moreno20solar

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Pad for future MEMS integration, HV buffer & solar cells chip, and SCµM (left to right).

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System Diagram

Abstract

We demonstrate a wireless temperature sensor consisting of only a chip with an ARM Cortex-M0 and radio, a mm-scale solar panel, and one surface mount capacitor. The radio lacks a crystal reference in favor of other on-chip oscillators to reduce cost and size. Without an absolute time reference, the on-chip oscillators need to be calibrated because their frequencies vary with supply voltage and temperature. An initial optical calibration procedure is used along with temperature-based compensation. This enables the radio to properly transmit standards-compatible IEEE 802.15.4 wireless packets over a temperature range from 35.5 C to 40.0 C. The standard deviation of the temperature estimate error relative to a reference is 0.28 C. The power for all of the components, including the radio and the microprocessor, is supplied by a solar panel on a CMOS chip under 200 mW/cm² of irradiation.

Introduction

Developing autonomous micro-systems is challenging because they integrate multiple subsystems, such as:

• power source

• low power electronics for sensing, control, and communication.

Background:

1. The Michigan Micro Mote is a 2×4×4 mm³ wireless imaging system with an integrated processor, radio, battery, and solar cells.

2. Fetik et al. demonstrated a 8.75 mm³ energy-autonomous temperature sensing system with a battery, solar cells, and an ARM Cortex-M3 operating at 7.7 µW.

3. Wu et al. use a Cortex-M0+ processor on a 0.04 mm3 programmable temperature sensor system featuring 2-way optical communication and a base-station generated clock reference.

Previous autonomous micro-systems usually use custom ASICs, optical communication, or external clock references.

The Single-Chip Micro Mote (SCµM), however, integrates a full microprocessor and crystal-free standards-compatible 802.15.4 radio.

Some applications for SCuM:

• H₂S gas sensor

• cm-accuracy 3-D localization sensor

• controller to actuate MEMS grippers

This work integrates SCµM, a solar panel (Zappy2), and an 100 µF 0805 capacitor into an autonomous solar-powered wireless temperature sensor.

This integration is challenging due to:

• energy constraints

• large voltage variations

• clock calibration requirements

System Description

Single-Chip Micro Mote (SCµM)

• a 3×2×0.3 mm3 system-on-chip

• designed to be alow-cost, low-power wireless sensor node

• features an ARM Cortex-M0 microprocessor, a standards-compatible 802.15.4 radio, and an optical receiver

• requires just a 1.5 V power supply to operate

• features several tunable CMOS oscillators, including:

  • 20 MHz RC oscillator used to generate the clock for the Cortex microprocessor

  • 2 MHz RC oscillator used as the chipping clock for 802.15.4 transmission

  • 32 kHz oscillator used as a low-power sleep timer

  • 500 kHz RF timer used to generate interrupts based on user-defined counters or the RF controller

  • 2.4 GHz LC oscillator used as its local oscillator (LO) to dictate the 802.15.4 channel frequency

• The LO frequency can be tuned by a 15-bit capacitive DAC, split into three 5-bit capacitive DACs called the coarse, mid, and fine codes.

• SCµM’s oscillators are sensitive to the supply voltage and temperature

Solar Cells

Transmitting a packet requires a lot of power, so we need to charge the VBAT capacitor between periods of radio operation. Normally, the CPU clock rate is at 5 MHz, but we decrease it to 78 kHz to further reduce SCµM’s power consumption in a low-power state.

System Operation

Optically Program and Calibrate

At 39 ◦C, SCµM is connected to an external 1.7 V VBAT source due to the sustained high-power radio-on period required during calibration. Once calibrated, a bright light is turned on to provide irradiation.

Local Oscillator Temperature Compensation

The first step for the calibration procedure used for this system involves determining which LC fine codes allow for proper radio transmission at which temperature.

1. As we vary the temperature with a hot plate, we continuously sweep through all 32 LC fine codes, measure the ratio of the on-chip 2 MHz and 32 kHz frequency counts over 100 ms, and transmit the ratio in a 14-byte 802.15.4 packet.

2. The coarse and mid codes were predetermined from an earlier calibration.

3. Throughout this process, we record the frequency ratios and LC fine codes of the packets that are successfully received by an OpenMote CC2538.

4. For each received packet, a reference temperature measurement is taken with a TMP102 digital temperature sensor (±0.5 ◦C accuracy) attached to a Teensy 3.6 microcontroller.

5. A linear model is then fit between the clock ratio and the LC fine code as shown below.

6. For subsequent radio operation, SCµM measures the clock ratio and then use this linear model to determine which LC fine code to transmit at.

Temperature Estimate Calibration

• This calibration is accomplished by continually transmitting the 2 MHz and 32 kHz clock ratio to an OpenMote across a temperature range controlled by the hot plate.

• SCµM continuously corrects its LO frequency using the linear model above.

• Meanwhile, temperature measurements are taken by a TMP102 digital temperature sensor. A linear regression is calculated between the 2 MHz and 32 kHz clock ratio and the reference temperature
  measured by the Teensy.

• SCµM then measures the clock ratio and uses this model to produce temperature estimates.

• This calibration is chip specific.

Temperature Estimate Operation and Accuracy

SCµM is programmed with the following procedure to test the accuracy of the temperature estimates:

1. Measure 2 MHz and 32 kHz clock counts over 100 ms using the RF timer and compute the ratio.

2. Use the model above to update the LC fine code.

3. Use the model above to estimate the temperature.

4. Decrease the CPU clock rate to 78 kHz for 1 s to charge the VBAT capacitor.

5. Increase the CPU clock up to 5 MHz to transmit a single 10-byte packet containing the temperature estimate.

6. Repeat from step 1.