Encoder velocity....

[thasatelliteguy]

[QUOTE=HenrikOlsson;126443]Hi,
For example, TMR1 is a 16 bit wide counter/timer. It's pretty much setup to be a "tick counter" at start up, all you need to do is enable it

T1CON.0 = 1   ' Start TMR1

From now on TMR1 will tick at Fosc/4 but you can scale that down by using the prescaler.

OK. So I am starting the clock ticker... How would I pull the current values from it? And then, since I only really need it when the machine is "lost", after my homing process, would it be to my advantage to disable it once finished, or just leave it running?

And I'm not sure what you mean about getting V without doing a lot of math. The math is simple enough... and I don't see how to find V any easier since V is a calculation of time and distance. I would love an easier way if you can show me. This is what I'm doing now...


homeElevation:
serout commout,T4800,"E"
Eencoder0Pos = 0
gosub elUP
pause(250)
if Eencoder0Pos = 0 then gosub moveerror
done = 0
Do
serin commin,T4800,cereal
if cereal = "*" then goto abort
Enewposition = Eencoder0Pos
newtime = millis()
if Enewposition > Eoldposition then Enewvel = (Enewposition-Eoldposition) * 1000 /(newtime-oldtime)
if Eoldposition > Enewposition then Enewvel = (Eoldposition-Enewposition) * 1000 /(newtime-oldtime)
Eoldposition = Enewposition
oldtime = newtime

if Enewvel < .5*Eoldvel then
gosub allSTOP
Eencoder0Pos = 3665
gosub elDN
do
serin commin,T4800,cereal
if cereal = "*" then goto abort
if Eencoder0Pos =< elHOMEpos then
gosub allSTOP
done = 1
exit
endif
pause(250)
loop
endif
if(Enewvel > .5*Eoldvel) then Eoldvel = Enewvel
if done = 1 then exit
loop
return

[HenrikOlsson]

Hi,

OK. So I am starting the clock ticker... How would I pull the current values from it? And then, since I only really need it when the machine is "lost", after my homing process, would it be to my advantage to disable it once finished, or just leave it running?

The timer, since it is 16 bits wide, consits of two 8 bit registers, TMR1H and TMR1L (check the datasheet). You read the registers as you read any other register in the PIC, for example TickCount = TMR1H * 256 + TMR1L. Since it's a hardware timer it'll keep running in the background without any impact to the rest of your software - as long as you don't want to use it for something else of course. Just remember that it's "only" 16 bits wide so it'll overflow "frequenctly".

And I'm not sure what you mean about getting V without doing a lot of math. The math is simple enough...

Well, yes, the math might not be very complicated from a human perspective but none of the devices you list as possible targets for your code does multiplication OR division (no 10, 12, 16 or 18 series PICs does division) in hardware so even though the actual equation/formula in it self (multiply by a thousand, divide by delta time) isn't "complicated" it's going to take the poor little PIC a lot of instruction cycles to get to a result (which MAY or may not matter). What I'm saying is that if you instead see to it that you get a new position update every x ms then all you need is a subtraction (current position - previous position) to get your velocity (in encoder counts per x ms).

If it's not possible to get a consistent interval between position updates then you ARE doing the right thing by calculating velocity based on delta time AND distance.

/Henrik.