Universal
Instruments Shortens GSM1 Design Cycle With VisSim
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 simula- tion
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.
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.
"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
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 & 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. |
RSS Feeds
VisSim/Real-TimePRO
