topic Re: How to input a time to a Function node? in LabVIEW

How to input a time to a Function node?

fourteen — Thu, 08 Apr 2010 09:34:43 GMT

How can i input a time input as the time to count elapsed when a condition turn to be true!

or- just how to input a time value in millisecond, i have tried to using the Tick count, but this can only produce one value, what want is a continuous time value, used to generate a voltage out put according to the time value.

Re: How to input a time to a Function node?

GerdW — Thu, 08 Apr 2010 09:44:24 GMT

Hi fourteen,

have a look at the Waveform datatype. This one includes a start time and dt between the samples...

Re: How to input a time to a Function node?

KateB — Fri, 09 Apr 2010 01:13:16 GMT

Use Build Waveform function.

Re: How to input a time to a Function node?

fourteen — Wed, 14 Apr 2010 08:59:18 GMT

Sorry, I am still not quite understand this function.

this time input is still not the real time ~ i mean, what i expected is that the graph will be generated continuously as the time pass away,

like continuously drawing out the graph!

but this waveform built is more like generate the profile instantly.

It's more like the one i have attached below.

The reason i am asking this is because i wanna to continuously input a position command to the motor according to the profile i have generate, and i want to this time to be the real time. Not the digital time in the computer screen that you have seen, but the real time.

and i have ask this question before, haven't got the answer yet!

http://forums.ni.com/ni/board/message?board.id=170&message.id=489245#M489245

Re: How to input a time to a Function node?

fourteen — Wed, 14 Apr 2010 09:07:26 GMT

LabView is such a powerful software, can't even generate a signal like a watch???

What i want is a watch, a timer, which can count time elapsed between two command that i send to the computer.

and this should give a series of number just like the watch(timer) did, according the the resolution that i have defined.

It's really unbelievable that it would be so hard for me to find a such program form all the labview functions, I really can not believable this!

Re: How to input a time to a Function node?

GerdW — Wed, 14 Apr 2010 09:12:08 GMT

Hi 14,

"it would be so hard for me to find..."

I also don't understand what's so hard to look at the time functions palette

From your attached block diagram:

You already use a "time resolution" which translates to the dt input of the waveform datatype.

You don't use any start time in your VI, so we suggested to use the system time...

"Not the digital time in the computer screen that you have seen, but the real time."

Could you please clarify the difference between "time in the computer screen" and "real time"?

For me there's no difference (atleast no difference bigger than ~200ms)

Re: How to input a time to a Function node?

fourteen — Wed, 14 Apr 2010 09:28:17 GMT

what i mean by ~~ the time you seen in the screen ~

is like a velocity profile, and you see that the X axis is for the time. and this is just the time that you defined ~

how are you going to let the machine move according to the number that you have entered??

I do have seen many functions under the timing, but i just can not find the one that i want.

Non of them seems being suitable for me.

For me, the main problem is how to command those time signals.

How to command the machine to move according to the time that i have defined.

it's like, at what time, i want this machine to do something ` but how can i tell the machine?

and i want this process being in a millisecond scale, in order to have a smoothly position profile.

Re: How to input a time to a Function node?

GerdW — Wed, 14 Apr 2010 09:36:12 GMT

Hi fourteen,

"how can i tell the machine?"

Read the manual of the machine!

Most "machines" don't work like "at time x move to position y", but rather "from current postion move with speed z (+ acceleration a) for time t". Think about it...

In the timing palette you find functions that give you the current time as timestamp and functions that give you a relative ms counter. Both values can be used for math functions. What else do you need?

Re: How to input a time to a Function node?

fourteen — Wed, 14 Apr 2010 10:24:24 GMT

what i mean by "at time X move to a position Y" is because the machine i am currently dealing with is just a simple DC motor, and i want to input a velocity profile, this profile will tell the machine to move at a certain speed at a specific time, and this command will last for a sample time!

Can the timestamp function return me the time in millisecond?? I didn't make it out.

For the relation ms counter, which function do you mention?

the tick count? the wait? wait until next ms multiple??

can those function return me the time elapsed just like the watch?

if all those are possible ~ can you show me a sample,

something like ~ the Vi will output 10

at 10s m and out put a 20 at 20s,

and continuously last for 2 minutes???

what i want is exactly like this ~ nothing about the machine~

i just want to out put some number at a certain time ! at the time interval for my case should be millisecond

Re: How to input a time to a Function node?

GerdW — Wed, 14 Apr 2010 11:01:32 GMT

Hi fourteen,

what kind of hardware (computer) do you use? Windows will not be suitable for (accurate) ms intervals...

Unless you use a waveform (with dt=1ms), including a curve for your output values. DAQmx is able to output that too...

- There's an ExpressVI named "Elapsed time", have you played with it?

- Otherwise take the current "tick count" and subtract an older tick count to get the elapsed time.

- Timestamps also include µsecond values, it's just a matter of formatting (or type conversion to DBL)...

