Marginally Clever Robots

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Building a 6DOF Joystick: Chopping up servos

2013-04-21 Dan

I want to provide a more concrete reason to get a Rotary Stewart Platform. So I figure the best way is to make a 6DOF joystick like this:

The method is pretty straightforward: build two stewart platforms. use one as the “input” and one as the “output”. Then take away the output, read the numbers from the input and report them to a PC as if it was a Human Interface Device (HID). Windows and OSX would recognize it. Then I would talk the Minecraft guys into providing support.

The first step is already half done: Rotary Stewart Platform is available now, if you want one. I asked Jonny Poole of InnerbreedFX how he built his and he said to open the servos, remove the circuit, and use the potentiometer already inside as a sensor.

.. and there it sat for about three weeks because I was completely stumped. The servo pot was returning garbage. I removed it completely from the servo, put it in a breadboard, and ran every combination of test I could think of. The multimeter returned sensible voltage on the middle pin when I put 5v across the other two and twisted the dial. but when I plugged it into an arduino with the following script:

[code language=”c”]void setup() {
Serial.begin(9600);
Serial.println("START");
}

void loop() {
Serial.println(analogRead(A0));
delay(100);
}[/code]

the numbers were all over the place. As I slowly turned the dial it would sometimes jump to 1023, sometimes to 0.

In the end I gave up and got another. Within about 2 minutes I had sensible numbers. In case you were wondering, sensible numbers look like this:

[code language=”c”]START
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
25
73
75
75
75
75
75
85
96
146
208
254
308
369
433
486
543
606
668
727
772
800
809
808
808
810
813
824
883
954
1006
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1006
960
916
862
807
749
681
616
561
499
436
372
317
258
193
141
99
52
4
0
0
0
0
0
0
0
0
0
0
0
0
0
0
8
37
75
119
160
205
260
308
350
397
470
551
622
693
766
827
878
928
967
997
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1023
1010
991
973
966
964
959
929
911
911
911
911
911
911
911
911
911
911
911
911
912
911
911
911
911
911
910
911
910
911[/code]

Next up I’ll convert five more and then connect two stewart platforms together.

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Vancouver Robotics Club, April 2013

2013-04-15 Dan

Always great stuff at the Vancouver Robotics Club. Next month is RC vehicles. I hope my HOG drive is ready by then. I’m already getting ready for the Vancouver Maker Fair 2013. Have you got your tickets?

Bob gives a short class on PWM signals and how they work with servos.

Christian has been building his first robot. It’s up and running around, which is huge step. Christian and his dad show up every month and always have smart questions.

Liz & Jonathan are make me a little envious. They can talk robots all day, every day. Between them they have enough great ideas to keep 6 people busy for a long, long time.

Liz & Jonathan own a lot of robots of various sorts. So many that they brought one of their custom robots in this box that was “just lying around”.

Crafting together a crawling robot. The parts required a bit of massaging – and by “massaing” I mean “drilling out the holes until they fit”.

At the VRC meeting I used my 3D printer to print a piece I designed the day before, and then when I got home installed it on the pan/tilt head I’ve been working on. It’s the one below the big gear that holds the motor and small gear in place.

Here’s a wide shot so you can see it better. The two gears on the top left are from a Greg’s Hinged Accessible Extruder. The one in the bottom center I designed myself.

Miscellaneous

Debugging a DIY Segway at the Vancouver Hack Space

2013-04-10 Dan

Al first contacted me by email. Apparently someone has been telling him that I’m the go-to guy for anything Arduino. He is not wrong. Sit right here and I’ll tell you about how we went to work on this machine and how far we got in an evening.

A Segway PT drives by sensing the way you tilt your body. The brain of the robot can sense which way is up and which way you lean. It does this with a gyroscope. This model also has an Dead Man’s Switch for safety: you have to hold the button down to make it go, meaning it can’t make any escape attempts. In order to test the parts early on there’s also a knob to make the robot turn and a knob to make the robot go forward and back. In theory you could drive this model without the robot brain, but that’s not as cool.

Al did what anyone would do. He carefully assembled the entire machine by following the steps in the manual, copied the code into the Arduino line by line, and when it was ready flipped the switch. One wheel started to turn right away and the other did nothing – and nobody was holding down the Dead Man’s switch!

To attack this problem efficiently we made a plan: if we know the fundamentals work then we can test higher-up stuff. First we have to check the mechanical and electrical connections match the diagram. Second, we have to check that the inputs work and the Arduino can “hear” them. Third, test that the brain can make the motors go the right way. Fourth, we know we have an input > brain > motors connection. The last step then is teaching the brain how to balance.

