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Skatebot – A Cheap Fun Development Platform

Robert L. Jordan

rljordan@airmail.net

Skateboard toy setup to run

Intro:

My recent web research on H-Bridges gave me an appreciation of their advantages over R/C servos for robot locomotion.

Also, I had wanted to try controlling a robot platform geometry other than the "standard" 2 driven wheels and "tailwheel".

Well I found a way

Using my daughter as an excuse to peruse the toy isle, I found a platform worthy of my time investment. It is called "Totally Extreme Skateboard" and is a 27 Mhz Remote Controlled toy.

Features:

Here is a list of the toy’s features:

Walmart stores in Dallas all stock the toy. It is also an inventoried item, which means they stock it in the central warehouse as well as on the store shelves. My guess is that it is available in many Walmart or Sam’s stores in the US.

My Goals for the project:

An HBridge platform for experimentation and learning about Pulse Width Modulation (PWM) and 4-wheel driven differential steering,

My experience with caterpillar style driven platforms left me disappointed. They work well for the purpose intended, dirt, but are hard on the bearings and drag a lot when turning. This makes even reasonable odometer type distance measurements impossible. Also, they consume a lot of extra power and run slow or bounce a lot. This platform and drive type seemed a logical way to resolve most of those issues and still have a low, stable, fast, and smooth robot. It proved to be just that!

Presentation:

Skateboard toy as per factory, in box

The platform:

 

Skateboard toy running via RF Remote control

RF Remote control for Skateboard toy

Skateboard Toy Base

Figure and Skateboard removed from the base

Base with Skateboard and figure removed

The PCB unscrews for easy access.

DC Supply for the Skate-bot

Note the PCB has three tabs near the ends of the board. These connected to the 3 –AA batteries inside the figure we removed.

Top of factory PCB

An internal wire antenna connected to the antenna spring clip on the top right in the picture It is required for a good long distance RF link . Since it is worked close and that was good for my use, so I did not use any external antenna.

In the original toy there batteries are in the base and three are inside the figure. They are all in series to give 6 AA X 1.5 Volts = 9 volts. The 3 batteries in the figure were connected via the spring loaded clips on the PCB. The B+ connection is on one end of the PCB and the B- connection (bottom right in the picture) is on the end of the board.

The original setup had 9 volts but caused the wheels spin a lot and made the car run way to fast to control with most of my sensors so I tried 4 AA X 1.5 volts for 6 volts. The 6 Volts gave me a little trouble when the motors were both turned on under a load. The processor would reset from the surge. Yes I know the 5 volt regulators need "overhead" voltage but I had to try it anyway.

Photo of factory wheels

I connected an external battery holder with two AA batteries on my final configuration. This gave me 3 AA batteries inside the base and 2 AA batteries on the rear for 7.5 volts. Five (5) AA batteries X 1.5 Volts = 7.5 volts). This gave plenty of power except in very hard stalls, which I wanted to avoid anyway. The foam tires added to this issue even with 9 Volts. So I settled on the 7.2 volts and no foam wheels.

To connect external batteries, I just connected the Red lead from the external battery holder to the B- lead on the base, and the Black lead to the B+ lead. Remember you are putting the batteries in the base and the 2 external batteries in series. You are not connecting DC to the board. If you follow this method you will have 7.2 volts of DC switched through the slide switch on the bottom of the robot.

Photo of external Battery mounting. Note my final setup uses 2 external batteries.

This rear view picture shows the external battery and the mounting screws. I used two 7/8 inch tapped standoffs and tapped them in the side for mounting the external battery box. I later built a customized aluminum angle bracket, to use instead.

To mount the battery bracket and PCB to the toy base, I over drilled the existing plastic holes so a #4-40 screw would pass through the hole.

After passing a #4-40 X 1.2 inch screw up through the hole I screwed it into the standoff which the battery holder was mounted onto. When both battery support standoffs were mounted and both front standoffs were mounted, I attached the PC using 2 more #4-40 by inch screws.

Bottom view or toy base: Battery box, switch, & standard wheels.

