Chapter 5 – DETAILED DESIGN  

 

5.1 CIRCUIT WIRING

5.1.1 Overview

An iterative prototyping approach is used in the design process of the circuit wiring. The schematic of the overall circuit diagram is first planned out using circuit design software. This same circuit layout will then be modified, such that the same connections can be produced by connecting all the circuit components to the breadboard. Reconfigurable breadboard circuits are then created to enable rearrangement of the components for a more compact and simpler design. Below shows an overview of the prototyping process involved.

 

5.1.2 List of circuit components used

The following are the various circuit components that are used in the building of the circuit prototype [A] Microcontroller module [B] Servo motor (Hitec HS-815BB) [ [C] Piezo electric sensor [D] Lead acid batteries [E] Solderless breadboard [F] PCB board [G] LED pads (12V) [H] MOSFET transistors [I] (IRF510)Resistors (1M Ω) [J] Wire connectors [K] Crocodile clips [L] Wires

 

5.1.3 Schematic of overall circuit

This is a simplified circuit diagram showing the connections between the various electrical components. As the digital output pins for the controller is capped at maximum 5 volts, external power supply is required for powering of the servo motors and LED, which are rated at 6v and 12v respectively. The used of MOSFTS transistors in this setup allow the microcontroller to control the high voltage LED circuit without direct circuit contact.
 

5.1.4. Breadboard Circuit Prototype

Breadboards are experimental tools used for building temporary prototype circuits. There are very convenient as they allow theoretical circuits to be tested conveniently without the need for soldering to a PCB board. A prototyping process that is iterative will benefit from the flexibility that the breadboard offers.Breadboards are experimental tools used for building temporary prototype circuits. There are very convenient as they allow theoretical circuits to be tested conveniently without the need for soldering to a PCB board. A prototyping process that is iterative will benefit from the flexibility that the breadboard offers.                                   

5.1.5 PCB junction board

After the circuit has been tested out, the final refined circuit design will be used in the fabrication of a PCB junction board. This junction board will be the used in place of the breadboard in the actual prototype.        


 

5.2 SERVO MOTORS

Servos are essentially a form of self-contained DC motors with built-in feedback control circuits and integrated gearing [7]. They are generally characterized by a fixed range of motion that can be precisely positioned bi-directionally. This makes them ideal for a wide range of applications that required controlled movements such as mechanisms in robotics

5.2.1 Calculations for servo motor selection

As the mechanism has a symmetrical range of motion when the servo turns within its 140° range, we can base our calculation on one-half of the motion. The load, in this case, involves overcoming the kinetic frictional force, f due to the rolling linear guides and the overall composite weight exerted on the runner block.             Formula used:

Ff = μ FN where Ff = force due to friction (N)

FN = normal force (N)

μ = friction of coefficient between Calculation of Servo torque required:

Frictional coefficient of linear guide, μ ≈ 0.003
Frictional force, Ffriction = μ FN = 0.003 x (0.1 x 9.81 ) ≈ 0.003 N

Using Newton’s 2nd law, Fservo = Ffriction ≈ 0.003 N

Servo torque, Tservo = Fservo x r = 0.003 x (18cm ) ≈ 0.054 N.cm
From servo selection chart;
Hitec, HS-815bb Servo Torque (@6V power source): 24.70 N/cm Rotational Range: 140°

5.2.2. Servo Control (PWM)

Servos are controlled through the use of pulse width modulation (PWM) of the digital signal sent from the digital output pin of a microcontroller. During the driving of servos, signals are sent at regular frequency of 50 Hz and the angle of the servo rotation will depend on the pulse width duration of each cycle. [7] The following table shows the various typical voltage pulse width values and its corresponding resultant servo angular position. In general, the longer the pulse width, the more speed and torque the servo produce.

