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ES050 – Introductory Engineering Design and Innovation Studio

ECE Case Study

Accelerometers in Interface Design Prof. Ken McIsaac

2008 11 19

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Outline

Review of generated concepts Design selection Accelerometer mathematics Accelerometer mechanics Accelerometer electrical theory

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Review of Generated Concepts Pendulum proposal

d

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Review of Generated Concepts Mercury Switch

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Review of Generated Concepts Motion capture

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Review of Generated ConceptsAccelerometer

“Something to measure acceleration” Untethered, inertial sensor

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Decision MakingNow, we have a set of concepts. What criteria would we use to choose between them?

Some thoughts I had:

Infrastructure (ease of installation)

Accuracy (How close to reality is the model?)

Resolution (How fine motions can we detect?) Form factor (weight/size)

Cost

#1

#3

#4

#5

#6

Range of motion (What can the user do?) #2

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Decision MakingEvaluating our choices:

Option Problems

Pendulum Infrastructure, range

Mercury Sw Range, resolution

Motion Capture

Infrastructure

Accelerometer Maybe none, IF we get the right one

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Accelerometer Mathematics A transducer is a device that measures a

physical quantity and turns it into an electrical signal

An accelerometer is a transducer that turns acceleration into voltage

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Accelerometer Mathematics We measure V=f(a), then:

Vfa 1

0vdtav 0pdtvp

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Accelerometer Mathematics

As it happens, this is inaccurate, but who cares?

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Accelerometer Mechanics

One dimensional model

So how do we transduce acceleration?

Stationary

Acceleration

Can we instead transduce this bending?

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Accelerometer Theory

New problem: measure bending

A

LR

A

LR A

L

1. Resistance of a wire varies with length

2. Length of a bent strut changes

So…the resistance of a wire bonded to the strut detects the bending.

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Accelerometer Theory

Proposed circuit:

+-

R1 (Fixed)R2 (Variable)

+

-

VoutVin

VinRR

RVout

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2

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Accelerometer Theory

Now, some math:

Vin

AL

R

AL

V

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Stationary

Vout

ALL

R

ALL

V

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Accelerating

Assuming LL Vin

AL

R

AL

V

1

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Accelerometer Theory

This will work, but it is hard to measure, so…

Strain gauges are thin wires folded and printed onto flexible foils. They magnify the deformation effect on resistance

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Accelerometer Theory

Accelerometer for into plane acceleration or pitch

Strain gauge

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Wii First Proposal

We can build something like this:

Acc 1

Acc 2

Acc 3

Computer monitors and integrates

acceleration data

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Design Evaluation

Our design will work, but:

It is too big

It is difficult (= costly) to manufacture

We need a design iteration to resolve these problems.