Joustbot–Step 2… and a few more


With the Parallax Robot and Microcontroller Expo just a week away I’m still trying to get a pair of jousting robots put together.  In this installment of the Joustbot saga I’ve got the basics of the robots assembled and will be able to do a little testing next week.

How did I get to this point?  Mostly by taking measurements, going to the hardware store, employing some hand tools, and applying a little paint.   To start I created some paper models of the battery pack, radio receiver, and carrier board, and laid them out with the motors and motor mounts.  This created a 2-D footprint that allowed me to try out different options for where things needed to go.  Since I wanted the carrier board visible and accessible I decided to stack that on top of the chassis and keep the battery, wiring, and receiver underneath it.

Step 2:   I  cut and bent all the metal pieces, drilled the holes, painted anything that didn’t move (and a few things that did), and put it all in a big pile for the photo-op above.

Step3:  My son is watching Top Shot these days, so I opted for the blue and red team colors.  I figure if I can get him to think I’m cool by incorporating his current interests the chance of him stealing my car when he’s a teenager is reduced.  In step three I assembled the chassis for each robot.  The black motor mounts are at the rear of the chassis.  The standoffs attached to the black motor mounts will carry the striker plate that detects when your bot has been hit by a lance (or anything else really).  The roller wheel, which is a bent and cut window roller from Orchard Supply Hardware is at the front.   With all the weight of the motors and the striker plate at the rear the chassis does a “wheelie”.  The battery pack will sit in the chassis at the front end, and I’m hoping will keep the front wheel on the ground (at least it did during an early test).  The chassis is about 4”x5” and was made from pieces of 4”x8”x0.125” stainless steel and some aluminum angle bar (0.75”x0.5”x cut to about 4” long).


Step4: Here’s a chassis with the motors mounted.  That battery pack sits in the open tray (Velcro will hold it in place).  The RC receiver which is a 2.4GHz hobby-type will be mounted with Velcro behind the motors on the motor mounts.


Step 5a:  Here is the rear of the mostly assembled Joustbot.  I’m going to try using the friction rollers instead of foam wheels to allow for smaller turn radii.  The striker plate (red rectangle) turned out pretty nice.  I hot-glued the plate to 4 momentary switches on the perforated prototyping board mounted to the standoffs.  If you hit the top, bottom, or middle of the plate it registers a strike. The striker plate switches are wired to a Parallax BASIC Stamp 2, and when it sees a switch closure (plate being struck) it sounds a buzzer, flashes an LED, and counts the hit.  At the end of the match you can press a button on the carrier board and it will count off the number of hits that bot took (they’ll be no guessing about who wins).  The BASIC Stamp 2 also limits the number of strikes to 1 every 500mS.  That eliminates the possibility of getting multiple hits due to switch bouncing.

The yellow wires connect a 2.4GHz RC receiver to the carrier board and Motor Mind C ANx/RCx inputs.  This allows you to drive the motors forward and reverse with separate RC channels.  For a future iteration I might try channel mixing (convert the 2 channels in a microcontroller so one is speed and one is direction), but for now this kind of skid steering should work.


Step 5b:  Here’s the front of the Joustbot.  I haven’t attached the lance yet, because I’m not sure what I want to use.  2” of 6-32 standoffs seems to work, but I want something that looks a little more medieval.  It’ll mount into a hole to the right of the front roller wheel.

You can see the battery and motor wiring at the terminal block.  I’m expecting  9.6V NiCad battery packs from DigiKey this morning and I should be able to solder them into the bots and try the first bout of jousting.   I built in a pretty gross charging circuit as well.  You can plug in a 12V DC wall-plug and a series resister will limit current into the battery pack.  It’ll do for now, but isn’t anything more than a temporary accommodation.


Next Steps:  These were put together in just a few days, and probably under 4 hours of actual work.  I spent most of that time measuring, cutting, and drilling.  The electronics were done in less than a half-hour since we relied on the off-the-shelf Motor Mind C, BASIC Stamp 2, and our MMC-BS2 Carrier Board.  I’m toying with the idea of taking this idea through an engineering process.   I’d like to design in a solid battery charger and much faster motors.  It might also be fun to place multiple striker plates on the design.

The main shortcoming I’m concerned about now is motor speed.  The motors operate fast enough at 12V and they were tested at that voltage early on.  Later I opted for a 9.6V battery pack due to its size and cost.  The bots will be under-powered at 9.6V, but I’m not sure if they’ll be so slow as to be no fun.  If they are there will be a simple fix, put in a bigger battery.

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