Electronic system for acquisition of mechanical parameters

Julio. C. Politti, Luis Di Pinto, Fernando D. Farfán, Cecilia E. Garcia Cena1

Departamento de Bioingeniería – FACEyT – UNT

1. Departamento de Automática, Ingeniería Electrónica e Informática Industrial

(DISAM) de la Escuela Técnica Superior de Ingenieros Industriales (Universidad

Politécnica de Madrid, UPM)

E-mail: jpolitti@herrera.unt.edu.ar

Abstract: The present work delivers the preliminaries results for a portable electronic

system capable of acquiring mechanical parameters using solid-state sensors. The

system measures inclination acceleration and angular speed using the Adis16201 and

Adis16100 from Analog Devices. The information is sent via a serial channel using

the SPI protocol to a dsPIC30f4012. There, the data is multiplexed and sent to a PC.

The data is displayed on screen by a program developed in LabVIEW 8.5. The system

can support a range between ±90º with precision below the degree using 8 bits words,

and accelerations up to ±1.7g with a precision below 5mg, also using 8bits words,

supporting the device up to 3500g.

1. Introduction

The study of the human movements in the last decades has been critical in the development of new

achievements into so diverse fields such as rehabilitation, medical assistance on patients with spastics

hemiparesis (paralysis on one side of the body) the industrial robotics, and also in the sports and the

entertainment industry. This study tends more and more to implement new portable systems that

enable the user to make movements with the least of restriction and load applied by the measurement

system, which are being new valuable alternatives to the costly traditional video-computing systems.

These systems, measure mechanical parameters such as acceleration, inclination, angular speed,

strength, among others. Introducing these data into specific computer models, it is possible to measure

and record the user’s movements for their later usage and study, or to be used in the control of

protheses, ortheses and robotic-assistance systems.

In the last decades, the sensors implemented to determine the mechanical variables have been

encapsulated as solid-state micromechanical devices reducing their dimension and weight noticeably.

This enabled the development of portable robotic devices that can improve the life quality on disabled

people [1-4]. Many ortheses, protheses and rehabilitation devices are quite advanced technologically,

but unreachable, by low resources people.

This work develops the required electronic for a low cost, portable system, capable of acquiring

mechanical parameters using solid-state sensors with MEMS technology. Such a system could be used

XVIII Congreso Argentino de Bioingeniería SABI 2011 - VII Jornadas de Ingeniería Clínica Mar del Plata, 28 al 30 de septiembre de 2011

in the study of human movements to measure inclination and acceleration. Then, with the obtained

data, inclination, positioning, acceleration and speed of the segmental and global mass centers can be

determined by an off-line processing. The system measures inclinations between a range of ±90º whit

a precision below the degree, and accelerations up to ±1.7g, supporting the device up to 3500g. the

measures are presented in a graphical environment developed in LabVIEW 8.5

Material and methods

A schematic diagram of the acquisition system used for the study of human movements can be seen on

picture N°1

Figure 1: diagram of the acquisition system for the study of human movements

The electronic system uses ADIS16201 and 16100 with MEMS technology (Micro Electro-

Mechanical System) to acquire the data. So is called a certain type of integrated circuits that embed,

on a single substrate, sensors, actuators and electronic measuring elements in length from micrometers

to millimeters. These devices characteristics are displayed next.

ADIS 16201: is a complete, dual-axis acceleration and inclination angle measurement system

available in a single compact package manufactured by Analog Devices. Its measuring range is ± 90º

whit lineal output and 12 bits resolution for inclination and ±1.7g 14 bits for acceleration. It also

includes a digital temperature sensor of 12 bits, and both, sensibility and frequency, can be controlled

digitally. The information is stored on internal registers that can be acceded using an SPI inter-phase.

The ADIS16201 offers the following embedded features, which eliminate the need for external

circuitry and provide a simplified system interface:

• Configurable alarm function

• Auxiliary 12-bit ADC

• Auxiliary 12-bit DAC

XVIII Congreso Argentino de Bioingeniería SABI 2011 - VII Jornadas de Ingeniería Clínica Mar del Plata, 28 al 30 de septiembre de 2011

• Configurable digital I/O port

• Digital self-test function

The ADIS16201 is available in a 9.2 mm × 9.2 mm × 3.9 mm laminate-based land grid array (LGA)

package with a temperature range of −40°C to +125°C.

The communication must be achieved through a 16 bits SPI interphase, in which a data is sent and

received simultaneously by each device.

A functional block diagram and pin configuration can be seen on Figure N°2

Figure 2: ADIS 16100: A) Acquisition system block diagram B) Pin Configuration

Figure 3 displays a complete diagram of the measure and acquisition system used:

Figure 3 diagram of the measure and acquisition system

The system acquires through de Adis16201 and the data is sent by a serial channel using the SPI

protocol to a dsPIC30f4012. This communication protocol requires only four wires to establish the

communication, which is synchronous and developed between a master devise, and a slave one. The

wires are defined as follows.

PC (software

Developed in

LabVIEW)

DAQ

ADIS

16201

dsPIC 30f4012 DAC

SPI

Comunication

Parallel digital

communication Parallel analog

communication

Serial digital

Communication

(USB)

(A) (B)

XVIII Congreso Argentino de Bioingeniería SABI 2011 - VII Jornadas de Ingeniería Clínica Mar del Plata, 28 al 30 de septiembre de 2011

SCK: serial clock. The master device controls this line with its internal clock signal, in order to

synchronize the slave device.

SDI: serial data input. It is the data input line. The slave’s data input line, must be wired to the

master’s data output line, and vice versa.

