MTS Series 793 - Manual (Page 21)

About Tuning
MTS Series 793 Tuning and Calibration Introduction 21
If you turn the steering wheel too little, and the car responds too slowly. If you
turn the steering wheel too much, the car overresponds. So the objective for a
good driver is to turn the wheels just right. If so, the car accurately follows the
line, the passengers have a smooth ride.
The test waveform
We want our servo loop to work just like the skilled driver, that is, to turn the
steering wheel just the right amount.
Now imagine the road transformed into the square wave shown in the diagram,
which is one type of test waveform. We want our test (the car) to follow the
desired command (the road) in all respects. That means the test system should
exert the precise force (or displacement or strain) that we want on the specimen.
The only problem is that different types of materials—from the softest to the
hardest—exhibit different reactions to the force or displacement or strain. Just as
we would tune a car differently for racing than we would for a weekly drive to
the store, the tuning differs too. A system properly tuned for a soft specimen will
go unstable if you install a very hard specimen.
The ultimate goal
Get the error signal to be as small as possible at all times (without compensators),
because:
• The error signal tells the servovalve to open.
• The larger the error signal, the more the servovalve opens.
• Therefore, if the error is zero, the servovalve is closed. This means the servo
loop is “satisfied” and all is well.
Remember: If the error is as close to zero as possible (actually maintaining zero
is impossible), it indicates that the system is closely following the command.