Manual

any given instant. When using a potentiometer, this can add from $25 (for a rotary pot) to
$100 or more (for a linear pot or LVDT).
Electronic FeedBack cards typically use what is called a ‘PID’ loop to control the
position of a movement. This type of loop is available on our new four channel PID-Quad
and BS-EFB Electronic FeedBack cards. The ‘P’, ‘I’ and ‘D’ stand for Proportional, Integral
and Derivative. Forget that. This is what they do:
The basic feedback loop is the Proportional (or ‘gain’). It is what is found on the simple
EFB cards like our EFB-Quad. A simple loop like this has been traditionally used for
controlling most analog functions in animated figures. This is because most figures’
positional requirements are not stringent enough to demand a higher performance
feedback loop.
The ‘P’ function compares the desired position with the actual position (as measured
by a potentiometer or other measuring device that is attached to the movement). The
difference is amplified and fed to a servo valve to open or close it as needed. If the ‘gain’
of the ‘P’ is turned up too high, then the movement will overshoot, and then try to get back,
and overshoot again, and again, and again..... This is what is known as oscillation, and it
is not a good thing. If the gain is tuned too low, then the movement will follow the
commands sluggishly.
The ‘I’ function is the one that is used to suck a movement in when it is too close to the
desired position for the ‘P’ function to open the valve enough to overcome the ‘stiction’ of
the cylinder. If the movement isn’t perfectly positioned, the ‘I’ function generates a slowly
rising voltage to the valve until it opens enough for the cylinder to move the last little bit to
the desired position. If the ‘I’ function is turned up too high, then the movement will
constantly seek the desired position, overshoot, and seek again. Unlike the oscillation that
occurs when the ‘P’ is set too high, the ‘I’ oscillation occurs at a low speed. Although
entertaining, it is rarely destructive. If the ‘I’ is adjusted too low, then the ‘dead band’
around the desired position will be wider.
The ‘D’ function is used so that you don’t have to turn the ‘P’ up too high to get the
movement to accurately follow fast changing commands. The ‘D’ gives an extra ‘kick’ to
the valve when a movement is commanded to start quickly. If it is set too high, then the
movement will start too quickly, overshoot the commanded position, and then slow down
as the ‘P’ error takes over.
Another adjustment that is available on our EFB-Quad or PID-Quad cards is a ‘Velocity
Limit’. This control allows you to set the maximum a valve can be opened by the
FeedBack card. On the EFB-Quad it allows you to set the gain a bit higher to control
oscillations by limiting the maximum opening of the valve. By raising the gain higher and
limiting the velocity, you can narrow the ‘dead band’ of the servo loop.
The PID-Quad also has adjustment to add a 60 Hz dither signal or offset (null) to the
output. The dither is used with some valves whose performance is increased remarkably
when such a signal is applied. The nulling allows you to center the spool of the valve
electronically if it wasn’t centered properly in the factory.
GILDERFLUKE & CO. • 205 SOUTH FLOWER ST. • BURBANK, CALIF. 91502-2102 • 818/840-9484 • FAX[phone]
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