Vaisala RVP900, User Manual, Page: 104 / 484

The above example illustrates typical values that might be used with a
Motor/
Integrator
servo loop. The feedback slope of 1.0
D–Units/KHz
means that a frequency
error of 100 KHz produces the full–scale (100
D–Units
) AFC output. But this is modified by
the minimum and maximum slew requests as follows:
• A 0 D–Unit output is always be produced when AFC is locked.
• When AFC is tracking, the output drive is always at least ±15 D–Units . Set this
minimum non–zero drive to the sustaining drive level of the motor actuator, that is, the
minimum drive that keeps the motor turning.
• When AFC is tracking, the output drive never exceeds ±90 D–Units . This parameter
can be used to limit the maximum motor speed, even when the frequency error is very
large.
The AFC
Motor/Integrator
feedback loop works properly even if the motor has become
stuck in a cold start, that is, after the radar has been turned off for a period of time. The
mechanical starting friction can sometimes be larger than normal, and additional motor
drive is required to break out of the stuck condition. But once the motor begins to turn, then
the normal AFC parameters (minimum slew, maximum slew, feedback slope) all resume
working properly. The algorithm operates as follows:
1. When AFC correction is applied, RVP900 calculates how long it would take to reach the
desired IF frequency at the present rate of change.
For example, if we are 1 MHz away from the desired IF frequency, and the measured rate
of change of the IF burst frequency is 20 KHz/sec, then it is 50 seconds until the loop
reaches equilibrium.
2. When the AFC loop is in Track–Mode, but the time to equilibrium is greater than 2
minutes, then the minimum slew parameter is slowly increased.
The idea is to gradually increase the starting motor drive when it appears that the IF
frequency is not converging toward the correct value, that is, the motor is stuck.
3. When the frequency begins to change, such that the desired IF would be reached in
less than two minutes, the
minimum slew
parameter is immediately put back to its
correct setup value.
4. The loop then continues to run properly using its normal setup values.
Manual frequency control (MFC) operates unchanged in both of the AFC servo modes.
When MFC is enabled in the
Ps command, it directly controls the output voltage of the AFC
D/A converter. The MFC mode can be useful when testing the motor response under
different drive levels, and when determining the correct value for the minimum slew request.
5.2.2
Mc
— Top-level Configuration
You can use this set of commands to configure general properties of the IFDR.
This is the Ethernet address of the remote IFDR module. For best results, attach the IFDR to
a dedicated Ethernet port of the host computer with no routers, switches, or hubs in
between.
IP address of networked RVP9/IFD: 10.0.1.254
RVP900 User Guide M211322EN-J
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