You
can use mechanical cams, gears, and drive shafts to provide the
repetitive motion profiles necessary for coordinated moving parts.
But then you have the problems that come along with mechanical
components. Electronic cams avoid such difficulties and offer many
advantages.
By John Rathkey and Steve Danek
The large machines involved in
manufacturing and packaging usually have several moving components.
The individual components, which move in a related or sequential
manner, must all be precisely coordinated with respect to the
overall machine process. The traditional approach to such
coordination relies upon a main drive shaft and mechanical cams,
but electronic cams can provide an attractive alternative. Not
only do electronic cams eliminate several of the problems
associated with mechanical systems, but they can offer important
new capabilities for machine design.
A Mechanical Approach
The components of a mechanical cam system can be a purely
mechanical approach which usually involves a large main motor as a
central motion source, providing power and a coordination
reference for the rest of the machine. Individual axes are driven
from this source with gears and drive trains. The gears
synchronize and determine the speed relationship of each
individual axis, and the drive trains deliver the motion to the
appropriate place. If the
required motion of an axis is not constant but needs to follow a
pattern, mechanical cams are used. The shape of the cam determines
the motion pattern of the cam follower with respect to the motion
of the cam driver. Individual axes may be started and stopped
using clutches and brakes. These are required to accelerate and
decelerate the load but need to be rugged enough to control the
axis within one cam cycle. In particular, a one-shot clutch is
required to bring the cam into synchronization with the rest of
the machine.
An Electronic Approach
The components of an electronic cam system can serve as a
drop-in replacement for the mechanical components. This electronic
approach assumes a large main motor as a central motion source,
measured by an encoder. It's a standard approach if other aspects
of the machine (e.g. conveyors) need the motor. If all motion can
be replaced by electronic cams, a small motor with an encoder or a
simple pulse train may serve as the synchronization source.
Each individual controller/drive
generates its own cam motion profile, based on its own stored
program, and the common synchronization source. Controllers can
also accept electronic inputs and assert outputs. This I/O
capacity gives much greater design flexibility and run-time
decision making power to a machine than could be achieved with
mechanical components.
The actual motion may be delivered
by a linear actuator or by a servo or stepper motor coupled with a
gearbox, belt, and pulley. These components are connected with
simple cables that are easily routed over great distances. This
allows the motor or actuator to be mounted exactly where the
motion is needed, eliminating the need for drive trains. The
size, power, and configuration of the motors and drives depend on
the machine requirements and may be different for each axis.
Problems Solved
Electronic cams avoid the inherent problems mechanical cam
systems have - particularly as required power and precision go up.
The mechanical parts are conceptually simple, but as with all
mechanics, they suffer the problems of wear, backlash, and
distortion. These problems directly affect the accuracy and
repeatability of the cam follower motion. With electronic cams,
these problems don't exist, resulting in precise, repeatable
control and reduced maintenance.
Power transmission is a big
mechanical issue. For each axis, the gear, drive train, cam, and
cam follower provide not only the speed and position coordination,
but they must also be substantial enough to deliver the required
power. If any part distorts because it is not rigid enough, the
position integrity of the follower is affected. Clutches and
brakes tend to be rough on the rest of the machinery, because of
the sudden jerk when they are engaged. With electronic cams, each
torque load is isolated from all others.
Electronic cams allow controlled
acceleration and deceleration in both directions without requiring
a spring loaded follower. The increased control gives better
precision and also allows higher speed moves on the individual
axes. This translates directly into better quality in overall
machine functions and increased throughput and higher productivity.
Pressure Angle
One concern for mechanical cam designers is pressure angle.
This is defined as the angle of contact between the follower and
the surface of the cam from the center axis. Pressure
angle causes extra stresses on a mechanical system and also makes
profile calculations more difficult. A large pressure angle can
cause damage to a mechanical system, limiting the maximum
acceleration. In an electronic system, the accelerations are
limited by the motor and motor drive, allowing for more aggressive
cam profiles.
There is a separation between the
follower and the cam at angles where the radius of the cam is
changing. This angle is dependent on the radius of the follower
and the rate of change in the cam radius. The mechanical designer
must use second order equations to accurately describe the motion
of the follower's profile, as it will not be equal to the profile
of the cam's radius. Designers of electronic cams do not have this
problem, because the profile they program will be the profile of
the follower.
Flexibility Advantage
Cam development time, part count, and machine cost may be
reduced with electronic cams. If the required cam shape is very
complex, the cam can be quite difficult to design and can be
expensive to machine and produce to required tolerances. For a
mechanical cam, each mistake involves dismantling the machine and
re-machining the part, whereas the electronic version is a simple
program change. The flexibility
and rapid changeover made possible by programmable electronic cams
is becoming increasingly important with today's emphasis on
Just-In-Time manufacturing and higher standards of customer
service. A machine built with mechanical components is limited in
function by the particular gears and cams installed. To process a
different product on the same machine can require significant
machine downtime and changeover labor as cams and gears are
replaced. By contrast, machines built with electronic cams can be
used to respond to a custom product order, or a small quantity
request for product, simply by selecting a new program. This
allows a given machine to produce a wide variety of products while
remaining fully utilized, regardless of changes in production
requirements.
Electronic Cam Design
Electronic cams are built from data about specific cam positions
and the corresponding follower positions. In some electronic cam
products, the controller does a simple linear position
interpolation between points to create a profile. The
corresponding mechanical cams have arc surface sections for each
pair of points and a sharp ridge between arcs. Many pairs of
points are required to even approximate a smooth surface. This
problem is avoided by including the cam surface slope in the data
used to calculate the cam profile. Each cam position has a
corresponding follower position and slope. The slope is simply the
rate of follower position change with respect to cam position
change, i.e. the ratio. These three pieces of data describe a
section of the cam, or a cam segment.
Other Advantages
The ability to turn on and off outputs is a useful feature in
many cam applications. For example, a machine may need to turn on
a glue gun at precise times in a machine cycle. A
mechanical system uses a programmable limit switch or a series of
switches attached to the line shaft. These switches are subjected
to the same wear and tear as the rest of the mechanical system. In
an electronic design, the switching can be embedded into the cam
profile.
Electronic cams also offer machine
protection. As happens in almost every product-assembly design, a
product may enter out of alignment, causing the machine to jam. In
a mechanical system, the line shaft still exerts its force on the
individual cam that has jammed and can cause damage to various
parts of the machine. In the electronic system, fault conditions
can be detected before damage occurs. Since an individual cam axis
is linked electronically, the drive and motor can be uncoupled
from the master. In many programmable controllers, this fault
condition can cause a branch to error handling code and the
machine can recover from the fault.
FREE TECHNICAL DATA
Readers of Product Design and
Development can receive additional information about electronic
cams by contacting Parker Hannifin Corp., Compumotor Div., 5500
Business Park Dr., Rohnert Park, CA, 94928.
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