Part 3: Sensors & Output

This project accompanies Part 3 of Servo Magazine's series, Beginner's Robotics on $50 a Month.  It covers building the IR sensors, the bump sensors and installing the LCD display.
The first part of the article can be found in the February issue of Servo Magazine.

The Part 3 kit is available here.

Click here for the Bascom source code used for  CircBot (zipped).

Main Board Connections

Click the image above to see a close up picture of the main board wiring.

IR Obstacle Sensor Circuit - Part 2

IR test circuit that was described in the magazine section of this article could be used as is for a sensor.  However, it would be nice to get as much light out of the IR LED's as possible since that would increase the range of the sensor.  The resistor used in the test circuit made sure that there was enough load on the sensor's GL pin for the sensor to operate properly.  Replacing the resistor with two more IR LED's will keep the GL pin loaded and will increase the amount of IR light.

Using the IR test circuit with three LED's would be sufficient if all you wanted was to tell if there was something immediately in front of the bot.  However, it is often useful to also be able to tell if the object in front of the bot is more towards the left or right side of the bot.  For example, a sumo bot needs to turn toward an obstacle (i.e. the opponent's sumo bot).

By making two sets of IR lights and placing one set on each side of the sensor, we can cause more light to be toward one side or the other of the sensor's field of view by putting power to one set of lights or the other. 

You could use output pins form the microcontroller to power the LED's. However, there is a limit to the amount of current a pin on the microcontroller can source.  Three IR LED's will require more current than the microcontroller can provide.  If hooked directly to a microcontroller pin, they would cause the power available to the rest of the chip to drop so low that the chip will reset.

To get around this problem, we will use an inverter chip as a buffer.  The inverter will limit the amount of current drawn from the microcontroller.  By running multiple inverters in parallel, we can increase the amount of current available form the inverter to run the LED's.  The table in Figure 11 shows how to wire a test circuit for the inverter.  Use the visible LED from the multiplexer test circuit to test the output of the two-inverter chains.  We'll make the IR LED chains when we build the bump sensors in the next section.

Figure 11 Inverter Wiring

 The Bump Sensors  

There will be objects that the IR detector will not see.  If a material either absorbs IR or is transparent at IR frequencies, the IR detector will not see it.  Bump sensors provide a method for your bot to sense these obstacles.

There are dozens of designs out there for making bump sensors.  They range from the simple to the complex.  On the simple end is a springy wire connected on one end to GND with the other end terminating in mid-air just in front of a contact pad which is wired to a terminal on the microcontroller (the terminal is also connect to +5V through a 100 KW resistor).  When something pushes the wire to contact the pad, the terminal on the microcontroller goes from reading 5V to reading 0V.  On the complex end there are multi-lever linkages with molded parts that surround a bot and activate different combinations of switches depending on where contact is made.

he bump sensors for this bot fall somewhere in between.  They use micro-switches which are pushed by a bumper made of ceiling hanging wire and electrical tape.  It works, but with a little creativity, you should be able to come up with better designs.  Use Google to search for bump sensor on the net for ideas.

We'll install the micro-switches and their circuitry first and then make the actual bumpers.  Figure 12 shows the small breadboard with the IR sensor, its LED chains, and the bump switch circuitry.  The table in Figure 13 describes how assemble the circuit in the previous figure. 

Figure 13 Front Breadboard Wiring

You will need to make some jumper wires to connect the small breadboard to the main board. Cut two 4-1/2 inch pieces from the six stranded cable which was left over from building the programming cable in last month's article.  Slide all of the wires out of the sleeves.   Cut one red and one black wire down to 2-1/2 inches.  Strip 9mm (about 1/4") off the ends of all the wires from one set and off the ends of the blue wire from the other set.  Break off a set of four header pins from the strip of header pins.  Solder the wires to the pins in the following order: Blue, Black, Red, Green.  Break off another set of three header pins.  Solder the following wires in order to these pins: White, Yellow, Blue.  Break off seven individual pins from the header strip.  Solder one to the other end of each wire you just attached to the sets of pins.

Plug the set of three pins in to the small breadboard in locations B1 through B3 with the blue wire in position B1.  Plug the set of four pins in the locations B27 through B30 with the green wire in position B27.