We disconnected the motors and then Al got out a sharpie and some masking tape. I gently pulled on a wire at the top of the steering controls and when the other end at the Arduino moved, he put a piece of tape on and labelled it. We did that through all the wires, which took about 20 minutes. You can see a ‘T’ (for turn) on a piece of tape next to Al’s index finger.

Al, being very clever, had drawn a schematic of how the wiring should be done. We took out all the wires from the red breadboard shield, effectively disconnecting all the controls. The breadboard has the gyroscope chip mounted in the center for easy connecting to the Arduino. The breadboard and the Arduino click together like a LEGO sandwich. Then we used the traced Dead Man’s wire and reconnected it to the pin in the schematic and went poking in the code. About 40 minutes later we had the light on the top of the breadboard turning on when the Dead Man’s Switch was pushed, and turning off when it was released. This was an excellent sign! We didn’t stop for high fives, there was a long way to go.

The next step was to get the knobs and the button all working at once. To do this I wanted to display on a screen the position of the knobs and the on/off of the button all at once. The best way I know to debug an Arduino is the Serial interface. In the code we have a value called steeringPot and another called gainPot. I wanted to be sure that when the steering knob is turned that only analogRead(steeringPot); would return a changing number between 0 and 1023. I also wanted analogRead(gainPot) to do work the same when the forward/back knob was turned. I told the Arduino,

Serial.print(analogRead(steeringPot));  Serial.print('\t');
Serial.print(analogRead(gainPot));  Serial.print('\t');
Serial.print(deadManSwitchState);  Serial.print('\n');

…This is where we hit the biggest bug of the night. We had the code reporting

1023 1023 1

over and over, no matter what we did to the knobs. We retested everything. The always resourceful Simon joined in with a multimeter and tested that the electrical signal from the knob potentiometers was rising and falling as the knob was turned. It’s his hand in the left of the photo when we tried removing the breadboard to see if it was somehow the problem. Everything seemed to be working, and yet the Arduino was deaf. For nearly and hours various people took a crack at it. In the end, it was a function I’d never used before called analogReference(INTERNAL); that was the culprit. Nobody caught that the AREF wire in the schematic was missing; the one in the board was removed because it wasn’t in the schematic; and the Arduino was waiting for voltages it would never get. We removed the line and suddenly we were getting numbers that made sense.

By then it was 11pm and time to go home. Al promised to work on connecting the 6DOF sensor over the week. If he does we’ll take another crack at it Tuesday night and you can read all about it here Wednesday afternoon.

I should probably mention along the way John cleaned a 1000 watt amplifier and started putting together the Solidworks computer; Miriam got her Makerbot Cupcake 3D printer moving; Jack was laser cutting; and there were at least 9 other people with as many projects going on that I didn’t even get to talk to. Come on down and check it out sometime, it’s really something.

Uncategorized

How cranes build themselves

2013-04-08 Dan

This is too good not to share. I never thought to ask the question until someone showed me the answer.

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Spray Paint Makelangelo?

2013-04-08 Dan

Chad writes,

“I have been playing around with draw bots this winter, and I really like your interface. I wanted to get your input on how difficult would it be to scale up the hardware and convert the pen to a paint can? I am a volunteer working with a city revitalizing effort. I would attempt to make a large scale Makelangelo Mural Bot to cover up gang related tagging on downtown buildings.

My thoughts are replacing the spindles with a chain sprocket, the thread would be #25 chain. allowing for long lengths and chain will reduce vertical sway.

Where I am confused how hard would it be to change out the adafruit board to a stepper driver board? Can your interface handle a much larger image? With a can of paint the speed of movement is an issue.

Any input would be greatly appreciated.”

Hi, Chad!

I’d stick with fishing line and increase the bobbin size, maybe get a local hackspace to print some gearing to handle the extra torque on the steppers. The bigger challenge will be the can holder/paint dispenser. The logic changes completely – instead of continuous lines and pen up/down it’s short taps on the nozzle to make dots, kind of like a giant pointilism piece. At the moment that logic doesn’t exist in the software.

It might also be easier to co-opt the tagging kids – pay them (in paint) to make actual murals with approved designs. They’re just trying to express themselves, right? If you give them ownership of the art they’ll defend it and feel pride in their community.

So far the only spraypaint polargraph I know of is Hektor. It only draws indoors. I guess the wind would be a problem for their model.

Anyone else want to convert their Makelangelo to use spray paint? Let’s talk in the forums.