Patent No. 5697076 Suspended carrier modulation of high-Q transmitters

 

Patent No. 5697076

Suspended carrier modulation of high-Q transmitters (Troyk, et al., December 9, 1997)

Assignee: Illinois Institute of Technology (Chicago, IL), United States of America as represented by the Department of Health and (Washington, DC)

Abstract

A method for on-off modulation of a transmitter coil current of a high-Q resonant circuit transmitter comprising the steps of sensing a zero-crossing of the transmitter coil current and substantially instantaneously interrupting the transmitter coil current, and a high-Q resonant circuit transmitter for carrying out said method.

 

Notes:

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a high-Q resonant circuit transmitter having means for on-off modulation of the transmitter coil current by which virtually no energy is lost during the current interruption such that, when the current is allowed to resume in the transmitter coil, its peak amplitude is not diminished. This permits the transition time between modulation states to be near zero. Such transmitters are suitable for use in a wide range of applications including data and power transmission for implantable micromodular devices for use in functional electrical stimulation for clinical application including reanimation of paralyzed limbs. Large numbers of these devices can be implanted and controlled by a single, external coil that transmits power and command signals by inductive coupling from a highly efficient power oscillator and modulator circuit in a wearable control box. The devices generally consist of a microcoil wound on a ferrite core, a custom IC chip, and a glass cylindrical capsule containing glass-to-metal feed through for electrodes at the ends.

2. Description of Prior Art

High-Q resonant circuits are often used as transmitters for inductively powered implanted electronic devices. However, high-Q circuits, by their nature, respond slowly to transients. By high-Q, we mean a resonant circuit transmitter having a Q greater than about twenty.

Several types of power converter topologies have been used in conjunction with high-Q resonant load networks for extracorporeal transmitters as part of implantable electronic systems. These include circuits based upon class A, class B, class C, and class E configurations. For most implantable systems, it is desirable to minimize power consumption by the transmitter circuitry. Therefore, high-Q resonant load networks are preferred, due to their inherently low real power consumption. This low power is a direct result of the low equivalent series resistance characteristic of a high-Q network.

Unfortunately, the use of a high-Q network, while enhancing power transfer from the transmitter to the implanted device also severely limits the bandwidth for data transfer to the implanted device. The data transfer is often accomplished by modulation of the transmitter carrier. The high-Q network limits the rate at which the peak amplitude of the transmitter carrier can be changed.

Class E resonant power converters offer high efficiency and consequently, in operation, Class E resonant power converters are characterized by low power dissipation, low junction temperature and high reliability. However, despite its extremely high-efficiency, Class E-type power converters are difficult to regulate and control. Class E power converters must be operated in a "lossless region". Otherwise, large amounts of power are dissipated in the transistor switching devices of the power converter, resulting in damage to these components. To maintain operation in the "lossless region", substantially zero voltage and zero slope conditions must be maintained at the switch-on times for the power switching device. One such Class E resonant power converter is taught by U.S. Pat. No. 5,179,511.

The '511 patent teaches a Class E resonant power converter comprising controllable switching means having non-conducting and conducting states, and drive means providing drive signals for causing the switching means to switch between its non-conducting and conducting states to supply direct current power to a load through a high-Q resonant load network. Sensing means for sensing load current and control means responsive to the sensing means for controlling the drive means to provide switching of the switching means between non-conducting and conducting states only when the amplitude of the voltage across the switching means is minimal and the slope of the voltage waveform for the voltage is substantially zero are also provided. In this manner, power converter operation is maintained in a minimal loss region.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a high-Q resonant circuit transmitter, the current of which is subject to instantaneous interruption with zero or substantially zero loss of energy from the circuit.

It is another object of this invention to provide a high-Q resonant circuit transmitter, the current of which can be turned on for a time as short as one carrier cycle, and turned off, that is suspended, for any time desired, including fractions of cycles.

It is another object of this invention to provide high-Q resonant circuit transmitters suitable for use with implanted devices which function as sensors, whereby the outgoing telemetry or data collection can be accomplished without interference from the transmitter.

It is yet another object of this invention to provide a high-Q resonant circuit transmitter in which the data rate for transmission of commands, in the form of on-off transmitter pulses, can approach the transmitter frequency while lowering the frequency of the transmitter to increase its efficiency. Using known modulation techniques, lowering of the frequency of the transmitter results in lowering of the data rate transmission. Accordingly, it is an object of this invention to provide a high-Q resonant circuit transmitter capable of operating at lower transmitter frequencies while maintaining data rate recovery at levels corresponding to higher transmitter frequencies.

These and other objects are achieved by a high-Q resonant circuit transmitter in accordance with this invention comprising control means for on-off modulation of the transmitter coil current whereby the transmitter coil current is substantially instantaneously interrupted at a zero-crossing of the transmitter coil current.

Further, in accordance with this invention, storage means for storage of the energy of the resonant circuit during the interruption of the transmitter coil current are provided. In this manner, all of the energy in the circuit is stored during the current interruption and resumption of oscillation with little or no transient behavior is possible.

Still further in accordance with this invention, switching means having conducting and non-conducting states are provided, which switching means are controlled by said control means to switch between the conducting state and non-conducting state in synchronization with the zero-crossings of the transmitter coil current.

Accordingly, a method for on-off modulation of a transmitter coil current of a high-Q resonant circuit transmitter in accordance with this invention comprises the steps of sensing a zero-crossing of the transmitter coil current and substantially instantaneously interrupting the transmitter coil current at said zero-crossing. The transition time between on-off modulation states in accordance with the method of this invention approaches zero. In addition, operation of the high-Q resonant circuit transmitter during the "on" period is independent of operation during the "off" period.