Patent Application
20060238068
The present invention relates, in general, to ultrasonic systems and, in particular, to methods and circuitry for driving a high-power ultrasonic transducer for use with a varying load.
Ultrasound technology is utilized in a variety of applications from machining and cleaning of jewelry, performing surgical operations to the processing of fluids, including hydrocarbons. The basic concept of ultrasonic systems involves the conversion of high frequency electric energy into ultrasonic frequency mechanical vibrations using transducer elements. Such systems typically include a driver circuit that generates electrical signals which excite a piezoelectric (or magnetostrictive) transducer assembly. A transmission element such as a probe connects to the transducer assembly and is used to deliver mechanical energy to the target.
Ultrasonic transducers include industrial and medical resonators. Industrial resonators deliver high energy density in order to substantially affect the materials with which they are in contact. Common uses of industrial resonators include welding of plastics and nonferrous metals, cleaning, abrasive machining of hard materials, cutting, enhancement of chemical reactions (sonochemistry), liquid processing, defoaming, and atomization. Usual frequencies for such operations are between 15 kHz and 40 kHz, although frequencies can range as low as 10 kHz and as high as 100+ kHz. Medical resonators include devices for cutting, disintegrating, cauterizing, scraping, cavitating, dental descaling, etc.
A transducer assembly for an industrial ultrasonic application may be referred to as an industrial ultrasonic stack, and may include a probe (or a sonotrode, or a horn), a booster, and a transducer (or a converter). The probe contacts the load and delivers power to the load. The probe's shape depends on the shape of the load and the required gain. Probes are typically made of titanium, aluminum, and steel. The booster adjusts the vibrational output from the transducer and transfers the ultrasonic energy to the probe. The booster also generally provides a method for mounting the ultrasonic stack to a support structure. The active elements are usually piezoelectric ceramics although magnetostrictive materials are also used.
Existing technology for driving ultrasonic probes has been developed for driving a system at one desired frequency and power level for a specific process. This known technology utilizes an electrical system based on a Silicon Controlled Rectifier (SCR). Typically, SCR's require a forced turn off system having a particular capacitor value to control and turn off the SCR which in turn limits the operating frequency of the electrical system. Also, the SCR systems are limited to much lower power levels which do not allow for the effective control of an ultrasonic probe at higher power levels. As used herein, a high power level refers to power levels of at least 500 Watts. For example, the SCR-based ultrasonic generators drive ultrasonic probes which are designed for a specific load such as molten steel. However, an SCR-based ultrasonic generator when used in a process which exposes an attached ultrasonic probe to varying load conditions, such as the processing of liquid hydrocarbons, limits the effectiveness of the probe in different liquids. This limited effectiveness is due to the loading effect different liquids will have on the ultrasonic probe. In addition, even for a given liquid, density and phase change effects can vary the loading on the ultrasonic probe.
There is therefore a need for a high-power and variable load driving circuit for an ultrasonic generator that does not suffer from the shortcomings of SCR-based ultrasonic generators.
The present invention provides an ultrasonic generator for driving a dynamic ultrasonic probe system for use with variable loads, at operating frequencies of up to 20 kHz and power levels of up to 60 kW. The system utilizes a Full Bridge Isolated Gate Bipolar Transistor (IGBT) system to drive ultrasonic probes at a resonant frequency at different and adjustable voltage, frequency, and current levels. As an ultrasonic probe experiences different loads the electrical power requirements will change. For example, during various hydrocarbon processing (e.g., desulfurization) techniques, such as those patented by the assignee herein, many different and varying loads are seen by an ultrasonic transducer as different fluids (e.g., such as different types of crude oils, diesel fuels, etc.) are processed. Various patented hydrocarbon processing techniques which are patented by the assignee herein are disclosed in U.S. Pat. Nos. 6,827,844; 6,500,219 and 6,402,939, the disclosures of which are hereby incorporated by reference herein. By using a system such as the Full Bridge IGBT based system, in accordance with the embodiments of the present invention, one can control the required variables such as frequency, voltage and current to effectively manage the performance of the ultrasonic probe for varying loads. The varying loads typically include different compressible and incompressible hydrocarbon fluids.
In one aspect, the embodiments of the present invention are directed to a high-powered (e.g., >500 W) ultrasonic generator for delivering high-power ultrasonic energy to a varying load. In one embodiment, the ultrasonic generator includes a variable frequency triangular waveform generator coupled with a pulse width modulator. The output from the pulse width modulator is coupled with the gates of an IGBT, which amplifies the signal and delivers it to a coil that is used to drive a magnetostrictive transducer. In one embodiment, high voltage of 0-600 VDC is delivered across the collector and emitter of the IGBT after the signal is delivered. The output of the IGBT is then a square waveform with a voltage of ±600V. This voltage is sent to a coil wound around the ultrasonic transducer. The voltage creates a magnetic field on the transducer and the magnetostrictive properties of the transducer cause the transducer to vibrate as a result of the magnetic field. The use of the IGBT as the amplifying device obviates the need for a SCR circuit, which is typically used in low powered ultrasonic transducers, and which would get overheated and fail in such a high-powered and load-varying application.