Now, we will bend the actual bumpers out of ceiling hanging wire.  The ceiling wire is fairly easy to bend into a desired shape using pliers.  It is also slightly springy so it will return to its original shape after the bot backs away from an object it has bumped.  The problem is keeping the ceiling wire bumper positioned over the micro-switch so that it will press it when something makes contact with the bumper.  We will take advantage of the fact that the breadboard holes are actually square.  The ceiling wire is 18 gauge, which is slightly larger than what the holes can take.  We will sand an end of the wire square to fit the hole. This will keep the whole piece form rotating.

However, the wire can still flex sideways.  To keep that from happening, we will double up the pieces and use electrical tape to hold them together.  Figure 14 shows how to make the individual pieces while Figure 15 shows them assembled.  Bending the bumpers is a bit of an art.  Take your time bending the pieces. The doubled pieces have to match close enough to be taped together

 Attaching the Vertical Breadboard

The flat of the angle piece on the front of the CIRC Bot will be where the small breadboard will be attached.  Position the small breadboard in front of the bot with the bottom of the board just above the ground as shown in Figure 15.  On the back of the board, mark the area where the board is making contact with the rest of the bot.  Use a knife to cut the wax paper which covers the double stick tape just the area where the board will touch the rest of the bot.  Stick the board to the front of the bot.

Take the black and red jumper wires and attach them to convenient locations on the lower +V (for the red) and GND (for the black) rails of the main board.  The blue wire that is next to the black wire goes to I23, the green wire goes to I24, the white wire goes to J15 and the yellow wire to A14.

 Adding a Second Level to the CIRC Bot

We are quickly running out of space on the CIRC Bot.  To make room for the LCD screen and for next month's parts, we need to add a second level to the bot.  Place standoffs in the four outrigger holes as shown and attach two angles to the top of the standoffs as shown in Figure 16.  Now use two-piece plastic rivets to attach a base plate to the top of the angles.  Take the logic tester circuit apart and place its breadboard sideways on the top plate at the back end of the bot with row one of the breadboard to the right when looking from the back of the robot.

 LCD Display Circuit

Header pins will need to be installed in the fourteen holes along the top of the LCD.  Please refer to the datasheet for the LCD which can be found on the Wright Hobbies website.  NOTE that there is some variation in the LCD's provided.  Some will have sixteen holes instead of fourteen.  You only need to add header pins to the first fourteen holes starting at the hole closest to the edge (marked "1" on the datasheet).  Snap a strip of fourteen pins from your header pins.  Solder the short end of the pins into the holes so that the long legs of the pins are facing away from the LCD Screen. 

The LCD will be installed on the second level so there will need to be a number of wires run from the first level to the second.  Cut a 4-1/2 inch piece from the six stranded cable which was left over from building the programming cable in last month's article.  Remove all but one inch of the outer sleeve.   Strip the ends of each wire.  Break off a set of four pins from the header strip.  Solder the following wires to those pin in this order: Red, Black, Blue, Green.  Break off another two pins from the header strip.  Solder the remaining two wires, Yellow and White, to those pins.  Fish the unsoldered ends of the wires through the center hole in the row of holes which is directly in front of the breadboard on the top plate.  Break off another set of four pins and solder to them the other end wires which were soldered to the other four pin strip.  The wires should be soldered in the same order: Red, Black, Blue, Green.  Finally, break off two single pins from the header strip.  Solder one of the pins to each of the remaining wires (Yellow & White).

Wire up the LCD to the top breadboard as described in the table in Figure 17.  Note that the wiring table substitutes a small jumper for the potentiometer shown on the schematic in the magazine portion of this article. In most cases, this jumper will set the LCD contrast to the correct level.  However, if you wish to adjust the contrast, you can use the potentiometer included in the kit.  The pins on the pot do not sit well in the breadboard so you will need to solder a header pin to each of the pins on the pot if you decide to use it.

Completed Top Level

Programming

As with last month's article, sample code to test your new hardware can be found both on the SERVO website and at http://www.wrighthobbies.net/guides/.

This month's program utilizes a timer interrupt.  A timer interrupt causes the program which it is in to stop what it is doing and go do what is called out in the interrupt service routine (ISR).  Once the ISR is completed, the program is returned to what it was doing before it was interrupted.  

You can use ISR's two ways.  One way is to put high priority actions in the ISR to make sure that they run no matter what the rest of your program is doing.  For example, we could have put the sensor read routine and the motor control case routine in the ISR.  This would have made sure that the sensors are read and their information is acted upon at a regular interval.  The problem with that is that the length of time spent in the ISR would be so long that it would be time to call the ISR again as soon as the routing has ended.

