Zero Field Magnetometer IOC

Purpose

The magnetometer reads magnetic field strengths from the beamline. This IOC takes the magnetometer signal, applies corrections and performs a check to make sure that the magnetometer has not been overloaded.

This IOC is designed to be used in conjunction with a sequencer IOC, which uses the field values to calculate corrections needed to maintain a magnetic field strength. When a new field value is reqiured, the sequencer will process the $(P)TAKEDATA PV of this IOC. When the new field values are available, this IOC processes $(PVPREFIX):ZFCNTRL_01:INPUTS_UPDATED.PROC, which it expects to allow the sequencer IOC to continue its calculations. This PV can be changed by setting the $(SQNCR) macro.

Corrections applied to the input fields

The maths which defines these operations is described in finer detail here.

1. Magnetometer range scaling

   • The three field components are multiplied by a scaling factor to transform the input on to the range of the magnetemeter. The scaling factor is set by the macro $(RANGE), and is held in the PV $(P)RANGE.

2. Applying offsets

   • Each component has a unique offset which is subtracted from the scaled data. These offsets are set as the macros $(OFFSETX), $(OFFSETY), $(OFFSETZ). These can also be set by the PVs $(P):X:OFFSET and from the OPI.

3. Multiply with sensor matrix

   • To obtain the corrected field strengths, the scaled and offset values need to be multiplied by a 'field sensor' matrix.
   • Macros define the sensor matrix: $(MATRIX_X_Y), where X and Y are the row and column indices.
   • PVs hold the elements of the sensor matrix: $(P)SENSORMATRIX:XY. These PVs can also be changed on the OPI.
   • The multiplication iteslf is done as an aSub record. This allows us to use the libraries available to us through GSL.
   • Set up using GSL vector and matrix structures, performing the matrix multiplication is a relatively straightforward call to a vector/matrix multiplication BLAS routine in GSL.
   • The documentation for GSL data structures used is here.
   • The GSL documentation for BLAS routines is lacking (see documentation). To get more in-depth information about the arguments used I would look up the routine in the BLAS documentation.

4. Output

   • The corrected field strengths are written directly to the PVs $(P)X:CORRECTEDFIELD from the aSub record.
   • The aSub record also calculates the magnitude of the corrected field and places it in $(P)FIELDSTRENGTH.

Overload

High field values will overload the magnetometer, which is detected in this IOC. The overload condition for the EMU magnetometer is:

max(measured_field) > magnetometer_range * 4.5

Where:

• max(measured_field) is the largest component of the 3 scaled (but not offset or matrix multiplied) input field strenghts.
• magnetometer_range is the scaling factor used in the magnetometer range scaling
• 4.5 is a constant taken from the EMU VI. This value may be different for other magnetometers.