DT-Ints latency and other interrupt conciderations

[HenrikOlsson]

Hi,
The way I do it is (basically) having an interrupt triggered by the USART when a character is received. The ISR takes the char and puts into an array that acts as a buffer, then it increments a "pointer" so it knows where in the array to put the next byte when it comes in. Finally it checks if the recieved char is a LF or CR - if it is it sets a flag, telling the main rountine that there's data to take care of.

The main routine sees the flag and looks at the first character in the array, if it's a "P" it knows it's supposed to set the proportional gain. Next it uses the "pointer" to determine how many charcters there are in the buffer and finally it uses ArrayRead to "convert" the correct number of ASCII characters in the buffer to a value.

In reallity it does a bit more than that but that's pretty much how I did it.

/Henrik.

[DDDvvv]

thanks for the prompt reply. it will probably take me 2 days to figure out some working code for just that, but i dont mind. i found out this is the best way to learn programming. meanwhile, im trying to design a h-bridge test board for a 2431. ill keep you updated on results.

thanks

[DDDvvv]

long time no see, henrik... hows your project going?
i managed to get my h-bridge working. im having trouble tuning the values. i spent all weekend trying to tune a motor at no load, and i kept getting overshoot, undershoot, oscillations, runaway, e.t.c.
im aware that tuning values are unique for every different model of motor, and i've tried all the tuning methods out there. im spoilt by those you-tube videos of pid motor position control.

how do you do it? ive tried the zieger-nichols, and every other tuning idea online, but to no avail. what kind of values do you use for your motors? ill take any parameters, no matter what motor you use, after spending a whole weekend tuning.

thanks

[HenrikOlsson]

Hi,
I usually (well always) do it "by feel"....

Leave I at 0 to begin with it, add P until you're starting to see severe overshoot or it starts to oscillate. When that happens start adding D to dampen it out. Once you dampened it you should be able to add some more P and then follow with even more D. Finally add I to get rid of any steady state error and increase the "stiffness".

On my "developmemnt system" with a smalish (30W) DC-motor, 500 line encoder (2000cpr) and LMD18200 H-bridge I have P:1500, I:100, D:2000, Ti:1

Remember that with very high gains there is a risk of overflowing the internal calculations (there's are no checks for that in PID routines). That would make the loop highly unstable and might cause the servo to run away. I'm not saying that is what is happening in your case, just that it's a possibility. It's also very imortant to set the output clamp to a proper value so you're not overflowing the PWM duty cylcle register.

What's the resolution of your encoder?

/Henrik.

[DDDvvv]

thanks for the reply. ill try your values. my encoder is only 100cpr on a yamamoto 24v motor, ripped off a photocopier machine. i have a bunch of other motors and encoders, upto 1500 cpr. does the cpr make a difference, other than more precision?

[HenrikOlsson]

Yes, it makes a hugh difference. Think about it...what if you only had four counts/revolution, then the motor could move 90 degrees without the system even knowing it. Same motor same settings but now with a 2000 counts/rev (500 lines) encoder, the 90 degree displacements now results in an error of 500 counts. This give the servo-loop a lot more "information" to work with which helps in stabalising the loop.

If that is a 25 line encoder (100cpr) then that may be a big part of your problem. Trying 'my' values with that motor/encoder probably isn't going tp help you one bit. If possible try with a higher resolution encoder.

/Henrik.

[DDDvvv]

oh no.. the encoder is 100 lines per revolution, giving a quadrature resolution of 400 cpr.

ill try the 1500 lines per revolution, thats 6000cpr! anyway, ill experiment with all the encoded motors that i have. thanks