Help with cybot light avoid code

[terrapinlogo]

I have hooked my cybot up to a 20x2 and am in the process of coding it with a light avoid program but I am having trouble with a certain line of code.

if rval = lval-50 or rval = lval+50 then goforward

I want to compare two variables and if they are within 50 of each other then it should go forward. With the code above I get a syntax error. Looking in the manual I can't see any command that would do what I want. Does an one have any ideas?

While I'm on the subject of this code, you may as well tell me if I have my hardware interrupt code setup correctly.

 

[BeanieBots]

Unfortunately, PICAXE BASIC does not permit calculations within an "if" statement.
You will need to calculate the two values, hold them in variables and then test against those.

 

[lanternfish]

Hi

This is not very elegant but overcomes your syntax error problem

	b3 = lval - 50
	if rval = b3 then goforward
	b3 = lval + 50
	if rval = b3 then goforward

If lval = 49 or lval = 206 then you will get an overflow error i.e. if lval = 49 then b3 will equal 255. And if lval = 206 then b3 will = 0.

How will this affect your program?

 

[hippy]

Another technique is ....

w0 = lval - rval
If w0 < ???? Or w0 > ???? Then ...

When lval >= rval the result will be $0000-$00FF ( 0 to 255 ), when when lval < rval the result will be $FFFF-$FEFF ( -1 to -255 ). A word variable has to be used to determine the subtraction has created a negative which appears as a large 16-bit integer through underflow wrapround.

 

[lanternfish]

I am wondering why you need this 'deadband'. And you appear to have no stop routine.

If you do not need the deadband then your program can be like this:

main:

	readadc lldr, lval	;read ldr values into variables
	readadc rldr, rval
	
	if lval < rval then goright				
	if rval < lval then goleft
	
goforward:
	low motorrr		;turn off reverse pins to prevent shorting out the hbridge
	low motorlr
	high motorrf	;turn motors on forwards
	high motorlf
goto main

(rest of code)

 

[terrapinlogo]

That will work perfectly hippy, thank you.

@lanternfish - The ldrs give slightly different values and if I didn't have the deadband then I believe the movement would be very jerky. I may be wrong but that is why I have it.

 

[hippy]

A simple way to measure light to left or right is to wire the two LDR as a potential divider, read via READADC and see what the reading is - it will be around 128 when both have equal light on them, above when light to one side, lower when light to the other.

 

[BeanieBots]

To expand on Hippy's suggestion, I find it helps to put a pot (of a few kohms) between the two LDRs with the wiper to the ADC input. This serves two purposes. One is to prevent a low(ish) impedance across the power rail when both are exposed to high light levels, the other offers a way of trimming the mid point value to exactly 128.

Also, don't forget, you can use the difference to produce a number which can be used to set different speeds. It does not need to be hard left of right.

 

[terrapinlogo]

The ldr section of the code is working in the simulator (haven't had time to hook the 20x2 all up yet), but the hint is not working as I hoped. It will trigger once but then wont trigger again. Is this some bug in the simulator or my code. I'm not exactly sure what I am supposed to have in the interrupt gosub to re initialize hint.

 

[hippy]

You need the 'return' rather than 'goto main' at the end of interrupt.
You need an additional 'hInt2flag=0' to clear the interrupt.