Documentation LucidControl RTD Input USB Module – Data Acquisition and Operation

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3 Module Operation

The acquisition circuit sources the RTD with a constant current and measures the voltage drop over the RTD which is proportional to the resistance and represents a temperature.
Fig. 3-1 explains the measurement procedure in Standard Acquisition Mode. The diagram shows two active RTD channels in order to keep the diagram simple.
The blue lines are related to measurement channel 1, the red lines to measurement channel 2.
It can be seen that the multiplexer is set to the appropriate input which sources the connected RTD with a constant current. Only one RTD is sourced with current at a given time. All active RTDs are processed subsequently by waiting the time T_Scan. After an input channel was selected the input waits for a configurable time T_Setup until the resistance of the RTD is stable and ready for conversion. T_Setup and T_Scan can be configured for each channel but it must be ensured that the times are chosen in a way that the scheduler is able to process each active channel.
Decreasing the values to not realistic timing may cause invalid values or may skip channels completely. The default values given in the table should only be changed if faster measurements are necessary or if the measured result is not stable.

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3.1 Data Acquisition Modes

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3.1.1 Inactive Mode

In Inactive Mode the RTD measurement is disabled and the channel is skipped.

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3.1.2 Standard Acquisition Mode

In Standard Acquisition Mode the RTD the inputs are measured as configured.

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3.2 Input Calibration

The LucidControl RT4 module measures analog signals (more precisely voltages) which are captured, conditioned and converted to digital values.

In contradiction to logical signals, where obviously only two states LOW or HIGH are possible, all analog voltages within a specified range are converted to their representing digital value.

The signal conditioning which is part of the measurement circuit contains components which are not free from tolerances (e.g. offset voltages of amplifier). These have to be compensated in order to measure a correct value.

The calibration functions of the RT4 module which are described in the following compensate these measurement tolerances.

Note
All modules are calibrated before shipping and it is not necessary to re-calibrate a new module nor is it necessary to calibrate it regularly!

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3.2.1 Full Calibration

The full input calibration consists of two measurements:

• Open Input Calibration where the IO terminals are left unconnected
• Short Input Calibration where the IO terminals are shortcut

On full input calibration both, the open and the short calibration must be executed.
After full input calibration has finished the module is able to measure all values with the specified accuracy.

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3.2.1.1 Short Input Calibration

For short input calibration the inputs must be shortcut as it is shown in Fig. 3.2.1.1-1. After creating a shortcut e.g. by connecting the terminals 1 and 2 of input channel 0 the short input calibration can be executed by using the LucidIoCtrl command line tool. For detailed information see section 3.2.3.

Example
The short input calibration should be done for channel 0 and the result should be stored for further usage.
LucidIoCtrl –dCOM1 –c0 –a --short –p –-quiet [ENTER]

In combination with the I/O Calibration command (-a) the short input calibration is specified (–short). Passing Parameter –quiet causes LucidIoCtrl to skip user confirmation before the command is started. Using Parameter –p makes the calibration setting persistent so that is used after a restart of the module.
After short input calibration for channel 0 has finished the remaining input channels can be calibrated the same way.

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3.2.1.2 Open Input Calibration

For open input calibration the input terminals must be left unconnected. In order to execute the open input calibration the LucidIoCtrl command line tool can be used. For detailed information see section 3.2.3.

Example
The open input calibration should be done for channel 0 and the result should be stored for further usage.
LucidIoCtrl –dCOM1 –c0 –a --open –p –-quiet [ENTER]>

In combination with the I/O Calibration command (-a) the open input calibration is specified (–open). Passing Parameter –quiet causes LucidIoCtrl to skip user confirmation before the command is started. Using Parameter –p makes the calibration setting persistent so that is used after a restart of the module.
After open input calibration for channel 0 has finished the remaining input channels can be calibrated the same way.

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3.2.2 Offset Compensation

In order to compensate the offset of an input channel the Parameter inRtOffset can be used.
This parameter allows configuring an offset resistance in 0.1 Ω steps which is added to the measured resistor. Since inRtOffset can also be a negative value, compensation in both directions to higher or lower values is possible.
For the RT4 module a 0 offset compensation can be made by connecting a load resistor.

Measured Resistance: R= R_M + R_Offset

Whereas R, R_M and R_Offset are values in 0.1 Ω resolution.

Since the temperature is approx. proportional T[℃] ~ R/10 * 0.256 [℃/Ω] the value R_M changes the resulting temperature by approx. 0.0256 [℃/Digit].

Example
When connecting a 1000 Ω resistor to the input channel 0 the resulting temperature should be exactly 0 °C. In this example it is +0.5 °C caused by outer influences e.g. the RTD itself. This offset value is compensated in the following.

With the equations above it can be calculated that the deviation of +0.5 °C results in an offset correction value of -19.53 (rounded -20, which means -2.0 Ω)
The following function call adjusts the measured value to the load of 1000 Ω representing a value close to 0°C and stores the setting:
LucidIoCtrl –dCOM1 –c0 –sinRtOffset=-20 –p [ENTER]

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3.3 Commands

Accessing inputs and outputs is a very common task which is mostly identical for all LucidControl modules. For this task input modules provide the commands GetIo for reading of a single value and GetIoGroup for reading of a group of values of the same type.

For more comprehensive information covering reading and writing of inputs and outputs please see this sections:

• LucidIoCtrl Commandline Tool (Read Command)
• General Lucid Control Documentation (Implemented Commands)

The following sections describe the commands supported by the RT4 module in detail.

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3.3.1 GetIo

This command reads the value of a RTD input.

Please see also the General LucidControl documentation for more information.

LucidIoCtrl Command Line Tool Example

Set T_Setup of input channel 0 to 25 ms and make the setting persistent.
LucidIoCtrl –dCOM4 –c0 –sinRtSetupTime=25 –p [ENTER]
Read T_Scan parameter of input channel 0
LucidIoCtrl –dCOM4 –c0 –ginRtSetupTime [ENTER]
-> inRtSetupTime=25

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3.4.5 inRtOffset

This IO Configuration Parameter configures the Input Offset Compensation value which is described in section 3.1.2.

LucidIoCtrl Command Line Tool Example

Set Input Offset Compensation value of input channel 0 to -2 Ω and make the setting persistent.
LucidIoCtrl –dCOM4 –c0 –sinRtOffset=-20 –p [ENTER]

Read Input Offset Compensation value
LucidIoCtrl –dCOM4 –c0 –ginRtOffset [ENTER]
-> inRtOffset=20

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3.4.6 inRtCalUm

This IO Configuration Parameter configures the full calibration value “UM” which is described in section 3.1.1. The value does not have a unit and even if it possible it is not recommended to change this value manually.

LucidIoCtrl Command Line Tool Example

Read Full Calibration Value UM of input channel 0
LucidIoCtrl –dCOM4 –c0 –ginRtCalUm [ENTER]
-> inRtCalUm=760

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3.4.7 inRtCalUrs

This IO Configuration Parameter configures the full calibration value “URS” which is described in section 3.1.1. The value does not have a unit and even if it possible it is not recommended to change this value manually.

LucidIoCtrl Command Line Tool Example

Read Full Calibration Value URS of input channel 0
LucidIoCtrl –dCOM4 –c0 –ginRtCalUrs [ENTER]
-> inRtCalUrs=760

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