Electronic damping system

An electronic damping system is disclosed which dampens mechanical resonant vibrations of a gantry beam end. The system includes a secondary velocity feedback loop in addition to the main servo motor-tach velocity loop. The resonant velocity of the beam is sensed from the movement of the end of the beam. The velocity feedback signal is summed as negative feedback with the velocity command signal from the position controller. The resultant sum of the signals are combined to form the velocity reference input command signal to a servo power amplifier. The servo power amplifier is connected to a main servo motor which controls the movement of the gantry beam in the Y-direction relative to printed circuit board.

FIELD OF THE INVENTION 
The present invention is directed to a system for electronically damping 
mechanical vibrations, and more particularly, to damping the mechanical 
vibrations caused by acceleration and deceleration incremental motion 
forces of a gantry beam positioning system. 
BACKGROUND OF THE INVENTION 
It is known to use a circuit board assembly apparatus having at least one 
movable carriage, i.e., gantry beam, mounted via bearings on guide rails 
for movement in one direction over a printed circuit board, while at least 
one component gripping device is mounted on the movable carriage for 
movement along the carriage in another direction. This device allows x-y 
positioning of electronic components held in the gripping device relative 
to a printed circuit board. Such an assembly is shown, for example, in 
U.S. Pat. No. 5,002,448, the contents of which are incorporated herein by 
reference. 
The gantry beam is driven by a servo control system. To optimize 
manufacturing efficiency when mounting components on printed circuit 
boards, it is desirable to position the components as quickly as possible 
relative to the circuit board. However, such high speed operation produces 
undesirable vibrations, oscillations, resonances and the like in the 
gantry beam. 
A particular problem is caused by the fact that the natural resonance 
frequencies of the gantry beam end change as the gantry beam load changes 
as the gripping device carried by the movable carriage traverses the beam. 
Thus, traditional systems for compensating for resonances, such as 
electronic notch filtering, would be very difficult to use in a gantry 
beam positioning system since the notch frequency setting would have to 
vary with the position of the load along the beam. 
SUMMARY OF THE INVENTION 
The present invention overcomes the above difficulties by creating a 
secondary velocity feedback loop for the main servo motor-tach velocity 
loop in which the resonance velocity of the beam is sensed from the 
movement of the end of the beam. The resonance velocity feedback signal is 
summed as negative feedback with the velocity command signal from the 
position controller. The resultant sum of the signals are combined to form 
the velocity reference input command signal to a servo power amplifier. 
This secondary velocity feedback loop is supplemental to the main servo 
motor-tach velocity loop in the servo power amplifier. 
The amount of secondary loop gain can be adjusted according to the optimum 
mechanical settling time required to produce high speed and high accuracy 
placement of components on printed circuit boards. Decreasing mechanical 
settling time in accordance with the invention ensures machine part 
placement repeatability and increased placement speed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A gantry beam electronic damping system in accordance with the present 
invention is shown in FIG. 1. Throughout the figures, like numerals are 
used to represent like elements. 
The system includes a gantry beam 16 supported for controlled movement in 
the Y-direction between linear bearing supports 10 and 50 over a printed 
circuit board 20. Ball or feed screw 12 and linear scale 14 coupled to the 
support 10 act as a mechanical rotary to linear velocity linkage converter 
for controlling the movement of the gantry beam in the Y-direction. At 
least one component placement device or gripping device 17 is mounted for 
controlled movement in the X-direction along the length of gantry beam 16 
by carriage 13. 
Rotary tachometer 18 is disposed at the end of the gantry beam 16. Beam 
velocity feedback signal 22 is output from the rotary tachometer 18 via 
damping gain amp 24 to summing junction 31. Together, damping gain amp 24 
and summing junction 31 form adjustable combining means 29. 
A position feedback signal 26 is output from linear scale 14 to the 
position controller 28, and a velocity command signal is output from the 
position controller to summing junction 31. The velocity resonant signal 
is summed as negative feedback with the velocity command signal and the 
resultant sum of the signals form velocity reference input command signal 
32 which is input to servo power amp 34. This forms the secondary loop 
supplemental to the main servo motor-tach velocity loop. Servo power amp 
34 drives servo motor 42, which in turn drives ball screw 12 to damp the 
vibrations of the beam, in a manner which will be described in greater 
detail below. 
As shown in the control loop diagram in FIG. 2, servo power amp 34 includes 
summing junction 44 which receives the tach feedback signal 40 from 
tachometer 38 coupled to servo motor 42. The tach feedback signal 40 and 
the velocity reference input command signal 32 are summed at summing 
junction 44, and the resulting signal is input to velocity loop error 
amplifier 46. The signal output from velocity loop error amplifier 46 and 
the current feedback signal 52 are summed at summing junction 48 and 
output to current loop error amplifier 50. The resultant drive signal 36 
is output to servo motor 42. The amount of secondary loop gain is adjusted 
at damping gain amp 24 according to the optimum settling time 
requirements. 
FIGS. 3A and 3B demonstrate the reduced settle time obtained using gantry 
beam electronic damping in accordance with the present invention. 
Experiments were conducted for a Y-axis displacement at 0.9 g's 
acceleration/deceleration and a selected carriage position. For purposes 
of the experiment, +-0.001 inches around the commanded position 
destination, the Y-axis was considered to be "settled". Settling time data 
was taken after the position controller signalled "reference trajectory 
completed". With no electronic damping, as shown in FIG. 3A, the 
mechanical settle time was 360 ms. measured with a similar linear scale 14 
mounted at end of gantry beam 16 opposite lead screw 12. With electronic 
damping in accordance with the present invention switched on, the 
mechanical settle time was 65 ms. 
The above is for illustrative purposes only. Changes can be made in 
accordance with the invention as defined in the appended claims. For 
example, the invention is not limited to the electronic damping of 
mechanical vibrations in a gantry beam, but may be used to dampen other 
elements which are subject to undesirable mechanical vibrations, 
oscillations or the like. Further, the invention is in no way limited for 
use with machines for mounting electronic components on circuit boards, 
but may be used in any environment in which undesirable vibrations occur. 
For further example, it is also contemplated that the system may include a 
linear scale feedback device mounted at the end of the beam. A position 
controller can digitally derive the resonant velocity of the gantry beam 
end from change in position over change in time, and perform the function 
of gain amp 24 and summing junction 31 via software. The resultant signal, 
i.e., the velocity reference command signal 32 is then input to the servo 
power amp 34 as discussed above.