System Board 6266
MAXREFDES82#: SMART FORCE SENSOR
Maxim’s MAXREFDES82# features a next generation industrial, smart force sensor.
Mounted on a quadrant of load cells channeled into a multi-channel, 24-bit analog-to-
digital converter (ADC), MAXREFDES82# senses weight and center of mass for objects
placed on the platform. The scale provides full-scale responses up to 780g. The user
interface provides an active display of center of mass on the platform, as well as total
measured weight. In addition to serving as a smart force sensor, the design works very
well as a ruggedized touch interface with force sensing. The system consumes less
than 50mA during normal operation and features a USB interface for quick evaluation
and integration.
The board is designed in a compact form factor for rapid evaluation or installation. Refer
to the
Details tab
for more information on the reference design. Design files and
firmware can be downloaded from the
Design Resources tab.
Features
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3D Human Interface Input
High Precision Weigh Scale
Low Power ADC
Compact
Competitive Advantages
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Rugged, Cost Effective, Flexible 3D Human-Interface Input Device
High Precision, High Resolution Measurements
Fast, Low-Power Data Acquisition
Applications
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3D Human Interface Input Device
Battery-Powered Instrumentation
Industrial Control and Measurement
MAXREFDES82# System Board
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MAXREFDES82# Block Diagram
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Details
Introduction
Technology marches forward, constantly and irrepressibly altering the world of human
to devices interfaces. From mobile device touch screens to weigh scales with electrodes
for body composition, machines continue to add more capability, and different ways to
gather their measurements. Every so often, a truly elegant concept emerges, merging
technology, design and simplicity. MAXREFDES82# is one such elegant concept.
This reference design is an industrial, smart force sensor. The design measures mass,
as well as the center of mass of the object on it. This combination of features and speed
means that MAXREFDES82# operates as both a weigh scale and a touch interface with
force sensing. Built with a plastic plate, the system ideally meets the needs of industrial
human machine interfaces (HMI).
The reference design features the MAX11254 24-bit, 6-channel, 64ksps delta-sigma
ADC with SPI interface. Four load cells are mounted on a quadrant plane to detect
touch forces applied on the flat plastic plate. Based on the forces applied on each
individual load cell, the system displays the total force magnitude, and the coordinates
of the force center. The force magnitude and coordinates are scanned approximately
each 10 milliseconds.
The system includes a thin-film-transistor (TFT) liquid crystal display (LCD) module,
digitally showing the forces on each individual load cell, and the total force applied on
the top plate. The LCD module also graphically displays the force magnitude and the
coordinates of the force center.
The MAXREFDES82# reference design consumes minimal energy and is powered by a
USB port. A PC-side graphical user interface (GUI) program is also included to facilitate
the human interface input demonstration and development.
The MAXREFDES82# reference design block diagram (Figure
1)
demonstrates how a
touch force and its movement are captured and used in a 3-dimensional (3D) human-
interface input device. The design includes source files for the microcontroller to enable
developers to quickly evaluate and customize the design for their specific applications
with minimal firmware or hardware changes. The board is designed in a compact form
factor for rapid evaluation or installation.
MAXREFDES82# System Board
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Figure 1. MAXREFDES82# reference design block diagram.
Detailed Description of Hardware
Figure 1 shows the MAXREFDES82# reference design system block diagram.
Figure 2
shows the assembly view of the design. Also refer to the schematic for circuit details.
Figure 2. MAXREFDES82# reference design assembly views.
When a touch force or weight contacts the surface of the top plate, the four strain-gauge
load cells deform and output electrical signals to the MAX11254 ADC. Based on the
distribution of the force to the four load cells, the system calculates the total force and
the position of the force's center of mass. The system continuously scans the signal
output of each of the four load cells, and an accurate force and its movement map can
be acquired. Because the system gathers and monitors not only the force position, but
also the force magnitude, the system is ideal for a 3D human interface input application.
The reset switch (SW1) resets the board, and the calibration switch is used to start a
calibration procedure.
The red light-emitting diode (LED) indicates the power supply is good. The flashing
green LED indicates the ADC is converting.
MAX11254 24-Bit, 6-Channel, 64ksps, Delta-Sigma ADC with SPI
Interface
The MAX11254 IC lies at the center of the system. The device is a 6-channel, 24-bit
delta-sigma ADC that achieves exceptional performance while consuming very low
power. Sample rates up to 64ksps allow preci¬sion DC measurements. The MAX11254
communicates through an SPI serial interface and is available in a small (5mm x 5mm)
TQFN package.
The device offers a 6.2nV/√Hz noise programmable gain amplifier (PGA) with gain
settings from 1x to 128x. The integrated PGA provides isolation of the signal inputs from
the switched capacitor sampling network. The PGA also enables the IC to interface
directly with high-impedance sources without compromising available dynamic range.
The device operates from a single 2.7V to 3.6V analog supply, or split ±1.8V analog
supplies, allowing the analog input to be sampled below ground. The digital supply
range is 1.7V to 2.0V, or 2.0V to 3.6V allowing communication with 1.8V, 2.5V, 3V, or
3.3V logic.
In this smart force sensor design, the MAX11254 is configured as follows:
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V
AVDD
= 3.6V, V
AVSS
= 0V, V
DVDD
= 3.6V, V
REFP
- V
REFN
= 1.8V
ADC sampling rate = 200sps
PGA gain = 128
Single-cycle conversion mode (SCYCLE = 1)
Sequencer mode = 2
Channel map = CH0, CH1, CH2, Ch3
MUX delay = 1024µs
The developers can easily change the configurations to their specific requirements in
the provided firmware.
