5.3 Rotor Speed Controller Accessory Operation
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VNMR 6.1C User Guide: Solid-State NMR
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of each acquisition, the initial spinning speed is noted. If, during acquisition, the speed
alters by more than 100 Hz and the interlock parameter
in
is set to
'y'
, acquisition is
halted.
The flexibility of rotor synchronization is mainly through the construction of pulse
sequences using rotor sync elements (refer to the VNMR User Programming Manual for
information on creating pulse sequences).
5.3 Rotor Speed Controller Accessory Operation
The Varian rotor speed controller accessory provides computer control of the spin rate of a
CP/MAS sample. By using the controller in a closed loop mode, the sample spinning rate
can be held constant to a few hertz or better over a long acquisition time (days or weeks).
During variable temperature operation, the rotor controller can keep the spin rate from
changing while varying the operating temperature. The setting of a desired rate is much
easier because fine control is provided for and slow drift is automatically compensated.
Rotor Speed Controller Hardware
The rotor speed controller accessory consists of the following hardware:
• Modified Varian VT pneumatics/tachometer box
• PC-compatible computer with 16-bit timer-counter and DAC cards
• Cabling to connect PC, pneumatics/tachometer box, and Sun workstation
The use of a PC computer provides an inexpensive, powerful, and dedicated processor for
rotor speed controlling tasks. The PC houses 16-bit DAC and counters for measuring the
rotor speed and supplying a 0 to 5 V signal to the pneumatics transducer inside the
pneumatics/tachometer box. Operator control of the PC and software takes place through a
RS-232 link to the Sun host computer.
Rotor Speed Controller Software
The rotor controller is run with the
rcontrol
software, which has a menu driven
interface. The main menu provides choices that initiate the following:
• Open loop
• Closed loop
• Configuration routines
• Exit from the
rcontrol
software
Open Loop Mode
In open loop (O) mode, the program requests a DAC value (0 to 65385) and then
continually displays the rotor speed on the screen. Open loop mode is useful when spinning
a sample up for the first time—perhaps, to check packing balance—or to calibrate the
electro-pneumatic regulator span and zero settings.