This program uses a different approach where a low priority action can only occur after the ISR has been called a certain number of times.  Inside the ISR is a counter that increments each time the ISR is called.  In the main program is an IF statement that is true only after the counter has exceeded a certain value.  The true part of the IF statement resets the counter and sends data to the LCD. 

The rest of the main program is short and executes fairly fast when the LCD is not called.  In fact, it executes so fast the LCD could not keep up if data was sent to it each time the microcontroller cycled through main loop of the program.  The interrupt counter and the IF statement keeps the LCD refresh rate down to an acceptable level and help speed up the rest of the main code.

Variations

The IR detector presented was about as minimalist as you can get.  Taking a look at the data sheet for the IS471F (http://www.junun.org/MarkIII/datasheets/IS471F.pdf), we can see that there are a lot of different subsystems included in the tiny sensor.  David Cook's Robot Room website has a page about building a modulated IR receiver out of component parts http://www.robotroom.com/Infrared555.html and another about building a transmitter http://www.robotroom.com/InfraredTransmitter.html.  Building these two circuits will give you a much better understanding of what the IS471F is doing.

Sharp also makes the GP family of IR distance sensors which are very popular in hobby robotics.  Take a look at http://www.acroname.com/robotics/info/articles/sharp/sharp.html for a discussion of these sensors, how they work and how to interface them to a microcontroller.

Pointing the IR sensor out from the robot makes it an object detector.  Pointing it down makes it either a line follower or an edge detector.  Like with visible light, different materials reflect different amounts of IR light.  Most (but not all) brands of black electrical tape reflect very little IR light while white paper, white fiberboard, and Melamine all are highly reflective in IR.  This allows a line follower to be built using one or more IR detectors.  For more information on line following, look at the article  "Line Following - A Guide to Using Sensors" in the guides' section of the Wright Hobbies website.

When you using an IR detector facing forward as an obstacle sensor, you want the bot to stop and turn when the sensor detects an object. When you use it facing downward as an edge detector, you want the bot to stop and turn when the sensor doesn't detect an object (i.e. the floor under it). This is harder than it sounds since a single sensor in the front of the bot near the centerline will only detect if the center of the front of the bot is over the edge.  It wells you nothing about whether an edge is near the wheels.  I advise using a short table with pillows around it and a friend standing on the other side of the table ready to catch the bot when you try out your edge detection design.

IR transmitter / receiver pairs are also used to make wheel encoders.  The Seattle Robotics Society has a good article on building an IR wheel encoder and using it for "dead reckoning" or a robot's position on their website at http://www.seattlerobotics.org/encoder/200010/dead_reckoning_article.html.

There is a lot more to the bump switch.  Chris and Dawn Schur's bumper logic web page, http://www.schursastrophotography.com/robotics/bumperlogic.html, goes into the benefits of a well thought out bumper design.

The micro-switches used in the bump sensors can also be used as input devices.  You can modify the BASCOM program to display a menu on the LCD screen.  Use the left bump switch to go up the menu and the right switch to go down.  Add two more buttons for enter and escape and you can create an interactive system on your bot.  Having such a system, you can create a program that allows the bot to do different things based on what has been selected on the menu.

 While the multiplexer chip does have a few ports left, you will once again run out of usable ports if you try to do multiple ideas from this section at one time.  However, if you look at the datasheet for the 74HC151, you will notice that there is also a 16 to 1 version, the 74HC150. This chip will allow you to try several of the variations suggested here.  It will require one more port on the microcontroller.  Do not use pins 2 or 3 (PD0 or PD1) on the microcontroller since they will be needed next month.  However, pins 9 & 10 are also still available.

 Conclusion

Your CIRC Bot is becoming more complex.  Last month, it had the hardware necessary to seek or avoid light - based on how you programmed it.  This month, it can also know when it has hit or is about to hit an obstacle.  If you strike out on your own and try some of the variations, it can also be made capable of line following, edge detection, or dead reckoning its position.  By mixing and matching the bits you have built on your CIRC bot and creating the right code, you can make a bot that can do maze solving, sumo wrestling, object retrieval, or line following.

 Next month, you will learn how to free your bot of the limitations of a single small microcontroller and give it the gift of "sight" with scanning sonar.