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Author: Jim York, Motion Control
Engineer: Universal Instruments
Universal Instruments, a leading
manufacturer of electronic circuit assembly equipment, turned to
VisSim, a Windows-based modeling and simulation package, to design
their new pick-and-place assembly system, the General Surface Mount
Application Machine (GSM1).
The GSM1 automatically picks surface mount
components and places them on printed circuit boards at speeds in
excess of several thousand components per hour.
It achieves precise component placement
through a fine pitch, closed-loop vision recognition system.
Accuracy is critical in lining up a component's leads over the
board's solder pads, as the distance between leads may be only 8 to
15 mils.
Critical to the design was enhancing the
head and Z-axis of the GSM1 to optimize the time it took to pick and
place a component, without causing the head to move so rapidly that
it would sacrifice accuracy. |
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"Based on the success of the GSM1 Z-axis
application, we use VisSim on every new servo system design. In
fact, modeling the system in VisSim is a required point on our
development checklist."
Jim York
Motion Control Engineer
Universal Instruments |
To ensure the head descended stably into
position, a short settling time of 10 ms was required. To attain this
time, we specified a high bandwidth for the servo motor governing the
axis (20 to 40 Hz for positioning, and 100 to 200 Hz for velocity).
However, as accurate as these motors are, the performance of the GSM1
would be limited mainly by its mechanical components.
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Universal Instruments' General Surface Mount
Application Machine (GSM1). |
Universal Instruments' General Surface Mount
Machine simulation in VisSim. Current, load distance, load torque,
load velocity and cable profiles are shown. |
The Challenge: Designing Speed and Accuracy in the Z-Axis
The servo motor for the Z-axis drives a shaft
via a timing belt, which allows a reduction ratio. To move the Z-axis, a
solenoid clutch couples a pulley and cable to the shaft. The other end
of the cable is connected to the Z-axis, along with a spring to keep the
cable taut. When the clutch is released, the Z-axis springs up to a hard
stop. If the spring does not have enough tension when the shaft
accelerates, the cable becomes slack and uncontrollable.
Our challenge was to design the mechanical
elements so they would support the head's acceleration and deceleration
rates, while maintaining tight control of the load as it was moved.
The Solution: VisSim
Our initial evaluation of VisSim proved that
it was not only easy to use but also powerful enough to model and
simulate a complex, nonlinear system, like the GSM1. Building the model
was simply a matter of dragging predefined function blocks off the
Blocks menu and into the work area, and wiring them together with the
mouse. We modeled the properties specific to the servo drive, such as
its position, velocity, and current control, as well as all the
parameters affecting the cable tension, including acceleration and
decelera- tion rates, load mass, spring rate, and friction. To achieve
the settling time characteristics of the servo mechanism, we also
included the proper gains and bandwidths. Component operating parameters
were entered directly to the appropriate blocks through pop-up dialog
boxes.
During simulation, we viewed the dynamics of
the cable's tension in plots and real-time graphs. As we entered known
values for acceleration, deceleration, and mass, their effect on the
tension could be immediately monitored. This allowed us to adjust the
spring rate so the tension was positive at all times.
Based on the simulation results, we built a
hardware prototype of the GSM1 Z-axis. After further testing,we released
the unit for production.
The Benefits
Using VisSim, we designed the GSM1 much faster
than if we had assembled a breadboard and performed physical testing.
In addition, the GSM1 model provided a high
degree of accuracy, allowing us to examine signals that would have been
too difficult to monitor in a breadboard.
Because we could view the entire dynamic
picture of the mechanical load, we designed the components to properly
support the acceleration and deceleration rates of the vertical axis,
ensuring tight control of the load while it moved up and down.
And, by validating the design through
simulation, we could identify the correct components before building the
prototype, shortening the design cycle significantly.
In the broader realm, VisSim will be used to
design and test improvements to our existing products. As machines
undergo changes to their mechanical and servo systems, we can test the
proposed changes by opening the corresponding VisSim model, making the
modifications, and running a quick simulation.
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