Chenling Huang

Calibration and Characterization of Self-Powered Floating-Gate Usage Monitor With Single Electron per Second Operational Limit

Self-powered monitoring refers to a signal processing technique where the computational power is harvested directly from the signal being monitored. In this paper, we present the design and calibration of a CMOS event counter for long-term, self-powered mechanical usage monitoring. The counter exploits a log-linear response of the hot-electron injection process on a floating-gate transistor when biased in weak-inversion. By configuring an array of floating-gate injectors to respond to different amplitude levels of the input signal, a complete analog processor has been designed that implements a level counting algorithm, which is widely used in mechanical usage monitoring. Measured results from a fabricated prototype in a 0.5-¿m CMOS process demonstrate that the processor can sense, store and compute over 105 usage cycles with an injection limit approaching one single electron per second and with a counting resolution of 5 bits. This paper also presents a calibration algorithm that is used for compensating the variations which arise due to device mismatch, power supply and temperature fluctuations. The maximum current rating of the fabricated analog processor has been measured to be less than 160 nA making it ideal for practical self-powered sensing applications.

An Asynchronous Analog Self-Powered CMOS Sensor-Data-Logger With a 13.56 MHz RF Programming Interface

Design and implementation of a hybrid energy scavenging integrated circuit (IC) is presented which includes an asynchronous self-powered analog sensor-data-logger (SDL) unit and a 13.56 MHz radio-frequency (RF) programming interface. The SDL unit operates on an event-based analog self-powering technique where the energy for sensing, computation and non-volatile storage is harvested directly from the signal being sensed. By exploiting operational primitives inherent in a controlled hot-electron injection mechanism, the SDL unit eliminates the need for voltage regulation, energy storage, ADCs, MCUs and RAMs which are commonly used in traditional energy scavenging sensors. Remote programming and data interrogation of the SDL unit are performed using an integrated 13.56 MHz RF back-telemetry interface. The interface consists of a 6-instruction set digital command and control unit based on a token-ring architecture; a high-voltage generator for programming floating-gate (FG) transistors; and an RF front-end unit for communicating with an external reader. We show that the self-powered design is suitable for integration with electro-capacitive transducers (e.g., piezoelectric transducers) that can generate open-load voltages greater than 5 V and drive currents less than 200 nA. Measured results from prototypes fabricated in a 0.5-μ m standard CMOS process demonstrate that the IC consumes less than 90 nA in the self-powering mode and less than 200 μW of power in the RF-powering mode with an interrogation distance up to 40 mm. By combining self-powering and RF-powering, we show that the sensor experiences minimum down-time and can continuously monitor and record level-crossing statistics of different attributes of sensor signals.

Rail-to-Rail, Linear Hot-Electron Injection Programming of Floating-Gate Voltage Bias Generators at 13-Bit Resolution

Hot-electron injection is widely used for accurate programming of on-chip floating-gate voltage and current references. The conventional programming approach involves adapting the duration and magnitude of the injection pulses based on a predictive model which is estimated by using measured data. However, varying the pulse-widths or amplitudes introduces nonlinearity in the injection process which complicates the modeling, calibration and programming procedure. In this paper, we propose a linear hot-electron injection technique which significantly simplifies the programming procedure, and can achieve programming accuracy greater than 13-b which is limited by the thermal noise from the injection process. The procedure employs an active feedback circuit which ensures that all the nonlinear factors affecting the hot-electron injection process are held constant, thus achieving a stable and controllable injection rate. Measured results using an array of floating-gate voltage reference prototyped in a 0.5-μm standard CMOS process demonstrate that the injection rates can be controlled from 0.1 to 4.1 V for the programmable voltage range. Using 50-ms injection pulses, we show that the average injection rate can be adapted from 6.9 to 250 μV/cycle.

An Ultra-Linear Piezo-Floating-Gate Strain-Gauge for Self-Powered Measurement of Quasi-Static-Strain

In this paper we describe a self-powered sensor that can be used for in-vivo measurement of the quasi-static-strain and also for in-vivo measurement of the L1 norm of the strain signal. At the core of the proposed design is a linear floating-gate injector that can achieve more than 13 bits of precision in sensing, signal integration and non-volatile storage. The injectors are self-powered by the piezoelectric transducers that convert mechanical energy from strain-variations into electrical energy. A differential injector topology is used to measure the quasi-static strain by integrating the difference between the L1 norm of the piezoelectric signal generated during the positive and negative strain-cycles. The linear floating-gate injectors are integrated with charge-pumps, digital calibration circuits and digital programming circuits to form a system-on-chip solution that can interface with a standard bio-telemetry platform. We demonstrate the proof-of-concept self-powered measurement of quasi-static strain and L1 norm of the strain signal using sensor prototypes fabricated in a 0.5- μm standard CMOS process and validated using a bench-top biomechanical test setup.

