SCIENTIFIC INSTRUMENT CONTROL WITH F#

Anton Tcholakov, Colin Stephen, Gavin MorleyDepartment of Physics, University of Warwick

@ant_pt

CUSTOM SPECTROMETER

CUSTOM SPECTROMETER

• Apply microwaves to a sample in a magnetic field

• Measure the reflected power

• Vary the magnetic field

• Changes in the signal help to characterise the sample

REQUIREMENTS

REQUIREMENTS• Concurrent control of multiple

instruments

• Real-time chart plotting

• Ability to save data and experimental parameters

• Modularity

• Robust error handling14.125 14.130 14.135 14.140

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Magnetic field (T)

DOMAIN MODELLING

• Records, unions and units of measure let us model hardware capabilities and experiment parameters clearly and concisely

• We get structural equality and serialisation for free

type Frequency = FrequencyInHz of float<Hz> type Amplitude = PowerInDbm of float<dBm> type StepSpacing = LinearSpacing | LogarithmicSpacing type Range<'T> = { Start : ’T ; Stop : 'T }

type FrequencySweep = | FrequencySweep of range : Range<Frequency> | FixedFrequency of frequency : Frequency

type AmplitudeSweep = | AmplitudeSweep of range : Range<Amplitude> | FixedAmplitude of amplitude : Amplitude

type StepSweep = { Frequency : FrequencySweep Amplitude : AmplitudeSweep NumberOfPoints : int Spacing : StepSpacing }

INSTRUMENT CONTROL

• Communication to hardware is inherently async

• async workflows are great metaphor for experiments

• Use asyncChoice (available in ExtCore) and railway-oriented programming to handle errors

DATA ACQUISITIONtype StreamingAcquisition = { Parameters : StreamingParameters Buffers : AcquisitionBuffers StopCapability : CancellationCapability<StreamStopOptions> StatusChanged : Event<StreamStatus> SampleBlockObserved : Event<SampleBlock> }

let run scope acquisition = asyncChoice { use acquisitionHandle = PicoScope.Acquisition.allocateHandle digitiser acquisition.Buffers

do! prepare scope acquisition do! startStreaming scope acquisition do! pollUntilStopped scope acquisition }

• Acquisition emits samples via Event<‘T>, so we can feed this straight into FSharp.Charting

• Use Rx transformations to implement signal processing

WHAT DO OTHERS USE?C / C++

• Low level, manual memory and thread management

Python

• Global Interpreter Lock can cause problems

• Pay the price for dynamic typing in large projects

C# (… in industry?)

LabVIEW

• Looks and feels like it belongs in the 90s

• … but has a vast library of instrument drivers

SUMMARY

• F#’s type system allows us to model our problem domain clearly and concisely

• Many of the tools we need are already available (async, Rx, FSharp.Charting)

• F# is very well suited to the task but lacks instrument libraries and user base