For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings.
Prior to the invention of the present ultrasonic generator, the prior art ultrasonic generators relied on Silicon Controlled Rectifier (“SCR”) technology. In these generators, the SCRs pulse current through an ultrasonic probe at a frequency of about 17.5 kHz. At this fast switching frequency, the SCRs can easily become overheated and fail. To address this overheating problem, the SCRs require a forced turn off system commonly know in the field of power electronics as “Forced Commutation.” This means that when a signal is delivered to the system to turn on the SCR, it will remain on for a specified amount of time after that signal is turned off. It is possible through forced commutation to make the SCR turn off faster. This forced commutation is required for a faster switching frequency of 17.5 kHz. Often due to this process the SCR becomes weakened and fails. Another problem with the SCR systems is that a specific capacitor arrangement is needed in order to make the forced commutation occur. The result of these added capacitors is a significant loss of power. The ultrasonic generator as developed by the inventors herein, requires a small amount of capacitance and thus is more reliable than the commonly used SCR-based systems. For example, the inventors herein have compared the novel IGBT-based generator with one that uses the prior art SCR technology, and report that the while the SCR-based system for the ultrasonic probe required a total input of about 3800 Watts, the ultrasonic generator in accordance with the embodiments of the present invention produces better results with the ultrasonic probe using only 2800 Watts. In addition to being more efficient than the commonly used SCR systems, the components, namely the IGBTs, in the generator are less costly and more readily available than the SCRs.
The ultrasonic generator in accordance with the embodiments of the present invention uses an IGBT rather than an SCR. The IGBT serves as an amplifier to magnify a pulse signal sent to the gates of the IGBT. The pulse sent to the gates of the IGBT is created from a variable pulse width generator. In one embodiment, this pulse width generator uses a variable frequency triangle waveform generator whose signal is sent to a comparator circuit with a variable reference voltage. The result is that by adjusting the reference voltage in the comparator circuit, the pulse width changes. This portion (e.g., the variable pulse width generator) of the generator is sometimes used with IGBTs to control A.C. motors. The variable frequency/pulse width signal is sent to the gates of the IGBT to be magnified. Variable voltage (e.g., in the range between 0-600 VDC) is delivered across the collector and emitter of the IGBT after the signal is delivered. The output of the IGBT is then a square waveform with a voltage of ±600V. This voltage is sent to a coil wound around the ultrasonic transducer. The voltage creates a magnetic field on the transducer and the magnetorestrictive properties of the transducer cause the transducer to vibrate as a result of the magnetic field.
The power driving circuit for the ultrasonic transducer in accordance with the embodiments of the present invention represents an innovation over previous driving circuits for ultrasonic transducers. In the circuit, the power components include matched IGBTs in a full bridge power configuration. As used herein, a full bridge includes two half-bridge push pull amplifiers. Each half bridge is driven by an asymmetrical rectangular pulse train. The two pulse trains, that drive the full bridge are 180 degrees out of phase and inverted. The symmetry (e.g., percent of positive and negative pulse components) of the pulses that drive each half bridge section can be configured for any desired ultrasound output power.
The IGBT-based driving circuit in accordance with the embodiments of the present invention is described below in further detail. The IGBT circuit includes the following main components, namely: a DC power source; an IGBT; a Gate Driving Circuit; and a Closed Loop Current Sensing Circuit. Each of these components is described in further detail below.
DC Power Source
The DC power source as used herein may be any power source which rectifies and filters standard (e.g., 60 Hz) AC voltage to be a DC voltage. Generally this power conversion is accomplished by increasing the line frequency by use of a thyristor or other such device. The high frequency AC is then rectified and filtered using a capacitor tank and/or a DC choke to eliminate AC ripple. The DC power source needs sufficient power to operate the largest load that the ultrasonic probe may encounter. Typically a DC voltage of up to 0-600V is suitable with an ampere rating of 50 A giving a maximum of 30 kW. Larger systems may be used producing voltages of up to 1200V, however the maximum voltage rating of the IGBT, which is typically 1200V, needs to be taken into consideration.
The DC power source is ideally connected to the IGBT through a polar capacitor bank with a large value in order to reduce switching spikes due to the extremely high operating frequencies and high voltages. The DC capacitor is sufficiently rated to handle the maximum voltage in the system and any voltage spike that may occur.
