Data Interval/Change Trigger: Difference between revisions

Latest revision as of 14:34, 27 July 2023

Phidget devices often send streams of sensor and state data to attached channels. The rate at which that data is delivered can be controlled by setting the DataInterval property of a channel. A threshold can be set to filter data that does not vary by more than a defined amount by setting the ChangeTrigger property of a channel.

Phidget device channels are initialized with a default DataInterval specific to the device and a ChangeTrigger of zero when they are attached. See the Phidget22 API for more information.

In most cases, it is likely that you'll simply use these properties as follows:

Keep the ChangeTrigger at zero and select the DataInterval based on how often you want your program to receive data. This configuration is useful in data logging applications and when data will be represented in a graph.
Or, keep the DataInterval at a minimal interval, and set the ChangeTrigger to the lowest relevant value for the quality you are measuring. This is useful for applications were you want to ignore values below a certain threshold or are displaying sensor values to be read in real-time.

Keep reading below for more details on these properties.

Data Interval and Data Rate

Data Interval

The DataInterval property, measured in milliseconds, determines the rate at which the device channel will report data back to your program. If a channel is open in more than one program (i.e. with remote connections over the Phidget Network Server), all instances of that channel will share one data interval, and changing it in one program will change it in all others. When this change occurs, a PropertyChange event will be triggered in each of the other programs so they can adjust accordingly if needed.

The minimum, maximum and default DataInterval varies for each device. Refer to the Phidget22 API and check the MinDataInterval and MaxDataInterval properties for your device to see the limits. In most cases, you will probably want to set the DataInterval to the minimum value to get as much data as possible. However, in some applications such as long-run data logging, you may want to increase DataInterval to make the resulting data set more manageable. You may also want to increase the DataInterval if you're using a PhidgetSBC or HUB5000, since they can be overwhelmed if too many channels are running at the minimum DataInterval.

Data Rate

DataRate is the reciprocal of DataInterval: instead of being measured in milliseconds between data samples, it is measured in hertz (the number of samples provided within one second). When you set the DataInterval for your device, the DataRate will also be set to the corresponding value and vice-versa. In the Phidget Control Panel, you can click on the DataInterval in order to switch between the two. In most cases, whether you use one or the other is a matter of personal preference. However, for devices that can provide data in faster than 1ms intervals, you must use Data Rate to get the most data because DataInterval is an integer and can't go below 1 millisecond.

While the Phidgets libraries will allow you to set any value within the specified minimum and maximum for your device, sometimes hardware limitations cause certain data rates to be unavailable. In this case, the nearest possible data rate will be selected instead. For more details on this behaviour, see the Advanced Topics section below.

Change Trigger

The ChangeTrigger property of a channel enables a filter that only passes data when that data has changed by an amount greater than or equal to the ChangeTrigger. For example, if the DataInterval was set to 0.7, new data would only be delivered to the channel when that new data differed from the previously delivered data by at least 0.7.

It can be useful to set a change trigger to reduce amount of data, only receiving data change events when meaningful thresholds are reached. For example, an application that displayed temperature on an LCD may only be interested on 0.5°C changes. By setting the ChangeTrigger to 0.5, you could avoid having a fast-changing number that is difficult to read, without sacrificing response time.

Advanced Topics

Why does a red number appear when I try to set the DataInterval or DataRate?

When you see a red number below the slider when you set the DataInterval or DataRate, it means that the DataInterval or DataRate has changed, and the red number is the actual speed that the channel is now set to. The number in black text in the box above it is what you previously had it set to.

There are two reasons why this can happen:

If you have the channel open in another program over the network server (or another instance of the Control Panel), changing these properties there will cause the Control Panel Example to update to the new value.
If you have a device that enforces restrictions on DataInterval based on how many channels are open, an you just opened another channel on the device or changed the DataInterval or DataRate on another channel, then this channel may have been adjusted to stay within the new limit.

The change is possible because a PropertyChanged event is thrown in both of these cases. If you want to use this event in your program, you can read more about it by going to the Phidget22 API and selecting the "PhidgetAPI" section for your device.