For an example you will have to wait a little bit

Message Edited by GerdW on 04-14-2010 01:01 PM

Re: How to input a time to a Function node?

fourteen — Thu, 15 Apr 2010 06:02:09 GMT

the tick count works just like a watch ~ but how to set the number to 0 when a press the stop button?

i use a indicator to indicate the time value ~ but this value is just keep on increasing, and can not return to zero.

Re: How to input a time to a Function node?

fourteen — Thu, 15 Apr 2010 06:25:48 GMT

For the tick count~ every time i stop the VI, when i restarted it, the output value will started from the last value when i stop it, how can i make it to started from 0?

Re: How to input a time to a Function node?

rolfk — Thu, 15 Apr 2010 06:30:14 GMT

Tick count is a timer that keeps running forever from the moment you startup the computer. Its absolute value is rather meaningless and has an interval of 4294967 s = 1193 hours after which the counter rolls over and starts from 0 again. However the most meaningful use of the Tick Count is the calculation of the difference between two values. Since it is an unsigned integer the result of a subtraction is correct even if the counter has rolled over in between (of course rolling over more than once can't be detected and therefore won't be possible to time in such a way).

Message Edited by rolfk on 04-15-2010 08:30 AM

Re: How to input a time to a Function node?

GerdW — Thu, 15 Apr 2010 06:30:14 GMT

Hi fourteen,

use a loop. Outside the loop you get a "start" tickcount value. In the loop you subtract the start value from the current tickcount to get "elapsed time" starting from zero. Simple math

TickCount just provides a counter. To get elapsed time you have to subtract two counter values...

Re: How to input a time to a Function node?

fourteen — Thu, 15 Apr 2010 08:23:24 GMT

I have managed to get a example for a tick count to reset the counter.

Right now i am facing another problem, as the lab computer so not have matlab, i have to use the function node to generate the trapezoidal profile!

i use Vmax, Amax, t, Pi as the input, and Po as the out put, and define

double p0, p1, p2, t0, t1, t2;

and the things i wrote in the function is below, but labview keep on show me that there are errors, like con not use double to define the data type, and the use of if else is also wrong~

can you do me a fever to point out where is the problem and how to correct this??

or which way i should go to solve this problem??

Can i use this to command the position continuously???

p0 = (Vmax*Vmax)/Amax;

t0 =sqrt(Pi/Amax);

p1=(Vmax*Vmax)/(Amax*2);

p2=Pi-p1;

t1= Vmax/Amax;

t2= Pi/Vmax;

if (Pi <=p0 && t<=t0) {

Po=0.5*Amax*t*t

}

elseif (Pi<=p0 && t0<t<=2*t0)

{

Po=Pi-0.5*Amax*(2*t0-t)*(2*t0-t)

}

elseif (Pi>po && t<=t1) {

Po=0.5*Amax*t*t;

}

elseif(Pi>P0 && t1<t<=t2) {

Po=p1+Vmax(t-t1)

}

elseif(Pi>p0 && t2<t<(t2+t1) )

{

Po=Pi-0.5*Amax*(t-t2)(t-t2)

}

Re: How to input a time to a Function node?

GerdW — Thu, 15 Apr 2010 08:32:30 GMT

Hi fourteen,

a formula node does not work with MatLab code (here you have to use the mathscript node), but resembles C code. It also has an extensive context help, where you could read a lot on syntax and supported functions!

All of the shown program could be done in pure LV functions (it mostly InRange-checks and simple math).

But it seems you are focused on your single approach to solve the problem.

You approach uses the current time to calculate the needed output. Could be done, but involves some overhead as you have to calculate for each and every time you want.

I would advise a different approach:

First calculate all needed outputs by creating a waveform (concat different waveforms like RampUp, Hold, RampDown). Then you simply need to index the value corresponding to current time! Also some output devices support to output a waveform, so you wouldn't need a loop anymore to output the whole "profile"...

Re: How to input a time to a Function node?

fourteen — Thu, 15 Apr 2010 10:11:49 GMT

Yeah, i have been trying to use basic labview structure to solve this problem also! but i think that if i can use a function node, or a matlab script, this would be easier.

And eventually i find out that this function node can not deliver what i want, and yet the lab computer do not have a matlab.

for your suggestion, the main problem, i have no ideal about how to index the value corresponding to current time, that's why even though i have come out the wave form, i do not know how to implant it. And that why i have been trying to find a time source to continuously input the time to get the position.

May be, i still have to go to the old way, use the basic logical functions in the labview to create the result i want.

All the problem that i was facing is because labview is such a huge thing, there are so many functions inside, and i only know very few of it, even i have read some of the books about labview, still get confused all the time.

just now, when i try to create a simple function according to the given simple, i still do not know how to get the icon that has been shown in the example, there are just so many options.

Thank you very much, i think i will still have many stupid questions to ask before i get what i want.

Best regards.