• Impedance Matching Woes

    JasMoH2 days ago 0 comments

    Piezoelectric element modeling

    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.

    Drive circuit modeling

    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:

    The problem

    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.

  • Purchasing Power ultrasonics - Demystified

    JasMoH4 days ago 0 comments

    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:

    • Power supply
    • Ultrasonic transducer
    • Output horn or mechanical coupling

    These three components generate electrical power, convert electrical power to mechanical vibrations, and direct the vibrations into your target.

    Power supply:

    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:

    1. Chinese ultrasonic cleaner power supplies. These cost around $30, output a fixed frequency of 28kHz or 40Khz, and have have no frequency adjustment, power adjustment, or any other “nice” features built in. They are by far the easiest and cheapest option available. I have purchased one and will be experimenting with it soon. Sources: ebay, alibaba, and ilk.
    2. Real industrial systems. These are the high end “cadillac” option. Common brands include Dukane and Branson, but others are available. You can pick whatever features, frequency, and power output you want, but they will cost you. I have not done an exhaustive search of actual sale price, but they typically list for around $1000 used, often more. These may be a viable option for a small company, but they are solidly out of my price range as a hobbyist.
    3. DIY. I’m still investigating the price point you can expect out of this, but this is what I perceive to be the only viable option for >100W supplies at sub $1k prices.

    Transducer

    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/:

    Purchasing options

    1. Flat, raw element ultrasonic bath disk transducers. ~$3, 35W, 40Khz. No housing, cooling, or even wiring....
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