Let's say I have a Boolean input which can switch its state automatically. Swichting from 0 to 1 and from 1 to 0 sets a cycle. To determine if that cycle is valid, the cycle time must be at least 10 seconds.
I can determine if one semicicle is invalid, but I can't figure out how could to be able to determine at the end of the cycle if it has been invalid.
This is what I have:
You seem to be saying if the status of the input signal is not stable for 10 seconds, you have an invalid cycle and you want to check for that.
To do that, you obviously need a timer triggered by each transition. (I'm not specifically familiar with your PLC but they're all pretty similar so I'll assume that's what you have, and you appropriately have two, triggered on different transitions.
The invalid transition condition for positive-transition is:
positive_input_transition_timer_q and not input
That is, the timer must be running and the input must go to the wrong state.
Now, the input might spike low then go high again immediately; you probably don't want lose the fact that you had an invalid transition. So what you really want to do is latch the detection of invalid transition.
I'm not good at pretty ladder logic graphics so I'll write in old style:
--+----[ ]----[TON]----+------[/]-----[/]------( )------
| Input Positive | Unlatch Input Invalid
| Transition | Invalid Positive
| | Positive Cycle
+--------[]----------+ Cycle
Invalid
Positive
Cycle
A corresponding ladder rung is needed for negative transition.
Presumably, some other bit of code will notice the InvalidPositive, respond appropriately, and then unlatch the condition.
Related
I am trying to understand how the auto rescheduling logic is supposed to work.
The recommended values in nagios.cfg are:
auto_reschedule_checks=1
auto_rescheduling_interval=30
auto_rescheduling_window=45
Supposedly this is after they realized the default values (same as above but with window=180) had the potential to have checks that never actually get executed (See https://support.nagios.com/kb/print-19.html).
But.. How are these new values any different?
How I understand it, every 30sec, it takes the next 45sec of checks, finds checks that overlap, and reschedules those evenly across the 45sec.
Isnt it possible for a check to indefinitely get rescheduled into that last 15 seconds of the 45sec window?
Am I missing something? Do rescheduled checks get an elevated priority so they arent rescheduled again? Reading through the code, it doesnt appear they are given any special treatment.
Seems to me that the only way to prevent that is for the interval and window to be the same.
Which means maybe a couple checks occurring right at each 30sec transition may go unnoticed,
but Id rather that than what I am presuming the behavior is with the "recommended" interval=30sec , window=45sec.
Any insights would be helpful!
I came across this code by Ganssle regarding switch debouncing. The code seems pretty efficient, and the few questions I have maybe very obvious, but I would appreciate clarification.
Why does he check 10 msec for button press and 100 msec for button release. Can't he just check 10 msec for press and release?
Is polling this function every 5 msec from main the most effecient way to execute it or should I check for an interrupt in the pin and when there is a interrupt change the pin to GPI and go into the polling routine and after we deduce the value switch the pin back to interrupt mode?
#define CHECK_MSEC 5 // Read hardware every 5 msec
#define PRESS_MSEC 10 // Stable time before registering pressed
#define RELEASE_MSEC 100 // Stable time before registering released
// This function reads the key state from the hardware.
extern bool_t RawKeyPressed();
// This holds the debounced state of the key.
bool_t DebouncedKeyPress = false;
// Service routine called every CHECK_MSEC to
// debounce both edges
void DebounceSwitch1(bool_t *Key_changed, bool_t *Key_pressed)
{
static uint8_t Count = RELEASE_MSEC / CHECK_MSEC;
bool_t RawState;
*Key_changed = false;
*Key_pressed = DebouncedKeyPress;
RawState = RawKeyPressed();
if (RawState == DebouncedKeyPress) {
// Set the timer which will allow a change from the current state.
if (DebouncedKeyPress) Count = RELEASE_MSEC / CHECK_MSEC;
else Count = PRESS_MSEC / CHECK_MSEC;
} else {
// Key has changed - wait for new state to become stable.
if (--Count == 0) {
// Timer expired - accept the change.
