The Robot

 

Main Board

The main board was designed with ISIS Proteus and ARES PCB 3D.

 

The main board consists of  2 h-bridges ICs, several capacitors and screw terminals, 1 led, 1 resistor, and a 7805.

All these components are needed to process the signals  to control the motors, regulate the voltage, power the the DC

Motors, and charge and distribute the correct power to the Xbee  and Arduino the boards, camara stand,  and camera.

First ill describe what is an h-bridge and then I’ll come back to describe why those capacitors are needed, the 7805,

the led circuit.

PCB

H-Bridges

An h-bridge circuit is basically a circuit allows to control the direction of the current. The popular version is built using 4 NPN transistors  as show in Figure 1.
The current flows according if there is power in the Power + and Power – terminals. The main board consists mainly of two h-bridges(L293d ) that move the motors according its logical inputs 1 and 2.

Using l293d chips was used instead of using transistors because implementing the h-bridges using transistor will require plenty of components for reverse voltage protection,
short circuit protection, and the low current/heat limitation of npn transistors.

An L293d has all the protection needed to drive two DC motors up to 3 Amps at 36 DCV.
Each l293d have two h-briges circuits inside mainly composed of OP Amps and several diodes that protect the circuit( Fig 3 ).
To control the direction of motors is just needed to the follow the next logic table:

Input1 Input2 Motor
0 0 Off
0 1 Clockwise
1 0 C-Clockwise
1 1 Off

For example, when input1 and input are off ( grounded ), the motor does not rotates; when input1 and input2 are 1 and 0 respectively the motor rotates C-Clock wise.

 

Fig 1

 

Fig 2


   Internal schematics of l293d

Fig 3

 

 

According to the datasheet and some bench tests the l293 has the following main characteristics:

 

Max Logic Voltage 12dcv
Min Logic Voltage 4.5dcv
Max Vs                               36dcv
Min Vs =>Min Logic Voltage
Max Peak Current  3 Amps
Max continuous current  1 Amp

 

 Capacitors 

The capacitors are needed because they serve one or/and of the following in the main board:
They protect l293d against reverse peak current when the motors stops or change of direction.
Although, l293d has protection against reverse current it has a maximum threshold of 3 Amps.
Using capacitors give the l293d extend their maxim threshold protection to about 3 timers.

0.1uF capacitor were used with the l293d because it creates a low-pass filter that filters the high frequency when the motors stop or change directions.

<br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br />
f_\mathrm{c} = {1 \over 2 \pi \tau } = {1 \over 2 \pi R C}<br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br />

Fc is called the cutoff frequency, which is basically the  above frequency that fill be filter out.
In this case C=0.1uF and R≈0.000001Ω because the traces along the l293d and capacitors are not superconductors.

Also capacitors are connected in paralleled with the power sources so current is almost constant.

7805

The purpose of this chip is to regulate the voltage 11.1VDC to 5VDC in order to use this chip some capacitors are also needed
so it filter out and make the constant the out current as shown in the next figure.

 Resistor

Why 200Ω and no 400 or 1k Ω?  The answers is in the Ohm’s Law and using Kirchhoff’s voltage law.

Our led has a forward bias voltage of 0.7VDC and max current of 20mA.

Our voltage through out of 7805 is 5VDC. So using mesh analysis leads to : (5v-0.7v )/0.20mAp

so R=215Ω but since nobody manufactures 215Ω resistors  a 200φ is used.

The Remote

 

 power management

Lets do some math to calculate the total power needed to operate the remote control.

LCD 100mAh
Xbee 60mW at 3.3 volts
Video Receiver 200mAh
Arduino Solo 50mA
Switches and LED indicators 0A
NiMH AA Battery 1.2v ( 2001mAh)

The switches and led indicators currents drain is neglected because they drain almost zero current ( No super conductors tough ). 

 

Total mAh =  20001 mAH at 9.6V.  It is at 9.6v because the batteries should be connected in series summing their voltages but keeping their amperage the same.

The xbee consumes 60mW at 3.3 V so   using Ohm’s law with power =>  60mW=3.3v*I     +>   I=18mA

The remote control consumes = 100mA + 18mA + 200mA+50mA = 368mhA 

Powering the the remote with these batteries in series makes the remote to lasts for at least 5 hours without recharing it.

Bills of Materials

Bills of materials for Main board without including costs of PCB copper board and the tools needed.

 

 

 

Code

The the code  for the robot and the remote was written in embedded C which is a variant of the standard C ANSI programming language. Our IDE of choice was Arduino 1.0.3

The code needs to be loaded into the robot’s Arduinoand the controller’s Arduino.

 To compile the code just select the target board and make sure you have installed the Arduino solo and pro drivers that are included with .zip file.

Robot’s code:xbee_robot_Final

Remote’s code:remote_code