Gears & Motors:

This view is with the top shell of the base removed. Three batteries, the gears, motors, switch and four wheel drive system is tightly integrated into the small base. Nice work!

In this view from the top you can see how the gear reduction system on the right is connected by a 1:1 gearing system to drive the other wheel on the same side.

 

The gear reduction system looks strong and runs quiet due to the brass to nylon gearing. Whoever designed this toy has had some experience! Axle bearings are nylon gear to plastic case on the outside and inside. They tend to have a little play but this is normal for these type gearboxes.

The wires you see connect the battery box, below.

The Factory PCB:

Top view of factory PCB

H-Bridge top view

PCB bottom view

Hacking the H-Bridge to be controlled by the BS2

This figure reveals the copper side of the Printed circuit board (PCB). It is clean

and easy to work on. The circuitry is split into the RF receiver on the left and the two full

two full H-Bridges on the right.

The custom IC that controls the H-Bridge is in the upper left corner of the picture. You can see on the right end of this IC, that the last two pins on each side route toward the right side of the PCB. These 4 leads connect through resisters to the transistors controlling the H-Bridge and motors.

If you cut the traces shown, you can connect your micro controller to the H-Bridge control leads to operate the motors.

Solder wires from the resistor ends of the lands to a connector on the added PCB. These leads connect directly to the OEM-BS2 (Basic Stamp) for controlling the H-Bridge, which controls the motors.

The H-Bridge connections to the Basic Stamp-OEM are connected to the development PCB on top of the Skatebot with a polarized, 4 pin, female connector. Use the male connector on the PCB. Be sure to connect the grounds between your controller and this PCB.

 

RF R/C or Skate-bot

We cut the PCB lands between the IC connections and the resistor connections.

Another option you have is to connect the 4 leads from the resistors to the center of a 4P4T switch. Connect the BS2 leads to one side of the switch and the 4 leads from the IC to the other side. This will allow you to run the car as a robot or as a remote controlled (R/C) toy. This option makes for some interesting contest "spoofing" if your friends have a good sense of humor. This might be a fun option to make your pet more interactive with your robot hobby, too.

If you do not use the RF side of the PC board, you might want to cut off the extra parts to avoid power consumption and RF issues. I did not have any problems leaving the circuit it tact.

PWM Note

Pulse Width Modulation or an H-Bridge can be a multitasking challenge on the BS2. As a development tool the OEM-BS2 is great. It is a single tasking MPU though . For contest I’ll likely add a co-processor to handle the PWM control of the H-Bridge driven motors and the wheel encoders.

I wonder if Al Williams has written any tip of the month or published any book ideas on using an SX processor for this use. If you are listening Al, I’d like a TTL serial interface with wheel encoder count accumulation and a reset command. Go for quadrature encoder support while you are at it so I can get at the direction the wheels are turning. OK! This is a blatant request for Al to fulfill my wish list. But don’t you want it too?

Processor:

The BS2-OEM from Parallax, Inc. is used as the Skatebot brain. I wanted a development platform and the BS2 was my choice as it is quick and easy to use. At $49 assembled or $39 kit, it is hard to beat as a standard processor. Also, in the BS2-OEM format I can unplug it and use it in other projects. Re-programming is quick and easy

The Vector board

This being a development platform, I chose to install a large PCB with a solderless breadboard on top. I brought out all connections from the BS2-OEM by using a 20 pin, right-angle, female, .025 square pin connector. All leads are connected to two rows of pin connectors so I can plug wires into them to get at the signals. Machine pin connectors work well for this and that is what I normally use.

This is a fast easy way to experiment with a platform. I can access any of my circuits by just plugging in jumper wires. I used phone wire. Notice that I have my H-Bridge leads on a connector to the PCB. I can wire them to the breadboard area for experimentation. Here they are wired to the OEM-BS2.

Two LEDs, a piezo sounder, and two (2) push buttons run to two 4 position pin connectors. Doing this with all the signals allows you to connect two (2) leads into any one signal, if required.

The "Z" shaped dual ribbon cable wire in the picture, connects the DC in connector at the top of the PCB (bottom view), to the BS2-OEM Vin connection.This Vin 7.2 Volts DC signal is available on the 20 pin connectors parallel to the MPU connector should I need it for powering servos or other devices.

