We’ve covered feedback devices before in our Blog in Motion
posts, but today we want to touch base on current trends we are seeing related
to feedback devices for servo applications. We asked Dan Wolke a few questions
about where we see the market heading:
Dan – what’s new in the world of feedback, as far as you are
have been some major changes in direction with Servo motor feedback
technology. First is 2 wire feedback for single motor cable
applications. The second is imbedded safety in encoders.
is driving the need for these changes?
years, Customers have been demanding single cable solutions for drive to motor
interface. This goal has been problematic because feedback signals
historically have had low signal to noise ratios. Putting the feedback in
the same cable as the motor power wires creates the perfect storm for inductive
& capacitive coupling of PWM generated EMC into the feedback signal.
Another issue is adding more feedback wires to the already large motor power
cable increases its size and bend radius, making the cable challenging to use.
has come out with HIPERFACE DSL® feedback that makes several improvements to
help minimize the negative aspect of single cable solution. They
increased the supply voltage from 5VDC to 9VDC, increased the signal to noise
ratio by almost 3:1, and reduced the number of additional wires added to the
motor power cable to one shielded pair. Kollmorgen has followed suite
with their SFD3 feedback creating a robust single cable solution.
are the challenges we are seeing?
hasn’t been that easy to implement and achieve the same application features as
two cable solution. Cable design and construction still creates large
bend radius (~6” bend radius for a cable with 14g motor power wire).
Motor connectors had to be changed to provide better shielding of the feedback
signals. Existing drive designs required external circuit boards to
process the signal. With the loss of the “sense” leads, voltage drop
becomes a concern on long cable runs.
about the area of imbedded safety in feedback devices?
second improvement in Servo motor feedback technology is around “Safety”.
Most OEMs are implementing IEEE
safety standards into their machine designs.
Safety standards use terms like “Known good components”, “Conformation”, and
“loss of any one safety component will not result in loss of safety”.
Using traditional feedback devices in the motors, additional components must be
added external to the motor and drive to meet these requirements. Now encoder
manufactures are designing the required safety functions into the motor’s
feedback device simplifying the system design.
encoder alone doesn’t solve the problem. The drive and cable must be
designed to support the safety encoder. There is additional cost
involved. And if the encoder and drive were certified separately, it
would be an incorrect assumption that they are certified together as a
HIPERFACE DSL® is a
registered trademark of SICK STEGMANN GmbH