SDO: serial data output. It is the data output line

SS: slave select. Generally it is an active-low selection, which means the master device can handle

many slave devices activating each one when it needs so. If there is only one slave device to

communicate with, this line is not necessary.

A schematic of a communication using the SPI protocol, is shown in Figure 4

Figure 4: SPI Master/Slave Connection

The SPI protocol has 4 different modes of work, selected by a couple of configuration bits. This

establishes the clock edge in which the output data is modified and the clock state in which input data

is read. In this work the mode (1,1), as descripted in the device’s datasheet, was used.

A timing schematics for a 16bits SPI communication using the (1,1) mode is shown on figure 5

Figure 5: Timing schematic for (1,1) mode

The transmission speed is of about 2 Kbaud, making it, up to 128 word/sec.

dsPIC30f4012: manufactured by Microchip, it is in charge of controlling all the data. It is a high

performance digital signal controller. developed with Harvard architecture.

XVIII Congreso Argentino de Bioingeniería SABI 2011 - VII Jornadas de Ingeniería Clínica Mar del Plata, 28 al 30 de septiembre de 2011

Between its characteristics we highlight: It handles 16 bits data with a reduced instruction set,

including an MCU instruction set (traditional microcontrollers instructions) as well as a DSP

instruction set (instructions specifically for digitally signal processing). It also includes an embedded

serial communication module that supports both SPI and I2C protocols. It has 2 AD conversion

modules that handle up to 6 conversion channels. It reaches a performance of up to 30 MIPS and has

48 KB of flash program memory, 2 KB of SRAM data memory and 1 KB of EEPROM memory.

The dsPIC will play the role of master device on the SPI communication, and it will be able to handle

many devices at a time. Data is sent through the PORTB to the DAC in a digital parallel transmission

DAC0808: Manufactured by NATIONAL INSTRUMENTS, is used to take the data from the dsPIC,

and represent it as an analog value sent to the data acquisition board. This allows us to introduce the

data through one wire, instead of eight. To point to which axis the inclination or acceleration belongs,

the data is multiplexed with a digital word introduces into the digital input port of the DAQ. This word

indicates which device, which kind of information and which axis, the data belongs to, and will be

controlled by the dsPIC.

DAQ: the DAQ used was the μDAQ-lite, manufactured by Eagle. It puts the data into the PC through

the USB port.

The information is processed in an environment developed in LabVIEW and is displayed on screen

on-line. Information is stored for a later study.

RESULTS

On Figure 6 the developed system with a single sensor plugged in is shown. The sensor, the DSPIC,

the DAC and the on-screen results, developed using LabVIEW 8.5, can also be seen on it. The system

is powered by two 9V batteries to obtain the needed voltages for the DAC to work. Each dspic30f4012

can handle two ADIS and read the inclination and acceleration values for both axes, x and y, of each

devices. After processing the information, it sends it to the DAC and to the PC consequently.

XVIII Congreso Argentino de Bioingeniería SABI 2011 - VII Jornadas de Ingeniería Clínica Mar del Plata, 28 al 30 de septiembre de 2011

Figure 6. Developed system. (A) Sensor Adis16201. (B) DSPIC30f4012. (C)

DAC8080. (D) Data visualization via LabVIEW software.

The obtained system works with a voltage of ± 9Volts, and less than 50mA of supply current. The rate

of transmission is 2Kb and it has the capability to measure inclination between ±90º with ±3º of

absolute error using 6bits words, and accelerations between ±1.7g with ±3.5mg of absolute error using

6bits words as well. The gyroscope has a range of ±300º/s with a precision of 10º/s.

The registered values for acceleration and inclination, for x and y axes by the inclinometer, during the

forearm flexion movement, are shown on Figure 7.

The registered values for x and y axes by the inclinometer, during the forearm flexion movement

(A)

(B) (C)

(D)

XVIII Congreso Argentino de Bioingeniería SABI 2011 - VII Jornadas de Ingeniería Clínica Mar del Plata, 28 al 30 de septiembre de 2011

Figure 7: Registered values for: A) inclination on y-axe. B) acceleration on y-axe C)

inclination on x-axe. D) acceleration on x-axe during the forearm flexion movement

Discussion and Conclusions

The ADIS16201 inclinometer output data is linear with respect to degrees of inclination and is

dependent on no forces, other than gravity, acting on the device. This requirement limits the utilities of

this sensor by the application on the human movements because the inertial acceleration is added to

the gravitational acceleration.

However, the sensor is able to be used on environment and application where the movements are

performed softly or even more in statics situations like ergonomics studies of human posture.

To conclude, there has been developed a portable, electronic equipment capable of simultaneously

measure, inclination and acceleration, in two normal directions as well as angular speed. Data can,

either be displayed on screen using a software developed on LabVIEW, or stored for a later off-line

study.

This system could be adapted to equipment to measure mechanical parameters in the study of human

movements. Such a system could also be applied in robotics field, industrial systems control, as well

as in assistancial medicine, rehabilitation and sports, to acquire mechanical parameters.

Acknowledgments

(A)

(B)

(C)

(D)

XVIII Congreso Argentino de Bioingeniería SABI 2011 - VII Jornadas de Ingeniería Clínica Mar del Plata, 28 al 30 de septiembre de 2011

This work was supported by grants from the Consejo de Investigaciones de la Univeridad de

Tucumán (CIUNT), Institutional funds from Instituto Superior de Investigaciones Biológicas

(INSIBIO), CONICET, and Ptroyectos-Semilla de Investigación, Desarrollo e Innovación, UPM

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XVIII Congreso Argentino de Bioingeniería SABI 2011 - VII Jornadas de Ingeniería Clínica Mar del Plata, 28 al 30 de septiembre de 2011