Stopwatches not in Sync?????

I have a program that logs (supposedly) 90 data points per second. I have a stopwatch that starts when the program opens and as each data point is logged the elapsed time of the stopwatch is also logged to keep track of the timing.

The data points are force values from a load cell with a rocket motor on top of it.

My program is telling me that my motor produced thrust for 1.923 seconds, but the video of the firing is telling me that it only produced thrust for 1.638seconds. The program had run for 393 seconds before the motor ignited, is it possible that after 393 seconds the stopwatch could be out of sync?


Ryan

Quote:

---

Originally Posted by rpmaurer

is it possible that after 393 seconds the stopwatch could be out of sync?

---

No it isn't.

Quote:

---

Originally Posted by MSDN

The timer used by the Stopwatch class depends on the system hardware and operating system. IsHighResolution is true if the Stopwatch timer is based on a high-resolution performance counter. Otherwise, IsHighResolution is false, which indicates that the Stopwatch timer is based on the system timer.

---

The Stopwatch class is as accurate as your computer is. You should consider the possibility of there being a delay in stopping the Stopwatch after the action being timed had stopped. Almost 0.3 of a second does seem like quite a lot though.

:( Well that's not good! Where would this delay be?

Or perhaps suggestions on getting a more accurate timekeeper?

Quote:

---

Originally Posted by rpmaurer

:( Well that's not good! Where would this delay be?

---

You tell us. Unlike you, we haven't seen your code.

Quote:

---

Originally Posted by rpmaurer

Or perhaps suggestions on getting a more accurate timekeeper?

---

Given that the Stopwatch is based on the most accurate timer available on the current system, there is no more accurate timekeeper.

How are you getting timing from the video? What are the specs on the computer running the program? What OS and version of .Net are you running?

I agree with what jmc has said.

Sorry, I really do have a bad habit of not posting needed information.

Using Corel VS X5 Pro to get the duration from ignition to burnout of the motor. Windows 8, Vb.net (2012 Express), 6GB RAM, i5 dual core.

Here is the code that is run when the serial port recieves data:

Code:

---

Private Sub GetData()
        Try
            If InvokeRequired Then
                Me.Invoke(New MethodInvoker(AddressOf GetData))
            Else

                'get voltage data from loadcell, main batt, and ampbatt
                inputstring = SerialPort1.ReadLine
                loadcell = Val(inputstring.Substring(0, 4)) / 1024 * 10
                inForce = (loadcell - zerovalue) * forcescale

                'add data point
                Datachart.Series("DataSeries").Points.AddXY(Math.Round(Timer.Elapsed.TotalSeconds, 3), inForce)

                'update chart
                If Timer.Elapsed.TotalSeconds > 10 Then
                    Datachart.ChartAreas(0).AxisX.Maximum = Timer.Elapsed.TotalSeconds
                    Datachart.ChartAreas(0).AxisX.Minimum = Datachart.ChartAreas(0).AxisX.Maximum - 10
                End If

                'respond
                SerialPort1.Write("1")
                Label_Time.Text = "Current Time (s): " + Timer.Elapsed.ToString

                'adjust chart y bounds
                If inForce > Datachart.ChartAreas(0).AxisY.Maximum Then
                    If firstdata = 1 Then
                        firstdata = 0
                        Exit Sub
                    End If
                    AutoAdjust = 1
                    TextBox_ChartMax.Text = inForce
                    Datachart.ChartAreas(0).AxisY.Maximum = inForce
                    AutoAdjust = 0
                End If

                'log data
                If logging = 1 Then
                    Count = Count + 1
                    Datachart.Series("LoggedData").Points.AddXY(Math.Round(Timer.Elapsed.TotalSeconds, 3), inForce)
                    DataGrid.Rows(Count).Cells(0).Value = Count
                    DataGrid.Rows(Count).Cells(1).Value = Math.Round(Timer.Elapsed.TotalSeconds, 4)
                    DataGrid.Rows(Count).Cells(2).Value = Math.Round(loadcell, 4)
                    DataGrid.Rows(Count).Cells(3).Value = Math.Round(inForce, 4)
                    If DataGrid.RowCount - Count < 10 Then
                        DataGrid.RowCount = DataGrid.RowCount + 500
                    End If
                End If
            End If

            'display batter voltage
            mainbatt = Val(inputstring.Substring(4, 4)) / 1024 * 10
            ampbatt = Val(inputstring.Substring(8, 4)) / 1024 * 10
            Label_AmpBatt.Text = Math.Round(ampbatt, 3).ToString + "V"
            Label_MainBatt.Text = Math.Round(mainbatt, 3).ToString + "V"

            'update data count
            datacount = datacount + 1

        Catch ex As Exception

        End Try

    End Sub

---

I guess timer is the Stopwatch??? I don't understand how you get a time stamp from Corel or see the logging of the Stopwatch. How does the SerialPort play into all of this? I see you have a blocking call to the SerialPort.

Ok, the program says that the motor burned (produced thrust) for roughly 1.95s. But in the video I only see a flame (burning rocket motor) for 1.63s. The thrust data is coming from a load cell through a serial port.


HEre is where it takes the load cell data and converts it to a force and then displays this on a chart at point (timer.elapsedtime, force)

Code:

---

'get voltage data from loadcell, main batt, and ampbatt
                inputstring = SerialPort1.ReadLine
                loadcell = Val(inputstring.Substring(0, 4)) / 1024 * 10
                inForce = (loadcell - zerovalue) * forcescale

                'add data point
                Datachart.Series("DataSeries").Points.AddXY(Math.Round(Timer.Elapsed.TotalSeconds, 3), inForce)

---

YEs, Timer is stopwatch, sorry for the confusion :o

There are only two lines there that could conceivably take any measurable time:

1) The Datachart line (the last line). This one involves a certain amount of drawing, so it could potentially be a bit slow, but the discrepancy you are seeing is greater than 0.1 seconds, so you'd see a flicker if this was the case....and it almost certainly isn't the case. A flicker might exist even if this thing wasn't an issue. A second Stopwatch starting before, and stopping after, that line would show how long it took. I doubt it will register more than 0 ms, though, so I don't think this line is the issue.

2) Serialport.Readline. This is roughly where I would expect the delay. ReadLine is a blocking call, which means that execution will stop at that line until something is available. If there is any delay in the operation of the serialport, this is where your delay could be coming from.

Any suggestions on getting around the ReadLine?

First off, I'd use a stopwatch to confirm that there is a delay there. Start the stopwatch before, stop it after, and check the elapsed milliseconds. After all, there's no point in hunting the wrong game.

That is one thing I want to check... will let you guys know if there is still a delay.

Does the video have a time stamp or are you timing it some other way? How is GetData being called?

Yes, video has a time stamp. GetData is called when the SerialPort recieves data.

Quote:

---

Originally Posted by rpmaurer

Yes, video has a time stamp. GetData is called when the SerialPort recieves data.

---

If you are using the DataReceived event handler it might fire several times before you have a 'line.' Does the data coming in have a specific format?

Yes, it sends as a 12 character string, where the first 4 characters are the voltage of the load cell, the second 4 are the voltage of one of the batteries, and the last 4 are the voltage of the final battery. Looks like this: 061206120612, splits into 0612 0612 0612, which translates into 2.5V, 2.5V, and 2.5V. The numbers are from 0 to 1024, based on a voltage range from 0V to 5V

Assuming that the handler isn't doing anything else, try it like this

Code:

---


    Private Sub SerialPort1_DataReceived(sender As Object, _
                                        e As IO.Ports.SerialDataReceivedEventArgs) _
                                    Handles SerialPort1.DataReceived
        If SerialPort1.BytesToRead > 12 Then
            'call to GetData here
        End If
    End Sub

---