You may have noticed the 14 position, .025 square pin connector running parallel to the large capacitor. I used this for an LCD while testing. It made the "Debug" commands available while the robot was scurrying around the floor.

Wheels:

Experiments were done with 3 different wheel types on the skate-bot.

The original wheels are wide red semi-clear plastic with a black firm plastic disk between the two wheel halves. It sticks out to touch the floor and act as tread with poor traction. You can see them in several pictures of the Skatebot in this article

I customized foam wheels from the hobby shop and merged them with pats of the original wheels. They were cut up and the inside disk and the center hub, with a square mating shape, were glued to the foam wheels. Then I used the original gray outside disk, which came with the foam wheels, to complete the custom wheels. This way they mounted very easily onto the axles. It was a lot of work and I do not recommend it. They got too much traction anyway which made sharp turns difficult.

The PVC disk wheels were turned from sheet material on a small lathe using a bolt and nut as the turning axle. Once they were round and the same size, I grooved the edge to hold the O-Ring in position and then stretched it around the edge. The wheels look nice and work well, but are hard to build so that they will mount onto the square end of the axles and not slip. Gluing a piece of PVC sheet cut round and punched square would work if you like these wheels. Due to the long axles, these wheels look a little strange from the front.

My favorite wheels are the original wheels with an added O-Ring for tread. The one I used was a #34 O-Ring, 1 & 1/4 inch OD, and 1/8 inch wide. This gives lots of traction and is easy to install.

The rubber like material on the wheels from the manufacturer was hard and ripple cut. This caused the Skatebot to slip and vibrate or bounce too much on hard surfaces. The added O-Ring solves this issue. It is glued around the original wheels on the inside next to the original hard rubber "tread". This keeps the hard rubber slightly off of the surface face and the O-Ring provides the required traction.

Sensors:

My preferred sensor is the bump bar with 2 bump switches and 2-4 IR object detection circuits.

I fashioned my bump bar into a "C" shape using thin gauge piano wire. I bent the ends down at right angle and plugged them into holes near the center of the solderless breadboard. I placed a loop of wire around the bump bar near the front ends of the solderless breadboard. These connect to the bump bar when it runs into something. In this way I could detect a ground from a left , right, or center (both left and right) bump.

The IR sensors I used worked so well that I later removed the bump bar, but it is always best to have if for your last line of defense on a robot. If you keep it you may want to put black heat shrink on the bump bar so the IR sensors won't see a reflection from it.

The IR object detection circuit is a set consisting of an IRED (IR LED) and a 38 KHz. demodulating IR sensor. Just use the FREQOUT command in the BS2 to drive the IRED. Then look at the IR sensor to determine if a reflection (low) has been detected.

These sensor are fast and allow "looking" at zones by enabling only the IRED for the direction you want to test. This also allows you to "look" for IR with your all you IRLED off, which can be used to prevent false readings. They work predictably well to about 12 inches. This is great for following a wall or detecting how close an object is.

Top view of Skate-bot

Parallax has some info that you may also find useful. They use IR Object detection on their BoE-Bot development/educational robot platform. The literature shows how to use the basic IR sensor. Reference Parallax, Inc. for their free info. A good place to start is in their Robotics! Text. See the references at the end of this document for the URL. They have some new IR stuff coming soon too. It should prove to be lots of help for learning about IR.

My results:

 

Side view of Skate-bot

Summary:

Great, cheap way to learn about H-Bridges and 4-wheel dual differential drive. Makes a fast fun platform as a robot or as a Remote Control toy.

Bottom view of Skate-bot

Software:

Click here to get a copy of the demo Skatebot software from my site

www.robotfun.com/skatebot/skatebot.bs2

James Vroman (james@vroman.com ) of the DPRG wrote a fun little demo program for the Skatebot. He has it go through its paces so you can see what it can do. Also, James has included code to support two sets of the Panasonic 4602 38 KHz demodulated IR sensors. We used 3 IR LEDs to "look" left, right, and straight ahead. These IR object detection components are the same that Parallax uses on their BoE-Bot robots. See the references in their Robotics! Text. They work well and are small and cheap. Digikey is my source.

