Let's Make Robots!

Big Chaser

Follow me carrying a 30kg child.

 

 

WheelMount1.jpg

2008-05-18

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.

2008-05-24

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.

02_WheelMount2.jpg

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!!

2008-06-01

 

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!

03_WheelMount3.jpg

2008-06-09

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.

Pisser.

05_Heavy_Duty.jpg

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.

2008-06-22

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.

 2008-07-07

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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BOA - great video!  Very smooth action (better than my driving).  Now for some wildly erratic questions.
1. Why did you mount the motors on the outside vs inside of your frame?
2. Why are you using a relay vs a full 4 FET H-bridge?
3.  What kind of RC are you using and what range does it have?

Is the drive train tight enough to make it balance? This video might give you some ideas.  The fact it can get itself up, if it falls over is a big plus.  Now it just needs to push the vacume around the house.

 

1) because the wheels came with built-in bearings, which I wanted to make use of. I have put an axle through them. I considered putting the axle on the outside of the frame, but decided that if I stood on it the frame would twist, so the axle runs the whole way from one side to the other.

2a) I don't have any P-channel FETs.

2b) I would prefer an H-bridge solution, but the only schematic I have only carries 4 Amps. I bon't like mechanical devices where electronic ones would work better (and make less noise).

3) It's a really old Acoms 2-channel for R/C cars circa the 1980s. I'm too yeller to find out what its range is but it is at least 200 metres.

There is absolutely no slack in the drive train and balncing is the ultimate goal. THIS one is brilliant. Check out his motorised unicycle!!

2a) You can do an H-bridge with all N-channel FETs but of course you would have to produce some negative voltage and feed 0V and -V to the motor and +V to turn the high sides of the bridge on (at least I think that's how it's done).

2b) That's interesting. I always thought that the amount of current an H-bridge could sink only depended on how much current the transistors and the snubber diodes could handle and not on how the schematic was designed. I would be very pleased if you could elaborate and explain what other considerations there are.

Big chaser looks like a really awesome platform!

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!!

I did not have audio - is the 400 Hz irritating? Is it loud - I imagine some of the sound depends on the physical makeup of the motor.

I have a full FET H bridge - I'm driving it with a 555 timer @ about 144 Hz - its kind of "not so pleasant", maybe its time to move over to a pic ;) .

I saw the little balancing bot before - it went down a high incline hill with dirt and loose rock and did not fall over (very impressive) - the ball bot is a neat concept too. It does not have to "turn" to go in any direction at any time.

Great project!

 

What current can your H-bridge sink? Do you have a schematic I could copy?

Let me give you a not so short summation of the long story. I decided a long time ago I wanted to build a "useful" bot. I had created ones that walked around in flames and ones that scared co-workers. I-robot's roomba is a pretty nifty floor cleaner, but i want something that will use my vacuum cleaner to clean the floor, or pick up my underwear, or run down to the grocery store and pick up some beer (get a wife has been the typical comment to that - but after 20 years of marriage, that solution does not always pan out).

I figured I needed something human size with human strength, if it's going to do human work - so a year or more ago I started Loki. I'm also interested in putting it together from off the shelf parts for the smallest amount of money. I was looking for a form factor which was pre-built to the dimensions of a human. The first thing I thought of was "electric wheelchair". I found one shortly after on craigslist.

This wheelchair is for a large person - and is powered by 2 (yes 2) deep cycle marine batteries in series. Next was a discovery in safety electronics, while trying to interface with the existing controller, I learned 3 things.
1. The safety electronics in wheelchairs are very impressive
2. The guts of the controller use 8 X IRF 2804 MOSFETS
3. You can control even more current by running the MOSFETS of the H-Bridge in parallel (can't do this with transistors because of the way heat affects current flow)

I fried the controller eventually and decided to make my own with 8 (4 for each motor) 2804 MOSFETS. Initially it did not have PWM - that was a big mistake. Then I tried doing the PWM in software (another big mistake - this manifested itself as erratic speed because I wasn't using realtime Linux and the task scheduler would get swapped with another task) - it was kind of a dumb process to tie up the cpu anyway. Another scary thing was parallel ports on PC do not stay low on boot up - (or there is no guarantee they do) - if your controlling your bot with it - then on boot the whole thing can lurch around in convulsions.

I got disgusted with the whole thing and started searching on the web for motor control schematics. I found a couple good ones, but unfortunately most were low current. Then I hit a gold-mine - OSMC Open Source Motor Control - it can sink 160 Amps CONTINUOUSLY ! It's in its 4th revision and comes with a great number of features (including overload protection).
Here is the list of parts and the artwork. I've had the pleasure of talking with Chris Baron who is a nice guy & founder of the project.

You would figure I actually built one of these ... sadly no. I believe my electronics manufacturing FU is not as strong as yours. So I ended up buying 2 "Simple H-Bridges" from Mr. Baron. I added a 555 PWM generator and am now controlling the motors through a NI DIO-96 PC board from ebay - (guaranteed low ttl voltage on startup).

I'd be curious to know what the people with strong electronics FU think of OSMC . I'd like to build one, but with limited time I've decided to concentrate on ranging, mapping, object detection, and remote tele-operation for Loki (there are only so many hours in the day - and some of which I'm not working on robotics, but doing more mundane chores like picking up my underwear off the floor)

I remember you told me the wheelchair / NI story before.

I like the board. I would leave 12 of the 16 FETs out in my case!! I only need a 10A controller. I recokon I could etch the PCB easily enough.

The circuit for the simple controller is identical to that given in the data sheet for the half H-bridge driver used! So, you'd like tothink it would be of an acceptable standard!

Why are the legs on fire? 

dramatic effect - it was actually intentional, although i'm not sure if anyone believes me

PIR was briefly considered as a person detector. The problem is that if the bot is moving, all the PIR will detect is that stuff in front of it is changing. It has been argued that we could periodically stop the bot and have it "look" but it would only "see" what had changed since it stopped.

Ultrasound was also briefly considered. It's facing some opposition, but it works on automobiles and I think it has a fighting chance.

Anyway, the first use for this platform has changed. I think it'll remain a 2-wheeler and that should be interesting enough.