Why am I getting data at a different DataInterval or DataRate than what was set?

There are a couple of factors that affect the rate at which data can be sent, such as ADC limitations and communication speed limits. Because of this, you may try to set the DataRate to 300Hz, but only get data at 250Hz. Usually the device will set the next-slowest possible rate than what was requested. If you want to know for certain what rate is actually being used, you can check the DataRate property after setting it.

You can also use the Phidget Control Panel to test this behaviour on your devices. When you click on the graph button beside your data readout, you can see the real data rate on the bottom of the graph:

The first number is the result of getting the DataRate value from the API, and the second number is the result of a manual time calculation of the data coming in (if the numbers don't match, it could indicate latency issues or a bug in the device firmware).

What happens when both Data Interval and Change Trigger are set?

In some complex situations, you may want to set both the ChangeTrigger and the DataInterval to non-trivial values. This will result in expected behavior: The channel will check at the DataInterval whether or not the new data differs from the last delivered data by an amount greater than or equal to the ChangeTrigger. If the data passes the change trigger filter, that data will be delivered, if the data does not, the channel will wait until the next data interval and check again.

The only complication is when you consider a situation where the value has spiked by a large amount and then returned to the baseline, in-between intervals. For example, suppose you have a voltage sensor and you've set the DataInterval to 5 milliseconds an the VoltageChangeTrigger to 3mV. The way this is handled depends on whether you're using a USB Phidget or a VINT Phidget:

USB Phidgets

In this example, the blue data points represent the real value of the voltage at any given time. The thick black bars represent the sampling interval, which is 5 milliseconds. When a USB Phidget samples at a slower rate than its default, it will average the last interval worth of samples together and report that value on each interval. This average is represented by the orange data point that lands on each interval line. It is this average that is compared against the change trigger to decide whether or not a data event fires or not. Let's step through each interval:

Interval 1 (0-5ms): The voltage stays mostly stable throughout this interval. The average at the end of the interval is 20mV, which is no different than our first sample. Since our change trigger is 3mV, an event does not fire at the 5ms mark.

Interval 2 (5-10ms): In this interval, the voltage spikes to 30mV and then drops back down. The average for this interval is 23mV, which is 3mV higher than our last event at 0ms. Thus, the change is sufficient to cause a data even to fire at 10ms.

Interval 3 (10-15ms): In this interval, the voltage drops down to 10mV and then rises close to 30mV. The average for this interval is 21mV, which is only 2mV off from our last event. Since this does not meet our change trigger, an event does not fire.

Interval 4 (15-20ms): In this interval, the voltage remains relatively stable at around 28mV. However, the average for this interval is 29mV, which is much higher than our last data event (23mV). This change of 6mV is greater than our change trigger, so an event fires.

VINT Phidgets

Let's see how a VINT devices behaves in the same example. Now, instead of averaging all of the values in the past interval, the VINT device goes to sleep to save power. That means it only cares about the data values on each interval line, and ignores all others.

Interval 1 (0-5ms): The device wakes up and sees a value of 20mV, which is no different than the starting value of 20mV. No event is fired.

Interval 2 (5-10ms): The device wakes up and sees 22mV, which does not differ enough from the last event of 20mV, as our change trigger is 3mV. No event is fired.

Interval 3 (10-15ms): The device wakes up and sees 29mV, which is enough of a change to fire a data event.

Interval 4 (15-20ms): The device wakes up and sees 30mV, which is only 1mV off from the last event, which is not sufficient to trigger a new event.

