Let's Make Robots!

Big Chaser

Follow me carrying a 30kg child.





The deal is, this 'bot will (hopefully) follow me around. For me the challenge is to do something bigger than Lego scale.


So far, I have attached a car windscreen wiper motor to a pneumatic sack truck tyre. It's going to be BIG. The clue is in the name.

The wheel consists of two metal plates which are bolted together. I made a couple of extra long bolts and bolted a bit of mild steel to it. Bolted to the middle of the bit of steel is a windscreen wiper motor from a Nissan Micra.

Next weekend, I'm off down to the junk yard / scrap heap / scrappie (depending on your geographical location) to get another wiper motor. When I have two fitted to some sort of a chassis, I'll report back.

I envisage something powered by a couple of big FETs under PWM control with DPDT relays to reverse direction.


Got my second wiper motor today. Found another scrap Nissan Micra down at my second favourite place (second only to the electronics store). Got the motor for $30. That's Not too bad. I could have haggled him down to $20, but I was in a hurry.


Remembered something else about these motors: They have a momentary switch internal to them which gets a hit every rotation. Cool! BUILT-IN SPEED SENSOR!!



It would appear that my eldest son has decided that my new robot platform would make an excellent trailer for him to tow his younger brother behind his bike. As soon as I obtain custody of it, I'm ready to retrofit the motors.


I may or may not keep the plastic seat which has been bolted on top. At this stage, I'm thinking "radio-controlled pram" or maybe just automatic pram which follows me around. At last - a robot with a purpose!



Built a new platform. It turns out the trailer is proving popular. New photo above. Hit a tiny problem. Not a showstopper. I mentioned a built-in SPCO switch in the wiper motor. I had hoped to use it as a rotation/speed counter. The problem is that the the motor "ground" wire is also the "normally open" contact of the switch. The plan was to connect the "normally closed" side of the switch to ground and run the switch common to the pic as an input (with pullup resistor enabled). Great when the N/C switch is closed, but for a brief second each rotation the switch changes over and the motor "ground" becomes switched into the PIC. Not a real problem for the PIC and if the motor is running "backward" (ie, ground is powered), the PIC will be able to increment a counter based onhe rising edge. HOWEVER, if the motor is running "forward" (ie ground is grounded) then the pic will detect no change.



BTW, this is 95kg (that's about 210lbs for our American friends) of ME

standing on the robot chassis. Early experiments show that it will easily plod along with a 25kg child sitting on it!!

I'm off to work on the circuit. I have a provisional one, but I want to separate the motor 12V from the TTL 5V onto two different PCBs.


I thought about it a bit more and decided the drive board needed its own logic supply. (See next post.) The reasons will become clear as I explain further my modular robot electronics concept.


New photo, new video. OKay, I wanted to prove that the controller board worked, so I programmed it to read the outputs from a radio control receiver. These are converted to 2's compliment byte values in the PIC and fed to the PWM controllers. The result is a pair of radio controlled windscreen washer motors with pneumatic wheels.

The platform currently has a "training" wheel. It seems likely this will be a feature for a while. I want to concentrate on the modular concept first, the person following, then the balancing act.

I'm very very happy with the motor control board and now I also have a good, usable routine should I ever build an I2C radio control receiver module.

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Yes. The guys at OSMC appear to have done just that using a dedicated driver IC.

For some obscure reason I had it in my head that the drain side could sink more current. Having thought more about it, this clearly cannot be the case as the current must also travel through the source. D'uh.

...but I'm no electronics expert and I bow to superior knowledge. So, I would be very pleased if I could elaborate and explain what other considerations there are!!

BOA: You are going to need some huge ass FETS to drive those motors! Dont forget about stall current too.
I have some mediocre FETs currently controlling the speed of the motors. The relays just switch the direction of the current.

 It seems you just need a couple of these:


Just stick one on the robot and as for the other...

Make yourself a belt, stick one of the N,S,E and W led's and sensors on the front, back and each hip on the belt.  You will have a little battery pack and PC board to stick in you pocket but that's it! --Should work great! 

www.rocketbrandcustom.com baby!!

The trouble with me is when I see something like that, I just want to make my own....

 I agree, but never the less, 4 (or more) sensor/transmitter sets mounted on a belt, (yes, let me toot my own horn...) is a great idea. I mean, none of this motion detector stuff or some crazy bunch of code to get a sonar sensor to "find an edge" stuff. -Just a silly belt with doo-dads sticking out! KISS!

AND!!... (This said in a cool echo God voice) The one who holds the belt, holds the power! 

www.rocketbrandcustom.com baby!!

Check out thecowgod's robot   http://letsmakerobots.com/node/1119  It uses a heat sensor or something to detect humans.

Ok, communication through the shout box can be somewhat strange so I'll just post here instead.

Just to recap: BOA shouted that he is using FETs rated for 40 A and that they get very hot when running 10 A motors, and this seems a little strange to me.

After checking out your schematic on the motor controller on this page, it seems that you're driving the FET gates directly from the PIC which I would guess means that the maximum voltage at the gates are ~5V. Since you're driving the n-channel FETs as switches to turn a 12 V potential on and off, you should use 12 V to turn them on and 0V to turn them on. If you use less than 12 V to turn them on, they will only be "half open" and the resistance across the drain and source (VDS) will be far greater than when fully open and so they will dissipate a lot more heat.

You can use optocouplers to bridge between the 5V from the PIC and the 12V from the motor voltage, but these have an upper limit to how fast they can be pulsed.

Ah, bother. This is a feature of FETs which I never understood properly. Is it the case that the drain/source voltage can only equal the gate/source voltage?

I don't really get this. I thought the FET would operate like a big switch if I supplied the +12V to the motor direct and used the FET to periodically ground the motor.

Assuming I've understood you correctly, couldn't I just use a transistor to open the FET gate?connect the base to the PIC, the emitter to the FET gate and the collector to +12?

1) Yes when using n-channel MOSFETs as switches I think you would want turn it on with a voltage at least as high as what is seen on the drain. If you'd like to read some stuff on how these things work , try wikipedia and search for "mosfet" - great description although a bit cryptical at times.

2) Well it is in fact doing that right now, but it's just getting a wee bit too hot :-). As far as remember it has to do with the n-channel not forming tight and narrow when the gate-source voltage is lower than the drain-source. I'm not really an expert on MOSFETs though.

3) I would definitely try that solution just to see if it works - but again I'm no expert. BJTs are current controlled whereas MOSFETs are voltage controlled and the MOSFET gate is a small capacitor so after a very short time when capacitor is charged, there will flow no more current into the gate. I don't know if it's a problem or not.