Lightning talk from F#nctional Londoners user group meeting 04/06/2015. Briefly discusses the instrument control software we have written in F# to control a custom experiment at the University of Warwick.

1. SCIENTIFIC INSTRUMENT
CONTROL WITH F#
Anton Tcholakov, Colin Stephen, Gavin Morley
Department of Physics, University of Warwick
@ant_pt

2. CUSTOM SPECTROMETER

3. 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

4. REQUIREMENTS

5. REQUIREMENTS
• Concurrent control of multiple
instruments
• Real-time chart plotting
• Ability to save data and
experimental parameters
• Modularity
• Robust error handling
14.125 14.130 14.135 14.140
-6000
-4000
-2000
0
2000
4000
6000
8000
10000
EPRsignal(arb.units)
Magnetic field (T)

6. 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 }

7. 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

8. DATA ACQUISITION
type 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

9. 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?)

10. LabVIEW
• Looks and feels like it belongs in the 90s
• … but has a vast library of instrument drivers

11. 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