Chapter 5. Solid-State NMR Accessories
77
VNMR 6.1C User Guide: Solid-State NMR
01-999162-00 C0402
The DAC value can be changed while continuously displaying the rotor speed by pressing
the following keys and simultaneous key combinations on the PC keyboard:
Closed Loop Mode
In closed loop (C) mode, the desired rotor speed is entered. The control algorithm then
takes over to control the rotor speed. While controlling is active, the DAC value, latest
increment to the DAC, and the difference of the rotor speed and the set point are displayed
each time through the control loop.
To stop the speed control process, press the q key. After the process is stopped, you can
enter a new rotor speed, save the data log file, or exit to the main menu.
To save the data log file (DAC and rotor speed values), choose the L option from the main
menu. You can specify the number of data points to log (4000 points maximum) and a
control loop divisor N. For example, if N is set to 10, a DAC and rotor speed pair is logged
every 10th time through the control loop. Log data is written to disk only after termination
of the closed loop control session and confirmation by the user.
Configuration Routines
The configuration (F) choice at the main menu displays the current gain settings used in the
closed loop control process and allows you to change them. Besides the gain values, the
configuration routines allow you to set the following:
• Loop delay, set in milliseconds
• Increment clamp value for the loop increment
The loop delay specifies a time delay between outputting to the DAC and reading the rotor
speed. Note that too short a time may lead to wild oscillations in the closed loop mode when
the gain settings are large.
The increment clamp value sets the maximum change to the DAC word and is a useful type
of “adaptive gain” that can allow gain settings that produce good control so long as the
change in set point is not too large.
Typical ranges for the gain settings are shown in
. G3 and G4 set to 0 produce good
control when the ranges shown in the table are used for the other gains.
Table 11. Rotor Controller Gain Setting and Typical Ranges
Actions
Keys
Decrease DAC value by 1 unit
f
Decrease DAC value by 100 units
l
Decrease DAC value by 1000 units
Shift-l
Increase DAC value by 1 unit
Shift-f
Increase DAC value by 100 units
h
Increase DAC value by 1000 units
Shift-h
Exit to main menu
q
Gain Setting
Typical Ranges
G1
4 to 5
G2
1 to 2