DebouncedKeyPress = RawState;
*Key_changed=true;
*Key_pressed=DebouncedKeyPress;
// And reset the timer.
if (DebouncedKeyPress) Count = RELEASE_MSEC / CHECK_MSEC;
else Count = PRESS_MSEC / CHECK_MSEC;
}
}
}
Why does he check 10 msec for button press and 100 msec for button release.
As the blog post says, "Respond instantly to user input." and "A 100ms delay is quite noticeable".
So, the main reason seems to be to emphasize that the make-debounce should be kept short so that the make is registered "immediately" by human sense, and that the break debounce is less time sensitive.
This is also supported by a paragraph near the end of the post: "As I described in the April issue, most switches seem to exhibit bounce rates under 10ms. Coupled with my observation that a 50ms response seems instantaneous, it's reasonable to pick a debounce period in the 20 to 50ms range."
In other words, the code in the example is much more important than the example values, and that the proper values to be used depends on the switches used; you're supposed to decide those yourself, based on the particulars of your specific use case.
Can't he just check 10 msec for press and release?
Sure, why not? As he wrote, it should work, even though he wrote (as quoted above) that he prefers a bit longer debounce periods (20 to 50 ms).
Is polling this function every 5 msec from main the most effecient way to execute it
No. As the author wrote, "All of these algorithms assume a timer or other periodic call that invokes the debouncer." In other words, this is just one way to implement software debouncing, and the shown examples are based on a regular timer interrupt, that's all.
Also, there is nothing magical about the 5 ms; as the author says, "For quick response and relatively low computational overhead I prefer a tick rate of a handful of milliseconds. One to five milliseconds is ideal."
or should I check for an interrupt in the pin and when there is a interrupt change the pin to GPI and go into the polling routine and after we deduce the value switch the pin back to interrupt mode?
If you implement that in code, you'll find that it is rather nasty to have an interrupt that blocks the normal running of the code for 10 - 50ms at a time. It is okay if checking the input pin state is the only thing being done, but if the hardware does anything else, like update a display, or flicker some blinkenlights, your debouncing routine in the interrupt handler will cause noticeable jitter/stutter. In other words, what you suggest, is not a practical implementation.
The way the periodic timer interrupt based software debouncing routines (shown in the original blog post, and elsewhere) work, they take only a very short amount of time, just a couple of dozen cycles or so, and do not interrupt other code for any significant amount of time. This is simple, and practical.
You can combine a periodic timer interrupt and an input pin (state change) interrupt, but since the overhead of many of the timer-interrupt-only -based software debounces is tiny, it typically is not worth the effort trying to combine the two -- the code gets very, very complicated, and complicated code (especially on an embedded device) tends to be hard/expensive to maintain.
The only case I can think of (but I'm only a hobbyist, not an EE by any means!) is if you wanted to minimize power use for e.g. battery powered operation, and used the input pin interrupt to bring the device to partial or full power mode from sleep, or similar.
(Actually, if you also have a millisecond or sub-millisecond counter (not necessarily based on an interrupt, but possibly a cycle counter or similar), you can use the input pin interrupt and the cycle counter to update the input state on the first change, then desensitize it for a specific duration afterwards, by storing the cycle counter value at the state change. You do need to handle counter overflow, though, to avoid the situation where a long ago event seems to have happened just a short time ago, due to counter overflowing.)
I found Lundin's answer quite informative, and decided to edit my answer to show my own suggestion for software debouncing. This might be especially interesting if you have very limited RAM, but lots of buttons multiplexed, and you want to be able to respond to key presses and releases with minimum delay.
Do note that I do not wish to imply this is "best" in any sense of the world; I only want you to show one approach I haven't seen often used, but which might have some useful properties in some use cases. Here, too, the number of scan cycles (milliseconds) the input changes are ignored (10 for make/off-to-ON, 10 for break/on-to-OFF) are just example values; use an oscilloscope or trial-and-error to find the best values in your use case. If this is an approach you find more suitable to your use case than the other myriad alternatives, that is.
The idea is simple: use a single byte per button to record the state, with the least significant bit describing the state, and the seven other bits being the desensitivity (debounce duration) counter. Whenever a state change occurs, the next change is only considered a number of scan cycles later.
