[Robo Build] Salahinator the Incinerator

Video's up:

http://www.youtube.com/watch?v=it1F9iqWxuc

(PS: couldn't get the beetle to turn to one of the chair's feet so I placed my finger as an obstacle)

Part 1: Understanding the Built of the Beetle Robot

Understanding Basic Concepts

Since I’m already writing the instructions on the built, I’ll write it so that even 8 year olds can do it (good science fair project for the kids in your families). Parental supervision is required for kids under 12 when it comes to soldering.

These are some of the basic concepts that should be understood for the robot built:

Electronic Terms: Voltage and Current

One of the best ways to understand these terms is by using the analogy of water circuits: voltage resembles the water pressure and current resembles the flow rate.

Think of a water tap and a pipe: when you open the tap (like connecting wires to terminals of a battery), the water is being pressured out of the tap => voltage, the electric pressure. The water flows from one side of the pipe to the other because there’s more water pressure on the first side than the second => current, the electric flow.

Batteries and Wires  Voltage and Current

Let’s think about a battery as a water pump with a nozzle connected to a set of pipes: the more water being pumped out of the nozzle, the more water flows in the pipes. Same thing for a battery connected a set of wires, the higher the voltage rating of the battery, the more current will flow in the wires. That’s also why the battery is known as the voltage or power supply.

If you have a pipe with water being pumped on both sides, how does the water flow in the pipe? The water flows from the side where the pressure is higher. Same thing with voltage and current in a wire: the current flows in the wire from the terminal with more voltage to the side with lesser voltage. And just like the total pressure that the pipe is being subjected to is the difference between the higher and lower pressure, the voltage reading across a wire is the difference of the higher and lower voltage readings: this is known as potential difference.

What if a battery is not connected to a wire? This is like having a water pump with a tap turned off: even if the tap is connected to a pipe, there will be no water flowing until the tap is opened. Connecting a wire to a battery is like opening the tap on the water pump. Current will flow out of the battery only when a wire (or conducting material) touches the terminals and the circuit is closed (more on this in a while). That’s why they tell you not to touch the terminals of a high-voltage battery: you will become the wire in that example which connects the battery to the ground, generating the most lethal closed-circuit known to mankind.

Opened Circuit, Closed Circuit

Currents only flow when the circuit is complete or closed i.e. when the current is able to move from the first terminal of the power supply, throughout all the circuit, and then back again to its second terminal. Any place where there’s a disconnection like a loose wire or an opened switch, the current stops flowing into the circuit. The latter case is known as an opened circuit.

Voltage Polarity and Direction of Current Flow

By convention, the current is said to flow from the positive voltage terminal to the negative voltage terminal i.e. from the terminal with higher voltage reading to the lower.

Soldering Techniques

The best way to teach someone how to solder is to show them how. The best illustration for this matter comes curtsey of “Madlab – Construction Step By Step” manual:

How to Solder?

Always make sure that whatever component you’re about to solder (whether a component on a perforated board or PCB, or a wire on a motor terminal), the leg of that component (or wire) should be as stable as possible so that the solder sticks in its place and not somewhere else (especially your hand).

A good way to make sure that the component is stable on a board is to bend the legs a bit outward like this:



For components off a board, twist the terminals to be soldered together and place the soldering iron just below them like this:



The correct way to solder is to heat the surface to be soldered via the soldering iron and place the piece of solder upon the heated surface and allow it to melt:



Correct Forms of Soldering

To ensure that the solder has reached the component’s leg and is holding it in place, wiggle the component a bit. You can directly sense if the solder hasn’t caught on. These are the forms of solder “humps”:



Notes:
1. Make sure that the soldering iron is sitting well on the handle of the housing (if you feel the housing is tipping over, place a counter-weight on it like a paper-weight).
2. Always be careful around the soldering iron when it is hot (always in front and to your right or left, according to the hand you’re using to solder with). Always keep it in your range of view.
3. If you burn your hand with the soldering iron, directly cool it off with running water for 10 minutes. Then use a gel for burns like MEBO (highly recommended) to ease the pain, and then cover the burn with gauze. If the burn is turns dark red or black, immediately go to a hospital.

Understanding the Components List and The Robot’s Operation

I’ve already mentioned the components list, but let’s “see” them and understand what they do.

1) 3V Motors: Obviously they are used for moving the robot, but there is one thing you need to know about motors: the polarity of the voltage supplied to them changes the direction of rotation on the motor shaft. Looking at the picture of the motors below:



You can see that I’ve soldered a red wire to indicate the positive terminal and a black wire to indicate the negative terminal. This lets me know that when I supply a positive voltage across the motor (higher voltage reading to red wire), the motor’s shaft turns clockwise. Otherwise, it would turn anti-clockwise. While the original design doesn’t indicate how the motor’s terminals should be connected, I’ve selected the wiring such that the rotation of both motor shafts enhances the movement of the beetle robot (smoother rotation and faster response when obstacles are detected).

