O objetivo principal da tese foi o desenvolvimento de sensores piezoresistivos baseados em matrizes poliméricas e o respetivo sistema de leitura. Tendo como finalidade ser usados em aplicações sujeitas a deformações elevadas. Os sensores piezoresistivos foram desenvolvidos e processados através de diferentes métodos, extrusão, screen printing e spray printing. Obtidos os sensores, realizaram-se testes mecânicos, elétricos e eletromecânicos, para verificar o seu desempenho e características. Para o método de extrusão, os compósitos foram desenvolvidos com SBS C401 e diferentes quantidades de CNTs, 0, 4, 6, 8 e 10%. Os sensores impressos por
screen printing e spray printing foram obtidos pela deposição de duas camadas, uma camada
condutora composta por nanopartículas de prata e uma camada ativa constituída por nanotubos de carbono e SBS.
A sensibilidade dos sensores obtidos por extrusão foi calculada a partir dos valores adquiridos pelo método de deformação uniaxial, para os sensores impressos realizou-se o método de quatro pontas.
Usou-se uma luva como prova de conceito para os sensores piezoresistivos desenvolvidos. Como conclusões específicas para os sensores piezoresistivos desenvolvidos temos as seguintes:
Os compósitos CNT/SBS apresentam maior tensão à rutura quando a quantidade de nanotubos de carbono é maior. A condutividade elétrica aumenta com o aumento dos nanotubos de carbono, o aumento abrupto da condutividade dá-se aos 5% de CNT.
Quando calculada a sensibilidade dos compósitos observa-se que apresentam deformações mecânicas e respostas elétricas relacionadas linearmente durante os ciclos de carga-descarga mecânica.
De modo geral todos os compósitos apresentam valores do GF superiores a 2.5.
As matrizes dos sensores piezoresistivos foram projetadas e desenhas de modo a apresentarem dimensões reduzidas.
A parte condutora dos sensores foi impressa com nanopartículas de prata através do método screen printing, a camada ativa foi impressa pelo método spray printing com tinta composta por CNT/SBS.
Os sensores foram caracterizados a partir do teste quatro pontas. Estiveram sujeitos a diferentes velocidades e deformações durante a realização dos testes.
Capítulo 8.
69
Feita a caracterização mecânica, elétrica e eletromecânica conclui-se que tanto os compósitos extrudidos como as matrizes impressas podem ser usados como sensores piezoresistivos.
Foi desenvolvido um sistema de leitura adaptativo para sensores piezoresistivos.
O circuito permite a leitura de 16×1 sensores, sendo um circuito multiplexado. As entradas dos multiplexers podem ser alteradas, medindo mais ou menos sensores.
O microcontrolador existente no circuito é responsável pela calibração dos sensores, controlo de todos os elementos integrantes e otimização do consumo de energia.
Os dados são registados e enviados em tempo real por RF e recebidos numa plataforma remota através de um módulo de receção USB.
O circuito foi validado experimentalmente na luva e nas matrizes dos sensores impressos.
Os materiais extrudidos com 8% de CNT foram cosidos numa luva e, num outro protótipo, foram impressos de forma a funcionarem como sensores piezoresistivos, sendo efetuadas as medições com sucesso.
Conclusões e trabalho futuro
70
8.2 Trabalho futuro
O material que constituí a luva usada como prova de conceito é bastante absorvente, devendo- se usar um material alternativo, mais plástico.
Concluir a impressão de sensores em todos os dedos da luva e conectar o circuito eletrónico. A luva além de permitir a determinação dos movimentos dos dedos poderia gerar estímulos elétricos sobre a forma de corrente elétrica de baixa frequência. De maneira a provocar a contração de músculos paralisados ou debilitados, resultando no movimento destes, podendo ser usada como forma de tratamento.
Os sensores podem ser aplicados em equipamentos desportivos, como fatos de taekwondo, estes têm apresentado algumas falhas na tecnologia que usam atualmente e uma possível melhoraria é o uso de sensores piezoresistivos. Na figura 5.1. é apresentada uma simulação do colete com sensores piezoresistivos impressos.
Figura 8.1 - Simulação de protótipo de um colete de taekwondo monitorizado com sensores piezoresistivos para medir a
71
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