@@ Line 1: / Line 1: @@
-There are two properties that give you control over how you get data from your Phidget: '''DataInterval''' and '''ChangeTrigger'''. By default, a Phidget will report data on a regular interval, on the smallest interval it supports. By setting new values for these properties, you can change how often or under what conditions a data event will fire.
+[[Category:Programming]] __NOTOC__
+[[Image:DataIntervalRate.jpg|link=|right]]
+Phidget devices often send streams of sensor and state data to attached channels.  The rate at which that data is delivered can be controlled by setting the <code>DataInterval</code> property of a channel.  A threshold can be set to filter data that does not vary by more than a defined amount by setting the <code>ChangeTrigger</code> property of a channel.
-== Data Interval ==
+Phidget device channels are initialized with a default <code>DataInterval</code> specific to the device and a <code>ChangeTrigger</code> of zero when they are attached.  See the {{Phidget22API}} for more information.
-To find the minimum and maximum possible DataInterval for your device, go to the {{Phidget22API}} and select the device, language and object you're using. Clicking on '''getMinDataInterval''' (or '''MinDataInterval''', in C#) will show you the minimum DataInterval for your device. Likewise for '''getMaxDataInterval'''.
+In most cases, it is likely that you'll simply use these properties as follows:
-You can also use these properties in your code when you want to set DataInterval to the minimum or maximum, but you don't know the numerical values. For example,
+* Keep the <code>ChangeTrigger</code> at zero and select the <code>DataInterval</code> based on how often you want your program to receive data. This configuration is useful in data logging applications and when data will be represented in a graph.
+* Or, keep the <code>DataInterval</code> at a minimal interval, and set the <code>ChangeTrigger</code> to the lowest relevant value for the quality you are measuring. This is useful for applications were you want to ignore values below a certain threshold or are displaying sensor values to be read in real-time.
-<code>myPhidget.setDataInterval(myPhidget.getMaxDataInterval());</code>
+Keep reading below for more details on these properties.
-would set the DataInterval to the maximum, regardless of what kind of Phidget <code>myPhidget</code> is (as long as it's a Phidget that supports setting DataInterval).
+==Data Interval and Data Rate==
-Again, by default, a Phidget's DataInterval is the smallest possible time interval (usually one millisecond). This means that data events would fire every millisecond. You can set DataInterval using '''setDataInterval''' (or using the '''DataInterval''' property in C#). Some USB Phidgets' DataInterval can only be set to multiples of 4ms or 8ms, so setDataInterval may round to the nearest permissible value in this case.
+=== Data Interval ===
+The <code>DataInterval</code> property, measured in milliseconds, determines the rate at which the device channel will report data back to your program. If a channel is open in more than one program (i.e. with remote connections over the [[Phidget Network Server]]), all instances of that channel will share one data interval, and changing it in one program will change it in all others. When this change occurs, a <code>PropertyChange</code> event will be triggered in each of the other programs so they can adjust accordingly if needed.
-If you want less frequent events, you can choose a larger value for DataInterval. Setting a larger data interval has differing effects for USB Phidgets and VINT Phidgets:
+The minimum, maximum and default DataInterval varies for each device.  Refer to the {{Phidget22API}} and check the <code>MinDataInterval</code> and <code>MaxDataInterval</code> properties for your device to see the limits. In most cases, you will probably want to set the DataInterval to the minimum value to get as much data as possible. However, in some applications such as long-run data logging, you may want to increase DataInterval to make the resulting data set more manageable. You may also want to increase the DataInterval if you're using a PhidgetSBC or HUB5000, since they can be overwhelmed if too many channels are running at the minimum DataInterval.
-'''USB Phidgets''' - Setting a larger DataInterval will reduce white noise on your measurements, because internally the Phidget will continue to sample at its minimum DataInterval and will average these samples together. For example, setting a 10ms DataInterval on a Phidget that normally samples at 1ms will cause 10 samples (gathered at 1ms intervals) to be averaged and reported in an event every 10ms. (Note: The only USB Phidgets that support setting DataInterval are InterfaceKits and Spatials)
+=== Data Rate===
