Motion profile generator.

[richard]

I have noticed another problem with my methods ,in that I'm calculating velocity by subtracting pos from old postion in the pid loop. when under acceleration this is actually average velocity not the actual (final)
velocity . I have not been successful in determining velocity without impinging on pid evaluation time.
using floating point routines with a 8bit cpu the highest pid rate I can achieve is 250 hz so average velocity is not good enough. might mean fp pid is not worth pursuing
general purpose might be a tall order . I have a leadshine dcs303 digital dc servo driver , it runs a dcm50202 servo motor / 500line en with no problems (can achieve 3000 rpm and move to pos +- 1 count) . It will not even come close to achieving servo lock on the other motors I'm using. my other motors have a really high inertia this makes them more predictable for my "slow" routines providing you don't try to exceed their accel profiles. the low inertia servo is problematic

[Ioannis]

I am not an expert on this subject but would try to help. Is it necessary to use the PID routine all the time?

Why not just call it just before the target point is close so that it will be used less and everything will be running much faster?

Maybe I am away from what is discussed here... Just a thought.

Ioannis

[HenrikOlsson]

Richard,
Yes, I think floating point is going to take too much resources and I'm not sure it'll actually solve anything. The position is still "set" and sampled at discrete intervals, there's no way to force it to "hit" a specific position at a discrete interval while still allowing "any" velocity and "any" acceleration. If target position is NOT to be violated then something else will have to give. Either change velocity so deceleration can start at exactly the correct spot. Or change deceleration so the position will end up at the correct spot once velocity reaches zero. I'm opting for the second....

I have successfully used my PID code with step and direction input running a motor with a 3600 line encoder at 4000rpm (960kHz step frequency), it'll position to 1 count without problem. The PID is executed at 1500Hz with processing power to spare. Here it is: ,

for more details see my webpage. But it's being commanded by an external motion control software which is what generates the motion profile. The DCS303 manual mentions built in motion controller for self thest with trapezoidal velocity profile which is exactly what I'm trying to achieve. Unfortunately the DCS303 manual doesn't say much about how its profile generator works and/or which set parameters it may "violate" (if any) to achieve its goal.

Ioannis,
YMMV of course but in my case I'd say yes, it's neccesary. It's not just a matter of moving from point A to B as fast as possible, I want the motor to move from point A to point B following the profile generated by the code discussed here as closely as possible. Without the closed loop in there the motor could potentionally stall in the middle of the move.

Fun stuff this....

/Henrik.

[richard]

henrik
will this profile generator acutally "control" the motor or just output pulses/dir to another step /dir controller ?
I'm thinking in the later case you have would heaps of time to do the sums and design a profile and then just output the pulses when prepared.
to accelerate just decrement/increment the pulse interval by a fixed amount , the first and last few pulses and maybe even the transition to constant velocity could all be given special treatment. that treatment would need to be customised for motor/load/inertia as would max velocity and acceleration. bit like mach3 in a pic ?

[HenrikOlsson]

Hi Richard,

The profile generator will generate the moving target setpoint that the PID filter servos the motor to, output of PID filter goes to PCPWM module which drives the H-bridge. Everything running on the same PIC. (Step/dir signals could also provide the moving target setpoint TO the PID filter (bypassing the profile generator) but for this project I'm not interested in generating step/dir signals.)

I think that generating a step/dir based profile would actually be easier (though generating step pulses at any decent frequency isn't easy on a PIC) because then you are basically controlling the motor down to the individual step level. For each "tick" you either issue a step or you don't so it's easy to "hit" the exact spot where deceleration needs to start in order to get to the target position without violating any of the settings.

When doing the profile generation, PID, PWM and feedback in the same PIC it's not as easy since the motor is moving more than a single count for each "tick". You tell the motor to move an arbitrary amount of counts per "tick" which so you're likely to overshoot the "sweetspot" where you need to start decelerating.

I hope that makes sense.

/Henrik.

[richard]

if you were to divide an acceleration move into a number of equally timed blocks,the number of blocks is high enough to ensure that the travel (encoder clicks?) for each block is always < 255 . each block would represent a relative target position based on required acceleration ,it will obviously turn out that the final block(the remainder) would violate the strict acceleration figure but it would always result in slightly lower acceleration ,i doubt that would matter much position being the more inportant criteria.
these blocks could be stored in an byte array , the setpoint for the pid routine would be incremented/decremented by the block travel[] at the block rate , and the preformance could be monitored by checking current position vs start pos+accumulated offsets at each block increment.
as long as the block duration is > than the pid update rate and you have enough memory for a large enough array , at worst the acceleration move could be divided into variable block rate stages to make things more manageable .