Limiting PWM in pbasic
 

Posted by Alan Federman at 2/11/2001 9:59 AM EST


Engineer on team #255, Odyssey, from Foothill HS, San Jose and NASA.



Which is the best way to limit the maximum amount of power to a PWM driven motor? is it: pwm1 = (pwm1 - 127) * .25 + 127 or pwm1 = (pwm1 MAX 160 MIN 94)

Re: Limiting PWM in pbasic
 

Posted by Matt Leese at 2/11/2001 1:59 PM EST


Other on team #73, Tigerbolt, from Edison Technical HS and Alstom & Fiber Technologies & RIT.


In Reply to: Limiting PWM in pbasic
Posted by Alan Federman on 2/11/2001 9:59 AM EST:



Well, you need to remember that PBasic doesn't support
floating point numbers. It only does integer math so
multiplying times .25 (if it even compiles) won't ever
do what you want. Dividing by 4 (or better yet, right
shifting two places) will work. This however just
scales down the output value. By using a MAX and MIN
you put a fixed limit and the slope of the input is the
same. When you scale down you change the slope of the
data. When scaling, be careful with being greater than
127 or less than 127 as PBasic also doesn't support
negative numbers so that will definatley give you bad
data. So for stricked limiting of values MAX and MIN
is better but if you want to slow down the robot
totally, scaling works better.

Matt

Re: Limiting PWM in pbasic
 

Posted by Mark Pierce at 2/13/2001 12:59 PM EST


Other on team #288, RoboDAWGS, from Grandville High School and X-Rite, Delphi, and others.


In Reply to: Limiting PWM in pbasic
Posted by Alan Federman on 2/11/2001 9:59 AM EST:



Another way to limit PWMs, especially if you are driving it from a joystick or analog input is to use a look up table. Build a table of values that has the desired shape and range of values. This also eliminates the need for deadband handling code and can be adjusted to give the operators more natural control. See Steve's page:

http://sharingfirst.mit.edu/users/st...%20Control.htm

That seems a bit complicated.
 

Posted by Joe at 2/14/2001 9:10 PM EST


Student on team #506 from St. Anthony's HS.


In Reply to: Re: Limiting PWM in pbasic
Posted by Mark Pierce on 2/13/2001 12:59 PM EST:



: Another way to limit PWMs, especially if you are driving it from a joystick or analog input is to use a look up table. Build a table of values that has the desired shape and range of values. This also eliminates the need for deadband handling code and can be adjusted to give the operators more natural control. See Steve's page:

: http://sharingfirst.mit.edu/users/st...%20Control.htm


After sitting looking at the code for about 45 I decided to come here looking for answeres. I saw your
post and investigated. It looks to me like you know what your doing well beyond my teams comprehension (were a rookie team with no programming experience). I decided to combine the ideas listed in a previous post (the one you responded to i think) with the tank drive system. Hopefully this will alow the tank drive to run at a slower, scaled speed. Please tell me if this works and if it has similar effects to what your program did.
PWM1 = (((((2000 + p1_y - p1_x + 127) Min 2000 Max 2254) - 2000) - 127) / 2 + 127)
PWM2 = (((((2000 + p1_y + p1_x - 127) Min 2000 Max 2254) - 2000) - 127) / 2 + 127)

notice the adition of the -127) / 2 + 127) at the end of both lines. This is untested and i'll be trying it out tomorrow. If anyone sees a problem or an even easier way please let me know.
also: if this works the degree of the scale can be adjusted by simply changing the / 2 to another number.

Re: That seems a bit complicated.
 

Posted by Mark Pierce at 2/15/2001 10:14 PM EST


Other on team #288, RoboDAWGS, from Grandville High School and X-Rite, Delphi, and others.


In Reply to: That seems a bit complicated.
Posted by Joe on 2/14/2001 9:10 PM EST:



What you have should work, be careful about changing the 2 and 127 to ensure that rounding errors don't cause over flow (this is a whole number only processor).

In my first post I was thinking about limiting PWMs used to drive devices other than the drive. We always want to have our drive motors to have max. speed available and reduce speed with gearing. I wasn't thinking about tank drive, but a look up table can be used to simplify this as well.

As an attempt at explaining:

First, build a table with 128 - 0 values, followed by 256 values that range from 0 to 254, followed by 128 - 254 values:

PWMTBL Data 000, 000, 000, 000, 000, 000, 000, 000 ' 0

(repeat the above line 15 times)

Data 000, 001, 002, 003, 004, 005, 006, 007 ' 128
Data 008, 009, 010, 011, 012, 013, 014, 015 ' 136
Data 016, 017, 018, 019, 020, 021, 022, 023 ' 144
Data 024, 025, 026, 027, 028, 029, 030, 031 ' 152
Data 032, 033, 034, 035, 036, 037, 038, 039 ' 160
Data 040, 041, 042, 043, 044, 045, 046, 047 ' 168
Data 048, 049, 050, 051, 052, 053, 054, 055 ' 176
Data 056, 057, 058, 059, 060, 061, 062, 063 ' 184
Data 064, 065, 066, 067, 068, 069, 070, 071 ' 192
Data 072, 073, 074, 075, 076, 077, 078, 079 ' 200
Data 080, 081, 082, 083, 084, 085, 086, 087 ' 208
Data 088, 089, 090, 091, 092, 093, 094, 095 ' 216
Data 096, 097, 098, 099, 100, 101, 102, 103 ' 224
Data 104, 105, 106, 107, 108, 109, 110, 111 ' 232
Data 112, 113, 114, 115, 116, 117, 118, 119 ' 240
Data 120, 121, 122, 123, 124, 125, 126, 127 ' 248
Data 127, 128, 129, 130, 131, 132, 133, 134 ' 256
Data 135, 136, 137, 138, 139, 140, 141, 142 ' 264
Data 143, 144, 145, 146, 147, 148, 149, 150 ' 272
Data 151, 152, 153, 154, 155, 156, 157, 158 ' 280
Data 159, 160, 161, 162, 163, 164, 165, 166 ' 288
Data 167, 168, 169, 170, 171, 172, 173, 174 ' 296
Data 175, 176, 177, 178, 179, 180, 181, 182 ' 304
Data 183, 184, 185, 186, 187, 188, 189, 190 ' 312
Data 191, 192, 193, 194, 195, 196, 197, 198 ' 320
Data 199, 200, 201, 202, 203, 204, 205, 206 ' 328
Data 207, 208, 209, 210, 211, 212, 213, 214 ' 336
Data 215, 216, 217, 218, 219, 220, 221, 222 ' 344
Data 223, 224, 225, 226, 227, 228, 229, 230 ' 352
Data 231, 232, 233, 234, 235, 236, 237, 238 ' 360
Data 239, 240, 241, 242, 243, 244, 245, 246 ' 368
Data 247, 248, 249, 250, 251, 252, 253, 254 ' 376
Data 254, 254, 254, 254, 254, 254, 254, 254 ' 384

(repeat the last line 15 times)

Then your equations can be re-written as follows:
READ 256+p1_y-p1_x,PWM1
READ p1_y+p1_x,PWM2

To adjust the scale, change the zeros and 254s to some other numbers centered on 127 and scale the middle numbers accordingly. We use a spreadsheet to calculate these, but doing it by hand isn't difficult either.

Good Luck,

Mark