PZT. Like other ceramics, it is brittle, and does not handle large changes in shape without cracking. Preload means clamping the ceramic to allow large forces to be created without large changes in physical size, thus decreasing the tendency to crack or break. The most common design is the Langevin, which uses annular ring ceramics clamped between two pieces of steel with a bolt. Here is a cross section image sourced from http://techblog.ctgclean.com/2012/01/ultrasonics-transducers-piezoelectric-hardware/: