169 lines
4.8 KiB
ReStructuredText
169 lines
4.8 KiB
ReStructuredText
.. _pt1000_processing:
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PT1000 Temperature Value Processing
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===================================
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The PT1000 temperature sensor is the sensing element used for determining the Reflow Oven Temperature.
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The PT1000 value processing is enabled by default and not intended to be turned off.
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PT1000 Value Sampling
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---------------------
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The following block diagram shows the processing chain of the temperature signal.
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.. drawio-image:: pt1000.drawio
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ADC
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~~~
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The internal ADC of the STM32F407 controller is used to sample the analog signal from the :ref:`hw_analog_fe`. The ADC is triggered by the hardware Timer *TIM2* each millisecond, which results in a sampling frequency of
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1 kHz. The ADC module provides an analog value `watchdog <Watchdog_>`_, which is used to detect wirebreaks and other hardware errors that result in a wrong resistance measurement.
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The sample frequency is controlled by
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.. doxygendefine:: ADC_PT1000_SAMPLE_CNT_DELAY
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whereas the ADC Peripheral module is defined by
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.. doxygendefine:: ADC_PT1000_PERIPH
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Prefilter
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~~~~~~~~~
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The analog value prefilter is used to filter outliers. It is triggered after a certain amount ``n`` of values have been sampled by the `ADC`_.
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The filter then removes the two most extreme values and computes the average of the remaining ``n - 2`` values. By default ``n`` is configured to:
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.. doxygendefine:: ADC_PT1000_DMA_AVG_SAMPLES
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Therefore, by default, the resulting datastream has a sampling rate of 1/6 kHz. This depends on the :c:macro:`ADC_PT1000_SAMPLE_CNT_DELAY` and ``n``
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.. _firmware_meas_adc_watchdog:
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Watchdog
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~~~~~~~~
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The analog watchdog supervises the measured value of the `ADC`_. It is configured by the following defines:
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.. doxygendefine:: ADC_PT1000_LOWER_WATCHDOG
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.. doxygendefine:: ADC_PT1000_UPPER_WATCHDOG
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.. doxygendefine:: ADC_PT1000_WATCHDOG_SAMPLE_COUNT
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The watchdog will set the :ref:`safety_flags_adc_watchdog` error flag.
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ADC Value to Ohm
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~~~~~~~~~~~~~~~~
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This block converts the analog value to an Ohm resistance value.
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The formula is:
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.. math::
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R(V) = \frac{V}{4096} \cdot 2500~\Omega
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The equation is implemented in
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.. doxygendefine:: ADC_TO_RES
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and applied during the `Exponential Moving Average Filter`_.
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.. _firmware_meas_adc_filter:
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Exponential Moving Average Filter
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The external moving average filter filters the measured resistance value. It's equation is:
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.. math::
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y[n] = (1-\alpha) y[n-1] + \alpha x[n]
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The filter constant *alpha* defaults to the define
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.. doxygendefine:: ADC_PT1000_FILTER_WEIGHT
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and can be changed in code using
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.. doxygenfunction:: adc_pt1000_set_moving_average_filter_param
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The moving average filter is considered unstable, if the input to output difference is greater than
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.. doxygendefine:: ADC_PT1000_FILTER_UNSTABLE_DIFF
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In this case, the :ref:`safety_flags_adc_unstable` flag will be set until the filter output converges within the range for at least
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.. doxygendefine:: ADC_PT1000_FILTER_STABLE_SAMPLE_COUNT
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samples.
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If the input value keeps changing or oscillating, the error flag will be permanently set.
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The moving average filter's output signal is the Low Frequency (LF) PT1000 resistance signal used for internal PT1000 measurements.
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Reading and Converting the PT1000 Value
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---------------------------------------
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Calibration
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~~~~~~~~~~~
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The functions
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.. doxygenfunction:: adc_pt1000_set_resistance_calibration
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:outline:
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and
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.. doxygenfunction:: adc_pt1000_get_resistance_calibration
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:outline:
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are used to set the resistance calibration internally. For a guide on how to calibrate the deivce, see the corresponding :ref:`usage_calibration` usage page.
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The calibration is calculated the following way:
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.. drawio-image:: calibration.drawio
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The final calibrated PT1000 resistance is calculated as:
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.. math::
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R_{PT1000_{corr}} = (R_{PT1000_{LF}} - O) \cdot (1 + \sigma)
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The default values, if no calibration is loaded / executed, are:
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======== =====================
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Offset Sensitivity Deviation
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======== =====================
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0 0
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======== =====================
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Get Calibration Corrected Value
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The PT1000 value is available through the following function.
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If a calibration is set, it is applied.
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.. doxygenfunction:: adc_pt1000_get_current_resistance
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Converting the Value
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~~~~~~~~~~~~~~~~~~~~
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The valid range for conversion is between
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.. doxygendefine:: TEMP_CONVERSION_MIN_RES
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:outline:
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and
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.. doxygendefine:: TEMP_CONVERSION_MAX_RES
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:outline:
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By default, the valid range is:
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.. math::
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1000~\Omega \le R_{PT1000} \le 2200~\Omega
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.. doxygenfunction:: temp_converter_convert_resistance_to_temp
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The cvonversion function is based on a lookup table with linear interpolation between the data points.
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The lookuptable is stored as a header file and can, if necessary, be recreated using the ``create-temp-lookup-table.py`` script.
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