Picking up and carrying around stuff is fun, but another claw/gripper? Can’t we do something new?
That is what I did. I sat down with the idea of Pythagorean triangles in my mind, and then I imagined a tower crane, with all these diagonal truss beams. Djing!
So I started laying out triangles and piecing them together. I’m pretty content with the result, which uses 3 different triangles.
A few caveats:
You need a turntable, I didn't dare putting so much stress on a single axle.
The hook and winch are not in the manual, I used 3 different ones, and I bet you need another 3 for your own projects.
At first I used my gamepad code to control the crane, but later I programmed it as well.
Tower cranes are usually used to build high buildings right? It’s maybe not as cool as flying robot builders, but I managed to let my crane build a tower.
The logic goes something like “turn around until the ultrasonic sensor reads less than 20cm, pull the winch down and up, go to starting position, go down and up, repeat”, and can easily be programmed in NXT-G or NBC.
Before you buy, remember that you need a turntable and of course a bit of string. Both the old and new model turntables should work with minimal modifications.
When using Robotic Invention System, or NXT-G for programming a robot, line following is usually done like this:
If the light is more than 50, turn left, else turn right.
This results in a slow scanning motion. It works fine for a first time, but soon, you’ll want to go faster.
I used to think that you just needed 2 light sensors, one on both sides of the line, so that you could go straight if both where white, and turn towards the one that becomes back. There is a better way.
When the light sensor is on the edge of the line, does it see black or white? In fact it sees a bit of both, so you get something in between. The trick is to think of the line as a gradient, like so.
If you put the NXT in the gray area, you can have a proportional steering function. Light gray means just a bit left, while dark gray means just a bit right.
Proportional, you say? Yes, we can just apply good old PID again!
// Define to which ports the sensor and motors are connected
#define LIGHTSENSOR IN_1
#define LEFT OUT_C
#define RIGHT OUT_A
// Define constants to tweak the algorithm
#define kp 100
#define ki 5
#define kd 30
// And another one to scale the final value
#define scale 10
dseg segment
// Light sensor reading
light word
// target light
target word
high word
low word
// The current error
err sdword
// The previous error
errold sdword
// The integral, all accumulated errors
errint sdword
// The deriviate, the expected next error
errdiff sdword
// Final pid value
pid sdword
// Temporary variable for calculations
temp sdword
temp2 sdword
// power to the motors
leftpower sdword
rightpower sdword
dseg ends
thread main
// Initialize the light sensor
SetSensorColorRed(LIGHTSENSOR)
// Get the time and start turning around
gettick temp
add temp temp 3000
OnFwd(LEFT, 50)
OnRev(RIGHT, 50)
// get light sensor reading
getin light LIGHTSENSOR ScaledValue
// set high and low to that reading
mov low light
mov high light
Circle:
// Get the light reading
// if it is more than high, jump to Higher
// if it is lower than low, jump to Lower
getin light LIGHTSENSOR ScaledValue
brcmp LT Lower light low
brcmp GT Higher light high
// else check if the time has passed
// Jump to Done, else go back to Circle
gettick temp2
brcmp LT Done temp temp2
jmp Circle
// set light to the new low
// jump back to Circle
Lower:
mov low light
jmp Circle
// set light to the new high
// jump back to Circle
Higher:
mov high light
jmp Circle
Done:
// we now have the max and min light value found
// calculate the center value
sub target, high, low
div target target 2
add target target low
Forever:
// Read the sensor and store it in light
getin light LIGHTSENSOR ScaledValue
// Substract the actual distance from the target for the current error
sub err target light // Proportional
// Add the error to the integral
add errint errint err // Integral
mul errint errint 0.8 // multiply by 0.8 to dampen it
// Sunstract the previous error from error
// so that we get the speed at which the error changes
sub errdiff err errold // Derivative
mov errold err // set the current error as he old error
mul pid err kp // Apply proportional parameter
mul temp errint ki // Apply integral parameter
add pid pid temp
mul temp errdiff kd // Apply derivative parameter
add pid pid temp
div pid, pid, scale // Apply scale
NumOut(0,0,target)
NumOut(0,8,light)
// saturate over 100 and under -100
brcmp LT, under100, pid, 100
mov pid, 100
under100:
brcmp GT, overMin100, pid, -100
mov pid, -100
overMin100:
// subtract pid from one of the motors
brtst LT, Negative, pid
OnFwd(LEFT, 100)
sub rightpower 100 pid
OnFwd(RIGHT, rightpower)
jmp Run
Negative:
OnFwd(RIGHT, 100)
add leftpower 100 pid
OnFwd(LEFT, rightpower)
Run:
jmp Forever
endt
Did you know that even the motors of the NXT use PID themselves to provide accurate control?
When I got the Ultimate Builders set, it came with instructions for a plotter. Mind you, the RCX had no integrated rotation sensors, so it used a lever rotating against a touch sensor. It even used a complicated construction to drive a pneumatic pump with one motor.
It came with a program to write “LEGO”, which I did not understand, because it was huge. And it didn’t work either, but that turned out to be because of a broken sensor… after I took the plotter apart.
I just found the video of a “Lego Master builder” introducing the model.
Anyway, a plotter was on of the things on my list that had to happen properly someday. The one you see here is my third attempt. The first two where too fast, too bulky, and had a very feeble arm holding the pen.
I actually made this model before the NXTbike, but I messed up the building instructions, so I had to do it again. I can’t recommend Lego Digital Designer for any serious Technic building, use LDraw instead.
The first thing I did, even before my first attempt, was writing a virtual plotter, so I could quickly code up the alphabet. I wrote the software in Python, so that I could use the Turtle module for the virtual printer, and nxt-python for the real job. The software can be found on Github.
I made this model before I realized I should minimize the use of non-NXT parts. Ironically, this model uses a few parts from the Ultimate Builder set, but easy workarounds exist for most parts. Check the parts list before you buy.
