109 lines
3.5 KiB
ReStructuredText
109 lines
3.5 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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.. blockdiag::
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:desctable:
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blockdiag {
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orientation = portrait;
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ADC[description="`Analog to Digital Converter <ADC_>`_"];
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WATCHDOG [label = "WDT", shape=endpoint, description="`Hardware Value Watchdog <Watchdog_>`_"];
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PREFILTER [label=Prefilter, description="`Prefiltering and Downsampling <Prefilter_>`_"];
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ADC2RES [label= "Val -> Ohm", description="`Conversion from ADC value to resistance in Ohms <ADC Value to Ohm_>`_"]
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MAVG [label="MAVG Filter", description="`Exponential Moving Average Filter`_"];
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RAW_HF [label="HF", shape = endpoint, description="High Frequency raw value reading"];
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PT1000 [label = "LF", shape = endpoint, description="Low Frequency PT1000 resistance value"]
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RAW_STREAM [label = "Stream", shape = endpoint, description="Raw value streaming"];
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ADC -> WATCHDOG;
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ADC -> PREFILTER [label="1 kHz"];
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PREFILTER -> ADC2RES [label="1/6 kHz"];
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ADC2RES -> MAVG;
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MAVG -> PT1000 [label="1/6 kHz"];
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PREFILTER -> RAW_HF [label="1/6 kHz"];
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PREFILTER -> RAW_STREAM [label="1/6 kHz"];
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}
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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.
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The resulting datastream has a sampling rate of 1/6 kHz.
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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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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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After initial startup and after each change of the filter constant, the filter will set the :ref:`safety_flags_adc_unstable` flag for a defined sample count of:
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.. doxygendefine:: ADC_FILTER_STARTUP_CYCLES
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