The DC power source preferably has a variable voltage control to allow for voltage adjustment during different loading conditions. Also, the voltage adjustment will allow for the opportunity to run an ultrasonic transducer at a lower power level, if desired. In one embodiment, the voltage regulation can be a simple potentiometer style with a manual interface. Alternatively, the voltage regulation is achieved via an analog voltage or current applied to a sensor circuit, or a digitally programmed interface. It is also preferable for the power source to have a maximum current limit control which will prevent the system from overloading.
Isolated Gate Bipolar Transistor
An IGBT is used to invert a DC voltage into a pulsed bipolar rectangular waveform. IGBTs are most commonly used for motor control in variable frequency drives. The operation of an IGBT is similar to most other transistors in that a bus voltage is applied to the collector and emitter, while a signal is applied to its gate. The DC bus is then pulsed at the applied bus voltage and frequency and duty cycle of the gate signal.
An IGBT for use with a magnetostrictive transducer, such as exists in assignee's technology, can be sized depending on the loads on the transducer. During switching of the IGBT, large current spikes exist due to the magnetostrictive load being highly inductive. Thus, the IGBT used is often highly over rated for these current spikes. For example, a typical magnetostrictive transducer may require 9-10 Amps RMS. However, the current spikes may be as high as 300 Amps for only 1-2 microseconds during switching. Thus, a suitable IGBT for this type of operation should have a current rating of 300 A and a peak current rating of 600 A.
IGBT Gate Driving Circuit
An important aspect of the successful operation of the IGBT is the proper driving of its gate. Common methods for controlling IGBT gates used in motor control are not sufficient for operating the IGBT in use with a magnetostrictive ultrasonic probe. Generally, a motor control gate drive circuit attempts to simulate an alternating current similar to standard 50/60 Hz AC found in wall sockets. Thus, the IGBT is pulsed with a varying duty cycle at a very high frequency. At a low duty cycle (e.g., 10%) there is a small amount of current, then as the duty cycle increases the current also increases. When driving an IGBT for use with an ultrasonic probe a DC bias exists for successful operation. The amount of DC bias can be directly controlled in a full bridge system by varying the duty cycle of the various IGBT gates as shown in
In order to produce this type of gate driving, a waveform generator is used. The waveform generator can be any standard waveform generator which is capable of varying the frequency and/or duty cycle of the generated waveform. In one embodiment of the gate driving circuit, a triangle waveform generator is used. For example, the triangle waveform is produced by an 8038 triangle waveform generator. The 8038 chip allows for pulse width control of the in phase and quadrature IGBT control waveforms, which impacts the power management of the full bridge IGBT circuit. In one embodiment, the driving circuit uses this circuit with variable frequency control and variable pulse width control. The triangle wave is sent to two LF 353 comparators that compare a preset voltage to the positive and negative triangle waveforms to generator the in phase and quadrature control waveforms for the full bridge IGBT circuit. The quadrature control waveforms for the full bridge IGBT circuit are generated such that while the positive triangle wave is greater than the preset voltage a pulse width controlled rectangular wave is generated, and while the negative triangle wave is less than the preset voltage the quadrature control rectangular wave is generated. In an alternate embodiment, the power driving circuit uses the Global Specialties 2 MHz waveform generator. This waveform generator may also use the basic 8038 triangle waveform generator with positive and negative comparators.
In operation, the full bridge circuit is driven by the gate driving pulse trains A and B, as shown in
As shown in
In operation, the circuit of
Because the IGBT relies on rectangular power pulses, the fast current changes in the inductor produce L*dI/dT caused voltage spikes. The problem of high voltage spikes requires IGBT with high voltage capacities above the average operating voltage in the resonant transducer circuit. While the full bridge parallel resonant driver is more power efficient than the SCR driven ultrasonic transducer, it produces spikes, while an SCR-based system does not produce voltage spikes. This is because the SCRs are only actively triggered in the positive state and are turned off in the commutation mode where the transducer resonates in the commutative mode.
In operation, the circuit of
In one embodiment, the voltage controlled oscillator is based on an 8038 chip which generates a full cycle square wave with positive and negative rectangular components. The output from the voltage controlled oscillator is separated into two positive and negative pulse trains as shown in
In an alternate embodiment, a VCO is not used. Instead of a VCO, a Hall effect sensors detect the positive and negative going zero current crossings. At the positive current crossing a Positive pulse is sent to the base of Q1 and Q4 in
As will be understood by those skilled in the art, other equivalent or alternative methods and circuits for driving a high-power and variable-load ultrasonic transducer according to the embodiments of the present invention can be envisioned without departing from the essential characteristics thereof For example, the IGBT gates may be driven by a pulse train produced by any suitable wave generating device or system as described above. Accordingly, the foregoing disclosure is intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.