2) Small paperclip and Wooden Pearl: It was used to form the tail of the beetle along with the wooden pearl or bead. As mentioned before, the tail allows the beetle to move forward. Without the tail, the beetle will move forward, but the whole weight load will be carried by the motor shafts. The tail lifts the beetle’s body a bit and takes some of the load off the shafts.



3) Big Paperclip: The big paperclip is cut in half (after being unfold), and then glued to the levers of the sub-mini lever SPDTs. They are the “feelers” on the robot: they sense the obstacles on the way. While the original feelers in the circuit are the levers on the SPDTs, the paperclip is considered an extension, sort of like antennas, and they give a longer range of sensing for the robot.

4) Batteries and Batteries Holder: All-in-one voltage source, main circuit switch, and beetle body.

5) Heat Shrink and Wires: The heat shrink shown below:



is normally used to cover the soldered wires for two purposes: a) aesthetic purposes and b) avoiding “shorting” of wires i.e. transfer of electrons from one set of exposed wires to another like so:



To use them as such, cut part of the heat shrink and fit it into the wire (but not close to the place where you’re about to solder) before soldering. After soldering, push the heat shrink over the soldered surface to cover it and expose the heat shrink to heat (I prefer using the tip of the soldering iron, just place it close to the heat shrink but don’t allow it to touch it) like so:



For this project, the heat shrinks are also used to cover the shafts of the motor and give the robot a smoother run over surfaces (without them, the robot can be moving in circles cause the motor shafts would be in direct contact with the ground and the frictional forces too much for the shafts to bear):



6) Sub-mini Levers SPDT (Single Pole Double Throw) Switches: These switches are the core of the robot steering process. Single Pole Double Throw: means that there is one pole that can swivel between two terminals (NO and NC) according to whether the lever is clicked or not. Taking a closer look at one of them:



You can see, hopefully, that there are three labels on the SPDT: COM, for common, NO, normally opened, and NC, normally closed. This is the internal diagram of the switch for when the lever is not clicked (using C for common instead of COM):



COM is the common terminal of NO and NC, current always passes through this terminal. NO is the terminal where no current runs through it (hence the term normally opened: C and NO are normally not connected) until the lever is clicked down. NC is the exact opposite of NO, where C and NC are always connected until the lever is clicked down. The internal diagram of the SPDT changes to the following when the lever is clicked:



The role of the switches in steering the robot is better understood by viewing the following diagram for the robot built (I’m not much of a drawer, so bear with me):



There are four possible scenarios concerning the switches:

1. When both levers of switches SW1 and SW2 are not clicked (normal run of the robot), the robot diagram comes down to the following circuit sketch:



The +9V is supplied by the terminals of the batteries holder: following the robot diagram, the current will start at the “+ve” terminal of the battery holder i.e. beetle body, into the NC terminal of SW1, out of the C terminal of SW1, into the blue wire, into motor M1, and finally the red wire labeled “+3V” (similar run for M2 but instead passing through SW2 and the green wire). The +3V is supplied by a wire that is soldered on the inner terminal of the batteries holder (where 2 of the 4 AA batteries are in series):



Moving back to the sketch, the potential difference across both motors is +6V, but note that while this potential across M2 is from the positive to negative terminals, the same potential difference is applied from the negative to positive terminals of M1. Hence, M1’s shaft rotates in the anti-clockwise direction while that of M2 rotates in the clockwise direction. This difference in direction of rotation prevents the robot from rotating to a certain direction; instead, it moves forward.

2. When only the lever of switch SW1 is clicked (left antenna of robot touches an obstacle), the robot diagram comes down to the following circuit sketch:



M1’s case here is the same as in scenario 1. However, for M2, the current will start at the red wire labeled “+3V”, into motor M2, into the green wire, into the C terminal of SW2, out of the NO terminal of SW2, and finally into the “-ve” terminal of the battery holder.

Even though both motor shafts will rotate in the clockwise direction, M2’s shaft would be rotating at a higher angular speed (due to the higher potential difference). This allows the robot to steer round the right and avoid the obstacle found on the left.

3. When only the lever of switch SW2 is clicked (left antenna of robot touches an obstacle), the robot diagram comes down to the following circuit sketch:



The cases of M1 and M2 are switched in comparison to scenario 2. Even though both motor shafts will rotate in the anti-clockwise direction, M1’s shaft would be rotating at a higher angular speed (due to the higher potential difference). This allows the robot to steer round the left and avoid the obstacle found on the right.

4. When both levers of switches SW1 and SW2 are clicked (both antennas of the robot touch an obstacle), the robot diagram comes down to the following circuit sketch:



This is where the robot design shows its weakness: though the robot should be backing away from the obstacle, it will actually charge forward but at a higher speed (due to the higher potential difference across both motors, the shafts will rotate at a higher angular speed).

Part 2 will be coming in a format of a link to a pdf file describing the steps of the robot assembly. Part 3 will explain the operation of the dark-detector circuit on top of the beetle, and Part 4 will be a link to a pdf detailing the circuit construction. Hopefully Part 2 comes out tonight.