5.2.3. Servo Programming

The PWM method is used by the <servo.h> library in the microcontroller IDE to control servos using its digital pin 9 and 10. The library <servo.h>consists of three predefined functions; attach(); write(); read()


 

5.3 PIEZO SENSOR

Piezoelectric films are force sensitive sensors which when flexed, momentarily produce a usable voltage difference. This voltage signal is then detected when it is sent via wire leads to the analog pin of a monitoring microcontroller where the voltage is being monitored. Common application of piezo sensors include; • Detection of contact force from impact (e.g. detection, measurement, counting number of impact events) • Sensing of vibration (e.g. detection of motion)

5.3.1. MOUNTING

The laminated Piezo film sensor adheres to the surface of the elastic cantilever beam of the target board subassembly. During actual operation, when the target surface is being hit by a projectile, both the target board and the sensor will experience the impact force from the striking projectile. Note that ruggedness of the target has to be ensured to minimize any unwanted false signals created from bounce back motion   .

5.3.2. Sensor Programming

In each cycle of the program, the micro-controller will monitor the analog input pin connected to the Piezo sensor for any momentary voltage jump. The voltage reading is monitored with reference to a preprogrammed threshold value for signal output to activate the LEDs as can be seen in the circuit diagram on the next page. As there are two sets of target (hostage and terrorist), there will be two sets of sensor and LED, each to indicate either a “hostage hit” or “terrorist hit”               


 

5.3 FRAME STRUCTURE

5.3.1 Aluminum profile frame

As the aluminum profile has to be cut to length by the supplier, dimension of the various lengths of aluminum profile have to be decided on earlier on in the project to prevent any delays in the progress. This also meant that any design changes made to the secondary structure such as mountings and the slider mechanism will have to be constrained by the dimension of the aluminum profile frame structure. Clarity of drawings in terms of overall dimensioning has to be made as clear as possible to prevent any miscommunication or even rework. Below shows a diagram used for communicating the specifications to the vendor. To prevent confusion, color codes are incorporated into the diagram to indicate the length specification

 

 

 

 


 

5.4 EXPLODED ASSEMBLY DRAWINGS

5.4.1 Final Assembly

5.4.2. Target mounting subassembly

To have a better idea of the mounting sequence of the fasteners and the various components that make up the subassembly, an exploded diagram is used below to illustrate the components position in relation to each other.                               

5.4.3. Servo mounting sub-assembly

 


 

5.4 FABRICATED PROTOTYPE

5.4.1 Aluminum Profile Frame Structure

 

 

5.4.2 Full Assembly

5.4.2 Full Assembly


 

5.5 WORKING AND OPERATION PRINCIPLE

1. Target practice platform is switched on.
2. Motor starts to generate random motion in the hostage target. This is to simulate the actual movements encountered during an actual hostage scenario.
3. Shooter enters the scene and engages target.
4. During training, the shot can either hit the ‘HOSTILE’ or the ‘HOSTAGE’ target.
5. Upon impact, piezoelectric sensor attached to each of the target converts the resultant vibration to changes in resistance values which is feed backed to the microcontroller.
6. The microcontroller sends an output signals to activate the corresponding LED light. This provides an immediate feedback to indicate an either “terrorist hit” or “hostage hit” status

Here you can create the content that will be used within the module.

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Overview

 

CHAPTER 1 –  INTRODUCTION

1.1 Background
1.2 Objective
1.3 Scope

 

CHAPTER 2 –  LITERATURE REVIEW

2.1 Design 1
2·2 Design 2
2.3 Design 3

CHAPTER 3 –  DESIGN REQUIREMENTS

3.1 Target User Analysis
3·2 Quantifiable Design Parameters

CHAPTER 4 –  CONCEPTUAL DESIGN

4.1 Functional Analysis Diagram
4.2 Morphological Chart
4.3 Sub-Function Selection
4.4 Conceptual Design Selection
4.5 Conceptual Design Weightage Profile
4.6 Concept Evaluation

CHAPTER 5 –  DETAILED DESIGN

5.1 Circuit Wiring
5.2 Servo Motors
5.3 Pizo Sensors
5.4 Frame Structure
5.5 Exploded Assembly Drawings
5.6 Fabricated Prototype
5.6 Working & Operation Principles

CHAPTER 6 –  TESTING & EVALUATION

6.1 Servo Issues
6.2 Power Supply
6.3 Piezo Sensor Issues

CHAPTER 7 –  DISCUSSION & CONCLUSION

7.1 Evaluation of Prototype
7.2 Advantages & Limitations
7.3 Recommendations for Future Work
7.4 Conclusion
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