A hybrid energy scavenging sensor for long-term mechanical strain monitoring

We had previously reported a self-powered floating-gate level-crossing sensor/processor where the energy for sensing, computation and storage was extracted directly from the input strain variations. However, self-powering was found insufficient for wireless interrogation and configuration of the sensor. In this paper, we present a hybrid energy scavenging sensor where self-powering is employed for long-term ambient mechanical strain monitoring, whereas data digitization, framing, telemetry and high-voltage floating-gate configuration/programming are performed remotely using RF powering. As a hybrid energy scavenger, the sensor can seamlessly harvest working energy from both vibrations and RF signals under different working conditions. Therefore, the sensor does not experience any downtime and can continuously operate by recording key statistics of the ambient strain signals. Sensor prototypes with an integrated 13.56MHz RF interface have been fabricated in a 0.5-µm standard CMOS process and the measured results verify the long-term autonomous monitoring capability of the sensor.

Self-powered CMOS impact-rate monitors for biomechanical implants

We have previously reported a novel self-powered piezo-floating-gate sensor that can be used for long-term monitoring of strain levels in biomechanical implants. In this paper, we extend this work to monitor impact-rates (rate of change of strain levels) which is important for predicting mechanical fatigue. We augment the piezo-floating-gate sensor with a filtering and triggering circuit that activates the ionized-hot-electron-injection (IHEI) only when the impactrates exceed predetermined threshold levels. Using multiple prototypes fabricated in a 0.5-mum standard CMOS process we characterize the performance of the sensor for mismatch and for its variability under different biasing conditions. Experimental results obtained using the prototypes demonstrate that the sensor can record different impact-rate levels over a duration of 105 cycles.

Low-threshold voltage multipliers based on floating-gate charge-pumps

The paper presents a low-threshold voltage multiplier circuit that can be used for harvesting energy from ambient radio-frequency (RF) signals. At the core of the circuit is a charge-pump based on CMOS floating-gate transistor diodes (FGTD) whose threshold voltage can be adjusted using indirect programming. We show that the diodes can achieve threshold voltages less than 50 mV, which is typically less than the conventional Schottky diodes fabricated in an equivalent process. A prototype of a 5-stage charge-pump is fabricated in a standard 0.5-mum CMOS process (Vth = 0.7 V and -0.9 V for nMOS and pMOS transistors respectively). Measurement results validate the functionality of the prototype for multiplying and regulating sub-threshold input signals. Using a 5-stage charge-pump we demonstrate operation at less than 300 mV input signal range with an operating frequency ranging from 1-4 MHz.

A self-powered static-strain sensor based on differential linear piezo-floating-gate injectors

In this paper we describe a self-powered micro-sensor that can be used for embedded measurement of static-strain. At the core of the proposed design is a linear floating-gate injector that can achieve more than 13 bits of precision in sensing, signal integration and non-volatile storage. The injectors are self-powered by piezoelectric transducers which convert mechanical energy due to strain-variations into electrical energy. A differential injector topology is then used to measure static-strain by integrating the difference between the signal energy generated during the positive and negative strain-cycles. We demonstrate the proof-of-concept using measurement results obtained from prototypes fabricated in a 0.5-µm standard CMOS process.

A temperature compensated array of CMOS floating-gate analog memory

Floating-gate transistors have been extensively used as analog memory elements in adaptive learning and neural systems. However, conventional techniques for storing and programming sub-threshold currents on floating-gate transistors are sensitive to temperature variations thus limiting their applicability to controlled environments. In this paper, we propose a temperature compensated floating-gate array which can be used to store and program currents down to nanoampere level. The core of the proposed current memory is a dual-channel floating-gate transistor based current reference circuit which uses a linear resistor in translinear loop. As a result the stored current is linearly proportional to the charge on the floating-gate and hence can be precisely programmed. The paper presents results from a prototype fabricated in a 0.5-µm CMOS process which validates the functionality of the proposed current memory cell.

A miniature batteryless health and usage monitoring system based on hybrid energy harvesting

The cost and size of the state-of-the-art health and usage monitoring systems (HUMS) are determined by capacity of on-board energy storage which limits their large scale deployment. In this paper, we present a miniature low-cost mechanical HUMS integrated circuit (IC) based on the concept of hybrid energy harvesting where continuous monitoring is achieved by self-powering, where as the programming, localization and communication with the sensor is achieved using remote RF powering. The self-powered component of the proposed HUMS is based on our previous result which used a controllable hot electron injection on floatinggate transistor as an ultra-low power signal processor. We show that the HUMS IC can seamlessly switch between different energy harvesting modes based on the availability of ambient RF power and that the configuration, programming and communication functions can be remotely performed without physically accessing the HUMS device. All the measured results presented in this paper have been obtained from prototypes fabricated in a 0.5 micron standard CMOS process and the entire system has been successfully integrated on a 1.5cm x 1.5cm package.