What is on the Horizon for Feedback Devices?

January 26, 2018, by Dan Wolke

Encoder ChoicesWe’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 seeing things?

There 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.  

 What is driving the need for these changes? 

 For 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. 

SICK 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. 

What are the challenges we are seeing? 

It 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.

What about the area of imbedded safety in feedback devices? 

The 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. 

Safety 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 system.  

 

HIPERFACE DSL® is a registered trademark of SICK STEGMANN GmbH
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