08-06-2013, 03:12 PM
A Generic Multielement Microsystem for Portable Wireless Applications
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ABSTRACT
An open-architecture microsystem that can be populated with
a variety of sensors and actuators is described. The microsystem
is designed for low-power wireless applications where small
size and high sensor accuracy are important. It consists of an
in-module microcontroller connected to multiple front-end transducers
through an intramodule sensor bus. An external interface
allows internally processed data to be output through either a
hard-wired input/output port or a radio-frequency transmitter. The
present microsystem is configured for environmental monitoring
and measures temperature, barometric pressure, relative humidity,
and acceleration/vibration. It occupies less than 10 cc, consumes
an average of 530 W from 6 V, and transmits data up to 50 m.
System features such as active power management, the intramodule
sensor bus, generic bus interface circuitry, and in-module sensor
compensation based on bivariate polynomials are discussed.
INTRODUCTION
Driven by rapid advances in microcomputers and global
connectivity, many of the most important emerging markets
for microelectronics require the ability to gather information
from the nonelectronic world [1], [2]. Examples include
health care (diagnostic and therapeutic devices, prosthetics),
automotive systems (smart vehicles and smart highways),
automated manufacturing [including smart very-large-scaleintegration
(VLSI) process tools], environmental monitoring
and control devices, defense systems, and many
consumer products. Using integrated circuit technology and
extensions of it, integrated sensors and microactuators are
being developed to provide the necessary input/output (I/O)
devices.
GENERIC MICROSYSTEM ARCHITECTURE
The open architecture shown in Fig. 1 partitions each
individual microsystem so it can be populated to fit the
requirements of different applications without limiting the
transducer technologies that can be accommodated. Each
microsystem can be used as part of a distributed network
of such devices connected through a system bus
to a remote host. This external bus may be either hard
wired or wireless. Within each microsystem node is signalprocessing
electronics that operates under stored program
control to perform preset tasks and respond to commands
received from a host system. In-module memory stores the
control programs as well as sensor-specific code that describes
the operation of each front-end device. The control
electronics connects to the transducer front end through
an intramodule sensor bus, which allows read and write
instructions to be issued to the transducers.
The Microcontroller
The microcontroller unit (MCU) used in such microsystems
should provide a microprocessor for signal processing
and stored-program control, I/O interfaces to both the external
system bus and the intramodule sensor bus, and an onchip
ADC and timing hardware to accommodate different
sensor data formats. Adequate on-chip read-only memory
(ROM), random-access memory (RAM), and electrically
erasable and programmable (EEP)ROM are important, as
well as overall power dissipation, which is important in
spite of the relatively low duty cycles in many applications.
For the present Cluster, the Motorola 68HC11 is used. It
has an 8-b processor, an RS-232 compatible UART-type
interface for external I/O, a synchronous serial peripheral
interface adapted for sensor bus communications, an 8-
b ADC for converting analog sensor data, signal timing
hardware for frequency-encoded sensor data, a variety of
on-chip memory, and a low-power mode that maintains
RAM data while drawing less than 50 A [18]. The active
power dissipation is about 90 mW with a 2-MHz bus clock.
EPROM (96 kb) is used to store control and sensor-specific
program code, EEPROM (4 kb) is used to store operation
and compensation data for individual sensors, and RAM (4
kb) is used for the temporary storage of sensor data and
program variables.
MICROSYSTEM COMPONENTS
The generic architecture introduced above defines a basic
set of control, communication, and front-end components
that will now be discussed in more detail to illustrate
issues important to microsystem development. These issues
will be presented as they relate to the prototype Cluster
[9]. This microinstrumentation system was designed to
demonstrate how the generic architecture could be used to
build a microsystem that is small in size, dissipates very
low power, utilizes wireless communication, and provides
highly accurate, fully compensated sensor data. As shown
in Fig. 3, the Cluster consists of external interfaces for
both hard-wired and wireless communication, a control
block containing a microcontroller and power-management
circuitry, and several front-end sensing nodes. The control
block is connected to the sensors through an intramodule
sensor bus and sensor interface chips.
Power Management
In battery-powered microsystems, it is important to minimize
power consumption wherever possible. In the transducer
front end, capacitive sensors should be used since
these devices offer high sensitivity yet consume no power
and can be read out rapidly using low-power circuit techniques.
For wireless operation, it is important to use lowpower
telemetry hardware, understanding the direct tradeoff
between power and communication range. While many efforts
are currently under way to reduce the power consumption
of MCU’s [20] and wireless communication devices
[21], system-level approaches to power management are
also important. A common method for conserving power
that was adopted in the Cluster is to power down unused
subsystems. Since the MCU here is periodically shut down,
a separate power-management chip (PMC) was necessary.
This all-digital complementary metal–oxide–semiconductor
(CMOS) circuit can implement all of the necessary functions
with less than 10 A of supply current. As shown
in Fig. 4, the PMC contains an on-chip clock generator
and timing hardware that can measure eight discrete time
periods between 15 s and 5 min based on a 4-b code
input from the MCU.
THE TRANSDUCER FRONT END
The transducer front end of a microsystem consists of one
or more sensing nodes. Each node contains sensors and/or
actuators with appropriate readout circuitry to interface with
the intramodule sensor bus. For open architectures, the front
end can be populated based on the requirements of the
selected application, allowing the same basic system to be
used for a variety of applications. In the Cluster, each
front-end sensing node has an assigned 4-b address code,
allowing up to 16 such nodes to be accessed within the
same microsystem. Each node can contain multiple sensors
and/or actuators, limited only by the sensor bus instruction
format. As described above, the data format used on the
Cluster contains a 5-b element address that allows 32
readable elements and 32 writeable elements within each
sensing node. Because the front-end transducers in the
present Cluster are all capacitive, a generic interface chip
can be used to connect all of these devices to the sensor bus.
This generic interface chip and the individual sensors used
for environmental monitoring in the Cluster are discussed
below.
Temperature Sensor
To measure temperature in close proximity to the other
sensors in the microsystem, a temperature sensor has been
integrated on the interface chip. This sensor provides data
that can be used to compensate digitally for the temperature
sensitivity of any other sensors connected through this chip.
The temperature sensor utilizes the temperature dependence
of the drain current of an MOS transistor in weak inversion
[40]. The charging current for the capacitively loaded
Schmitt input stage of a ring oscillator is set by a p-channel
MOS transistor biased for subthreshold operation. Since this
charging current is temperature dependent, the frequency of
the oscillator provides a measure of the local temperature.
A typical device displays a sensitivity of 4 ms/ C at 60 C
and 33 ms/ C at 20 C with a resolution better than
0.5 C across the tested range. Although the sensor is highly
nonlinear, it is easily calibrated using the digital techniques
discussed later in this paper. While many integrated circuit
temperature sensors exist [41], this technique provides a
direct digital output and very low power dissipation, and
can easily be implemented in a standard CMOS process.
CONCLUSION
This paper has reported a microinstrumentation system
for the measurement of environmental temperature,
barometric pressure, humidity, and acceleration. It uses a
generic open architecture that permits it to be customized
for a given application through the choice of front-end
sensors and through the control software resident in the
embedded microcontroller. The MCU periodically scans
the sensors, calibrates and compensates their data, and
communicates the resulting information to the outside world
using either a hard-wired system bus or a wireless link.
The scan rate is programmable and adaptive.