This has the benefit of responding to changes immediately. It also allows different make-debounce and break-debounce durations (during which the pin state is not checked).
The downside is that if your switches/inputs have any glitches (misreadings outside the debounce duration), they show up as clear make/break events.
First, you define the number of scans the inputs are desensitized after a break, and after a make. These range from 0 to 127, inclusive. The exact values you use depend entirely on your use case; these are just placeholders.
#define ON_ATLEAST 10 /* 0 to 127, inclusive */
#define OFF_ATLEAST 10 /* 0 to 127, inclusive */
For each button, you have one byte of state, variable state below; initialized to 0. Let's say (PORT & BIT) is the expression you use to test that particular input pin, evaluating to true (nonzero) for ON, and false (zero) for OFF. During each scan (in your timer interrupt), you do
if (state > 1)
state -= 2;
else
if ( (!(PORT & BIT)) != (!state) ) {
if (state)
state = OFF_ATLEAST*2 + 0;
else
state = ON_ATLEAST*2 + 1;
}
At any point, you can test the button state using (state & 1). It will be 0 for OFF, and 1 for ON. Furthermore, if (state > 1), then this button was recently turned ON (if state & 1) or OFF (if state & 0) and is therefore not sensitive to changes in the input pin state.
In addition to the accepted answer, if you just wish to poll a switch from somewhere every n ms, there is no need for all of the obfuscation and complexity from that article. Simply do this:
static bool prev=false;
...
/*** execute every n ms ***/
bool btn_pressed = (PORT & button_mask) != 0;
bool reliable = btn_pressed==prev;
prev = btn_pressed;
if(!reliable)
{
btn_pressed = false; // btn_pressed is not yet reliable, treat as not pressed
}
// <-- here btn_pressed contains the state of the switch, do something with it
This is the simplest way to de-bounce a switch. For mission-critical applications, you can use the very same code but add a simple median filter for the 3 or 5 last samples.
As noted in the article, the electro-mechanical bounce of switches is most often less than 10ms. You can easily measure the bouncing with an oscilloscope, by connecting the switch between any DC supply and ground (in series with a current-limiting resistor, preferably).
I'm creating a card game in pygame for my college project, and a large aspect of the game is how the game's AI reacts to the current situation. I have a function to randomly generate a number within 2 parameters, and this is how long I want the program to wait.
All of the code on my ai is contained within an if statement, and once called I want the program to wait generated amount of time, and then make it's decision on what to do.
Originally I had:
pygame.time.delay(calcAISpeed(AIspeed))
This would work well, if it didn't pause the rest of the program whilst the AI is waiting, stopping the user from interacting with the program. This means I cannot use while loops to create my timer either.
What is the best way to work around this without going into multi-threading or other complex solutions? My project is due in soon and I don't want to make massive changes. I've tried using pygame.time.Clock functions to compare the current time to the generated one, but resetting the clock once the operation has been performed has proved troublesome.
Thanks for the help and I look forward to your input.
The easiest way around this would be to have a variable within your AI called something like "wait" and set it to a random number (of course it will have to be tweaked to your program speed... I'll explain in the code below.). Then in your update function have a conditional that waits to see if that wait number is zero or below, and if not subtract a certain amount of time from it. Below is a basic set of code to explain this...
class AI(object):
def __init__(self):
#put the stuff you want in your ai in here
self.currentwait = 100
#^^^ All you need is this variable defined somewhere
#If you want a static number as your wait time add this variable
self.wait = 100 #Your number here
def updateAI(self):
#If the wait number is less than zero then do stuff
if self.currentwait <= 0:
#Do your AI stuff here
else:
#Based on your game's tick speed and how long you want
#your AI to wait you can change the amount removed from
#your "current wait" variable
self.currentwait -= 100 #Your number here
To give you an idea of what is going on above, you have a variable called currentwait. This variable describes the time left the program has to wait. If this number is greater than 0, there is still time to wait, so nothing will get executed. However, time will be subtracted from this variable so every tick there is less time to wait. You can control this rate by using the clock tick rate. For example, if you clock rate is set to 60, then you can make the program wait 1 second by setting currentwait to 60 and taking 1 off every tick until the number reaches zero.
