Closed-loop step motor control using absolute encoders Page: 3 of 16
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Closed-Loop Step Motor Control Using Absolute Encoders
by
J. Steven Hicks
Michael C. Wright
Oak Ridge National Laboratory
Instrumentation and Controls Division
Oak Ridge, Tennessee 37831
Abstract - A multi-axis, step motor control system was developed to accurately position and control the
operation of a triple axis spectrometer at the High Flux Isotope Reactor (HFIR) located at Oak Ridge
National Laboratory. Triple axis spectrometers are used in neutron scattering and diffraction experiments
and require highly accurate positioning. This motion control system can handle up to 16 axes of motion.
Four of these axes are outfitted with 17-bit absolute encoders. These four axes are controlled with a
software feedback loop that terminates the move based on real-time position information from the absolute
encoders. Because the final position of the actuator is used to stop the motion of the step motors, the
moves can be made accurately in spite of the large amount of mechanical backlash from a chain drive
between the motors and the spectrometer arms. A modified trapezoidal profile, custom C software, and
an industrial PC, were used to achieve a positioning accuracy of 0.00275 degrees of rotation. A form of
active position maintenance ensures that the angles are maintained with zero error or drift.
1. INTRODUCTION
As with many problems in engineering, there is often more than one solution or approach that can
be made to work. User familiarity, backward compatibility, future compatibility,. and budget all had an
influence on the final system design. Starting with the right components is the key to successful systems
integration. Often the most cost effective solution is a complete integrated turnkey system from a
distributor or value added re-seller. However, sometimes unique system requirements do not match up
with COTS (Commercial Off-the-Shelf) hardware well enough to allow an outright "plug and play" solution.
A good systems integrator can adapt COTS hardware with custom software or interfaces to meet the
customers' "wants" as well as needs. As the end users' desired capabilities evolve over time, and
requirements change, custom solutions also have the added benefit of future flexibility. The downside to
developing one's own hardware and software is the added cost of development time. The rest of this
paper will describe the need for and then the design of a closed-loop step motor control system that
exceeded customer requirements.
. Managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy under contract
number DE-AC05-96OR22464.
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Hicks, J.S. & Wright, M.C. Closed-loop step motor control using absolute encoders, article, August 1, 1997; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc694549/m1/3/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.