Specifications

5.Actuator (cylinder/motor)
– Size the area for dynamic and static forces (remembering
the 1/4 to 1/3 P
S
requirement of the Servo or Proportional
Valve in the dynamic case).
– Calculate the resonant frequency and adjust the actuator
areas and valve size, if necessary, to optimize accuracy.
(Increased area plus increased natural frequency improves
accuracy.)
– Recognize the 2% to 20% breakout friction of different
seals and their effect on position resolution.
– Manifolds should not contain air pockets. If they do,
you cannot flush the air out of the manifold, leading to
a “soft” system.
– Keep the cylinder full area/rod end area ratio ² 2:1 to avoid
greatly differing extend and retract velocities.
Note:
5.1 Actuator Connection to Load and Frame:
– There should be no free play (a practical limit in a
position loop would be 3 to 10 times less than the
required position accuracy).
– The mechanical stiffness should normally be 3 to 10
times higher than hydraulic stiffness to avoid degrading
performance.
– Gearing down decreases inertia felt at actuator and hence
increases natural frequency (and with it system response
and accuracy). However, gearing down could lower stiff-
ness and introduce play/backlash due to the gears.
6.Feedback Transducer
Closes the loop and its characteristics are of paramount
importance, e.g.:
– Linearity
– Threshold (resolution) and hysteresis
– Drift with temperature or time
– Frequency response (it must be 3 to10 times faster than
the slowest element in the system).
6.1 Transducer Placement:
– Placing at the actuator output eliminates many control
problems (by excluding secondary spring-mass systems and
play), but may not provide accuracy at the point required.
7.Servoamplifier
– The dynamics of the analog electronics are always better
than the Servovalve and spring-mass system.Therefore, they
can be neglected.
– Some digital systems, however, lack the level of dynamics
that are needed. In order to see if this is a problem, check
the following:
(i) That the update rate of the PLC is a maximum of 20
times faster than the frequency of the valve.
(ii) That the update rate of a digital-to-analog converter,
which is required for Electric Feedback Servovalves,
is faster than the valve. A rule of thumb is that the
converter should be a minimum of 20 times faster,
and preferably 100 times faster than the 90˚ frequency
of the valve.
(iii) Use of 12 and 16 bit digital-to-analog converters.
Anything slower could compromise the valves
resolution.
(iv) The last stage to the Servovalve is a current output.
– Use of compensation techniques (Proportional, Integral
or Derivative) can be reviewed when selecting the
Servoamplifier. (Note that 90% of position loops can be
handled by a straight ‘P’ controller, and the simplicity of
set up and troubleshooting a ‘P’ controller is invaluable).
– Avoid placing the amplifier close to electric motor
controllers or other components that generate high
electromagnetic fields – consider shielding if necessary.
– Interconnection to the command signal and feedback trans-
ducer should use shielded cables to minimize interference.
(Ground only the chassis end to prevent ground loops.)
8.Conclusion
To lay out a design for a Servosystem means taking care of
minimizing lags in the control chain. (In addition to the usual
design requirements of strength, fatigue life, ease of mainte-
nance, ease/cost of manufacture, etc.)
Lags may be caused by:
1. Free-play/backlash/stick-slip
2. Free-time constants of components
The time constant of the valve can be selected. However, the
time constant of the actuator-mass system is dependent upon
the control of hydraulic and structural stiffness and the mass
of moving parts.
extend velocity full area
retract velocity rod end area
=
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