Like I said this is very basic so you will probably have to change it to fit your program slightly, but it should do the trick. Hope this helps you and good luck with your project :)
The other option is to create a timer event on the event queue and listen for it in the event loop: How can I detect if the user has double-clicked in pygame?
I'm looking for a way to disable the PID.vi from running in Labview when the event case container is false.
The program controls motor position to maintain constant tension on a cable using target force and actual force as the input parameters. The output is motor position. Note that reinitialize is set to false since it needs previous instances to spool the motor.
Currently, when the event case is true the motor spools as expected and maintains the cable tension. But when the event case state is toggled the PID.vi seems to be running in the background when false causing the motor spool sporatically.
Is there a way to freeze the PID controls so that it continues from where it left off?
The PID VI does not run in the background. It only executes when you call it. That said, PID is a time-based calculation. It calculates the difference from the last time you called the VI and uses that to calculate the new values. If a lot of time passed, it will just try to fix it using that data.
If you want to freeze the value and then resume fixing smoothly, you can use the limits input on the top and set the max and min to your desired output. This will cause the PID VI to always output that value. You will probably need a feedback node or shift register to remember the last value output by the PID.
What Yair said is not entirely true - the integral and derivative terms are indeed time dependent, but the proportional is not. A great reference for understanding PIDs and how they are implemented in LabVIEW can be found here (not sure why it is archived). Also, the PID VIs are coded in G so you can simply open them to see how they operate.
If you take a closer look at the PID VI, you can see what is happening and why you might not get the response you expect. In the VI itself, dt will be either 1) what you set it to, or 2) an accumulation of time based on a tick count stored in the VI (the default). Since you have not specified a dt, the PID algorithm uses the accumulated time between calls. If you have "paused" calculation for some time, this will have an impact on the integral and derivative output.
The derivative output will kick in when there is a change in the process variable (use of the process variable prevents derivative kick). The effect of a large accumulated time between calls will be to reduce the response of this term. The time that you have paused will have a more significant impact on the integral term. Since the response of the integral portion of the controller is the proportional to the integral of the error over dt, the longer you pause the larger the response simply because because the the algorithm is performing a trapezoidal integration over dt.
My first suggestion is don't pause the controller - let the PID do what it is supposed to do. If you are using it properly, then you should not have to stop the controller action. But, if you must pause the controller action, consider re-initializing the controller. This will force the controller to reset the accumulated time term and the response in the first iteration will be purely proportional.
Hope this helps.
I have been making a labview program for kids to moniter energy production from various types of power sources. I have a condition where if they are underproducing a warning will fire, and if they are overproducing by a certian threshold, another warning will fire.
I would like to time how long throughout the activity, each type of warning is fired so each group will have a score at the end. This is just to simulate how the eventual program will behave.
Currently I have a timer which can derrive the amount of time the warning is true, but it will overwrite itself each time the warning goes off and on again.
So basically I need to to sum up the total time that the value has been true, even when it has flitted between true and false.
One method of tabulating the total time spent "True" would be exporting the Warning indicator from the While-loop using an indexed tunnel. If you also export from the loop a millisecond counter value of when the indicator was triggered, you can post process what will be an array of True/False values with the corresponding time at which the value transitioned.
The post processing could be a for-loop that keeps a running total of time spent true.
P.s. if you export your code as a VI snippet, others will be able to directly examine and modify the code without needing to remake it from scratch. See the NI webpage on the subject:
http://www.ni.com/white-paper/9330/en/
I would suggest going another way. Personally, I found the code you used confusing, since you subtract the tick count from the value in the shift register, which may work, but doesn't make any logical sense.
Instead, I would suggest turning this into a subVI which does the following:
Keep the current boolean value, the running total and the last reset time in shift registers.
Initialize these SRs on the first call using the first call primitive and a case structure.
If the value changes from F to T (compare the input to the SR), update the start time.
If it changes from T to F, subtract the start time from the current time and add that to the total.
I didn't actually code this now, so there may be holes there, but I'm leaving that as an exercise. Also, I would suggest making the VI reentrant. That way, you can simply call it a second time to get the same functionality for the second timer.