For some good advice and neat robots, see "Vroman's Notebook" at james.vroman.com.

 

Specifications

Three (3) in base and two (2) on rear

The original unit used 6 AA cells (9 V)

Wires just push into the machine pin sockets used

In the sample software

Part List

Qty Description Retail


1 Parallax, Inc. BS2-OEM computer kit $39.00

This has the same function as the Basic Stamp 2 (BS2) plus it has a nifty small PCB with a 20 pin edge connector for plugging into your project boards.

1 Totally Extreme Skateboard R/C toy $25.00

Wall-Mart, Toys-R-Us, etc.

Electronic Hardware:

1 4 X 6 vector PC board (RS 276- ) or equivalent

2 Momentary contact push buttons

1 Solder-less bread board

1 Dual AA Battery holder ( I used the aluminum version)

1 Spool of tinned bus wire. 22 or 24 gauge

Electronics:

1 Piezo sounder (Speaker)RS 276- or equivalent

(40ohm or 100ohm speaker can be used instead. See

circuit examples in the Parallax, Inc. BS2 manual Ver.1.9)

2 LEDs one green & one Red best, 2 red OK

1 0.1 cap, marked 104 often. 1/10 spaced

1 10 uF 16Vdc Capacitor

1 3300 uF 16 Vdc, or greater, Capacitor

Radial or leads one end OK.

2200 uF @ 16 Vdc may work OK.

2 220 ohm resistor

2 10K ohm resistor, button pullup

4 1K ohm resistor, for H-Bridge interface

1 100K Ohm resistor, DC measurement circuit

1 220K Ohm resistor, DC measurement circuit

 

Connectors

1 20 pin, Right angle (R/A) .025 pin connector

Used to plug in OEM BS2 module

2 20 position machine pin, 1/10" spaced, connector

Used for plugging in 22-24 gauge wires

2 5 position, machine pin, 1/10" spaced, connector

2 4 position, machine pin, 1/10" spaced, connector

These are used for plugging in 22-24 awg wires for

access to the H-Bridge connections and the I/O

circuits on the board; buttons, LEDs, piezo,etc.

1 2 position male polarized connector, wire connect

1 2 position female polarized connector, PCB mount

This is used for DC Voltage connection

1 4 position male polarized connector, wire connect

1 4 position female polarized connector, PCB mount

This is used for connection of the H-Bridge leads

The Skatebot robot competes well with robots in the $200 range.

Estimate total cost of all Skatebot parts is $85 retail!

References;

  1. Walmart stores sell the toy Totally Extreme Skateboard" about $25.00

These are 27 Mhz of 49 Mhz versions Remote Controlled toy.

Customer service number 1800 WALLMART

2. Reference Parallax, Inc. free info, Robotics! Text.

You can view or download or purchase their info. Take a look at:

http://www.stampsinclass.com/ then select the download menu option. Click on your choice to view and it will come up in postscript viewable (.pdf) format. Also, you can download their code for experiments with the BS2 or BS2-OEM.

3. Thanks to Kevin Ross for his "The Basics – Robot Software" article in the SRS Encoder. I needed the advice.

4. Skatebot Demo software by James Vroman. www.james.vroman.com or email James at james@vroman.com

5. Robert L. Jordan (Bob) www.robotfun.com or email at rljordan@airmail.net

 

About the author:

I (Robert L. Jordan) work in Richardson, Texas for MCI Worldcom in Internet Training & production. My passion and 2nd job is robotics in education, thus my website name.

I get to do a lot of demos for kids which makes all the hard work worth while. Being a Parallax Dealer has helped keep the robot cost low, too.

It has been my good fortune to work with many great people in the Dallas Personal Robotics Group and to serve as Secretary for 1999-2000 (www.dprg.org). I give credit for many of my ideas to their loyal support. It’s just easier to learn from others mistakes. After all It’s "harder than in looks", according to our Roger Arrick originated DPRG motto. But remember. I'ts also more fun than you can imagine!