DT-Ints latency and other interrupt conciderations

[HenrikOlsson]

The protoype, on which I managed to get 60kHz+ yesterday, is still running on 20Mhz which makes me believe I should be able to reach my target of 100Khz even after adding the "front end" and some other features.

But right now these serial routines is making me go nuts....

[Ioannis]

Well, if you go from 20 to 40 MHz then your nice 60KHz is made with no pain 120KHz. That is more than 20% of your target, leaving you with a margin for the serial communication.

Why not try this first, to ease the pain?

Ioannis

[DDDvvv]

once again, thanks for the wonderful pid filter. im still troubleshooting, and my main problem is this;

 MAIN:                                             
     IF POSITION < SETPOINT THEN                  ;decreasing the setpoint                
     pid_Error = setpoint - position            ;calculate error
     ENDIF
     IF POSITION > SETPOINT THEN                  ;increasing the setpoint
     PID_ERROR = 65535-(POSITION - SETPOINT)    ;calculate error                                                              
     endif         
     gosub pid                                  ;take error to pid filter  
     DIRECTION = pid_out.15                     ;determine direction
     PID_TEMP = ABS PID_OUT
     DUTY = PID_TEMP                            ;load duty cycle from pid_out
     select case DIRECTION                      ;direction bit to control    
     case 0                                     ;  hpwm 1 for forward or hpwm2     
     GOSUB FORWARD                              ;  for reverse to corresponding
     case 1                                     ;  h-bridge circuit 
     GOSUB BACKWARD                             ;
     END SELECT
     LCDOUT $FE, $80, "POS=" ,DEC5 POSITION, " SP=" ,DEC5 SETPOINT; temporary
     LCDOUT $FE, $C0, "DT=" ,DEC5 DUTY, " ERR=" ,DEC5 PID_ERROR   ; temporary                 
     goto Main

my calculated pid_error jumps from 65534 to 00001 , skipping two numbers; 65535, and 00000. i have the recommended position calculation,

 position = 256*poscnth + poscntl

this runs from a dt_interrupt (thanks Darrel Taylor). i tried to change setpoint and position variables to byte, instead of word, but pid filter works on word variables, and all calculations get messed up. any solutions? this has been my biggest headache on this project, for the last three weeks. and any help from anyone will be highly appreciated. thanks in advance.

[HenrikOlsson]

Hi,
It "skips" 0 because when Position=Setpoint (ie. Error should be 0) neither of your < and > evaluations are true so it won't even calculate the error. Why it skips 65535 I haven't figured out though. Can you explain why you have those two comparison statements?

All you really should need to do is pid_Error = Position - Setpoint or Setpoint - Position if you prefer it the other way around, it doesn't matter as far as the PID routine is concerned.

Let's say pid_Error = Position - Setpoint (all three are 16bit variables):
If Position is 10000 and setpoint is 9000 the error will be calculated to 1000
If Position is 9000 and setpoint is 10000 the error will be calculated to 64536 (-1000)
If Position is 1 and setpoint is 0 the error will be calculated to 1
If Position is 0 and setpoint is 1 the error will be calculated to 65535 (-1)

Just pass it to the PID-filter, it figures out the sign of the error.

/Henrik.

[DDDvvv]

THAT FIXED IT!!

man, thanks, and pardon my blindness. i did not know that the pid filter takes care of all the math! every little bit! the more reason of printing these pid filter code, framing it and hanging it on the wall next to the family portraits!!!

i had it like that to fix a problem, when using two hpwm's the duty cycle would shift over to pwm2, from pwm1, and vice versa at position roll-over, no matter where setpoint was. that problem has dissapeared also.

thats one hurdle over, and now over to implementing the power control on the same chip, to drive a h-bridge driver.

this chip has 8 pwm outputs (4 complementary pairs) ill try as much as possible to implement full motor control from this same chip, as opposed to using a separate driver, like the lmd18200.

thanks
david m

[HenrikOlsson]

That's great!
Yeah, the 18Fxx31 has some very cool features indeed. I intend to scale up as well once I get the basics going. My prototype uses the LMD18200 and I've abused it quite a bit, still haven't managed to let the smoke out of it though :-)

Keep us posted on the progress!
/Henrik.

[DDDvvv]

hi guys
spent all weekend, maintaining the marriage, NFL playoffs and designing the board for the servo.
attached is the board im trying to design. its nearly done, needs to be optimised, and copper flooded.
im using eagle pcb pro.
i decided to go the old school way; adjust pid variables using pots, and display them on a 20x2 lcd. later on, lcd will be replaced by serial communication, for serial lcd, or pc serial port.

there are seven ports representing; i_term, output_clamp, integrator_clamp, minimum duty cycle, proportional, integral and derivative.

i went to fire up my cnc pcb router, to make this board, and found out that my x-axis is completely dead. needs a new sla7078 unipolar driver. this is what im trying to replace with this servo project.

ill probably order some 7078's, or use press and peel blue (thats how i was making boards before the router came along)

im using the power control module on the 18f4431. its way much better than ccp pwm. its configured to use channels 0,1,2,3, in independent mode, so 0/1 produce same duty, and turn on at the same time, while 2/3 is off, and vice versa.
the fet drivers are ncp5181, of which i could find at mouser.com, and has a bunch of application notes on the web.
still optimising and trimming down the code, which is still undergoing extensive testing.
have fun everyone....