+DataRate is the reciprocal of DataInterval: instead of being measured in milliseconds between data samples, it is measured in hertz (the number of samples provided within one second). When you set the DataInterval for your device, the DataRate will also be set to the corresponding value and vice-versa. In the Phidget Control Panel, you can click on the DataInterval in order to switch between the two. In most cases, whether you use one or the other is a matter of personal preference. However, for devices that can provide data in faster than 1ms intervals, you must use Data Rate to get the most data because DataInterval is an integer and can't go below 1 millisecond.
-'''VINT Phidgets''' - Setting a larger DataInterval will reduce current consumption, since VINT devices will actually "sleep" in between intervals. Unlike USB Phidgets, VINT devices will not take samples while they sleep, so when they wake up to fire an event, it will be an instantaneous value for that moment in time, not an average of all values during that interval.
+While the Phidgets libraries will allow you to set any value within the specified minimum and maximum for your device, sometimes hardware limitations cause certain data rates to be unavailable. In this case, the nearest possible data rate will be selected instead. For more details on this behaviour, see the Advanced Topics section below.
 == Change Trigger ==
-As with DataInterval, you can find your minimum and maximum ChangeTrigger values in the Phidget {{Phidget22API}}. The methods are '''getMin***ChangeTrigger''' and '''getMax***ChangeTrigger''' where '''***''' is replaced with the quality being reported by the Phidget. For example, a voltage input would use '''getMinVoltageChangeTrigger'''.
+The <code>ChangeTrigger</code> property of a channel enables a filter that only passes data when that data has changed by an amount greater than or equal to the <code>ChangeTrigger</code>.  For example, if the <code>DataInterval</code> was set to 0.7, new data would only be delivered to the channel when that new data differed from the previously delivered data by at least 0.7.
-Again, you can use these methods to easily set the ChangeTrigger to the minimum or maximum, like so:
-<code>myPhidget.setVoltageChangeTrigger(myPhidget.getMaxVoltageChangeTrigger());</code>
+It can be useful to set a ''change trigger'' to reduce amount of data, only receiving data change events when meaningful thresholds are reached.  For example, an application that displayed temperature on an LCD may only be interested on 0.5°C changes.  By setting the <code>ChangeTrigger</code> to 0.5, you could avoid having a fast-changing number that is difficult to read, without sacrificing response time.
-By default, the change trigger of a device will be zero. This means that any new value, even if it is the same as the previous value, will be reported in a data event. As a result, you will get regular data events on the interval you specified with the device's DataInterval.
+== Advanced Topics ==
+{{UGC-Entry|Why does a red number appear when I try to set the DataInterval or DataRate?||
+[[File:redDataInterval.jpg|link=]]
-When you set the ChangeTrigger of a Phidget using '''set***ChangeTrigger''', where '''***''' is the quality being reported, you will create a threshold that is checked on every interval. If the value has changed by an amount greater than or equal to the ChangeTrigger you specified, a data event will fire. For example, if you had a temperature sensor and you set the '''TemperatureChangeTrigger''' to 0.7, you would only get data events whenever the temperature changed by 0.7°C or more, compared to the previous event that fired. This is particularly useful when you have a user readout for a value. If you had the '''TemperatureChangeTrigger''' at zero, the tenths and hundredths digits of the temperature readout would be fluctuating so quickly on the screen that the user may have a hard time reading it. By increasing the ChangeTrigger to something more relevant like 0.5°C, the readout would change infrequently enough that it would be much more readable.
+When you see a red number below the slider when you set the DataInterval or DataRate, it means that the DataInterval or DataRate has changed, and the red number is the actual speed that the channel is now set to. The number in black text in the box above it is what you previously had it set to.
-== Setting Both Properties ==
+There are two reasons why this can happen:
+* If you have the channel open in another program over the network server (or another instance of the Control Panel), changing these properties there will cause the Control Panel Example to update to the new value.
+* If you have a device that enforces restrictions on DataInterval based on how many channels are open, an you just opened another channel on the device or changed the DataInterval or DataRate on another channel, then this channel may have been adjusted to stay within the new limit.
-In many cases, it is likely that you'll simply use these properties as follows:
+The change is possible because a <code>PropertyChanged</code> event is thrown in both of these cases. If you want to use this event in your program, you can read more about it by going to the {{Phidget22API}} and selecting the "PhidgetAPI" section for your device.
+}}
+{{UGC-Entry|Why am I getting data at a different DataInterval or DataRate than what was set?||
+There are a couple of factors that affect the rate at which data can be sent, such as ADC limitations and communication speed limits. Because of this, you may try to set the DataRate to 300Hz, but only get data at 250Hz. Usually the device will set the next-slowest possible rate than what was requested. If you want to know for certain what rate is actually being used, you can check the <code>DataRate</code> property after setting it.
-* Keep ChangeTrigger at zero and select a DataInterval based on how often you want your program to get data. This is useful for applications when you're logging data that is being processed in real-time or will eventually be represented in a graph.
+You can also use the Phidget Control Panel to test this behaviour on your devices. When you click on the graph button beside your data readout, you can see the real data rate on the bottom of the graph:
-* Keep DataInterval at the minimum possible interval, and set ChangeTrigger to the lowest relevant value for the quality you're measuring. This is useful for applications when you want to ignore values below a certain threshold or you're simply displaying sensor values for the user to read.
-However, in some complex situations, you may want to set both ChangeTrigger and DataInterval to non-trivial values. This behaves as you might expect: The device will check on every specified interval whether or not the new value differs from the last event by an amount greater than or equal to the ChangeTrigger. If it is, it will fire a data event. If not, it will wait until the next interval where it'll check again.
+[[File:realDataRate.jpg|link=]]
+The first number is the result of getting the <code>DataRate</code> value from the API, and the second number is the result of a manual time calculation of the data coming in (if the numbers don't match, it could indicate latency issues or a bug in the device firmware).
+}}
+{{UGC-Entry|What happens when both Data Interval and Change Trigger are set?||
+In some complex situations, you may want to set both the <code>ChangeTrigger</code> and the <code>DataInterval</code> to non-trivial values. This will result in expected behavior: The channel will check at the <code>DataInterval</code> whether or not the new data differs from the last delivered data by an amount greater than or equal to the <code>ChangeTrigger</code>. If the data passes the ''change trigger'' filter, that data will be delivered, if the data does not,  the channel will wait until the next ''data interval'' and check again.
 The only complication is when you consider a situation where the value has spiked by a large amount and then returned to the baseline, in-between intervals. For example, suppose you have a voltage sensor and you've set the DataInterval to 5 milliseconds an the VoltageChangeTrigger to 3mV. The way this is handled depends on whether you're using a USB Phidget or a VINT Phidget:
-=== USB Phidgets ===
+{{hiddenh3|USB Phidgets}}
@@ Line 58: / Line 72: @@
 '''Interval 4 (15-20ms):''' In this interval, the voltage remains relatively stable at around 28mV. However, the average for this interval is 29mV, which is much higher than our last data event (23mV). This change of 6mV is greater than our change trigger, so an event fires.
-=== VINT Phidgets ===
+{{hiddenh3|VINT Phidgets}}
 [[Image:intervalchangeVINT.jpg|link=]]
@@ Line 73: / Line 86: @@
 '''Interval 4 (15-20ms):''' The device wakes up and sees 30mV, which is only 1mV off from the last event, which is not sufficient to trigger a new event.
+}}
 == Further Reading ==
@@ Line 80: / Line 94: @@
 [[Polling vs. Events]] - Your program can gather data in either a polling-driven or event-driven manner. Learn the difference to determine which is best for your application.
-[[Using Multiple Phidgets]] - It can be difficult to figure out how to use more than one Phidget in your program. This page will guide you through the steps.
+[[Using Multiple Phidgets]] - Learn how to use more than one Phidget in your program. This page will guide you through the steps.
 [[Logging, Exceptions, and Errors]] - Learn about all the tools you can use to debug your program.
-[[Phidget Network Service]] - Phidgets can be controlled and communicated with over your network- either wirelessly or over ethernet.
+[[Phidget Network Server]] - Phidgets can be controlled and communicated with over your network- either wirelessly or over ethernet.
+[[Best Phidgets Practices]] - Good programming habits that will save you from common problems when writing code for your Phidgets.

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