Application Digest 7465

Dynamic modeling of couplers

David A. Kaiser, Parker Hannifin, Compumotor Div., Rohnert Park, CA

Motion control users size systems by assuming an infinitely stiff coupling between motor and load. To find the torque required to make a move, the user calculates total inertia as the sum of motor and load inertia, neglecting coupler torsional stiffness.

If load damping is small, and the motor is excited at a frequency, the model shown indicates that the load will try to be become unstable. This point is the model's anti-resonant point.

Experiments done by Parker Hannifin engineers with a bellows-type coupler and a beam-style coupler demonstrate close correlation between this model of the coupler and measured data. The anti-resonant point of the bellows-style coupler is higher in frequency and lower in amplitude than that of the beam-style coupler. Also, the bellows coupler resonated at a higher frequency than the beam coupler, for a given load/rotor inertia mismatch.

In high-performance servo systems, the data indicate that coupler torsional stiffness should be higher. In lower-performance systems, a beam-type coupler might represent a more cost-effective solution.

To contact a Parker Hannifin applications engineer, call Dave Kaiser at 800-3558-9068, extension 2526, or fax 707-584-8015.

Rotary encoders or resolvers?

Peter Polak, Dr. Johannes Heidenhain GmBh, Traunreut, Germany

In the early 1990s, microprocessors and DSPs made digital control of servomotor speed and current possible. Resolvers and rotary encoders generally provide the feedback needed to determine rotor speed and position. Which type of sensor is better suited for this task: resolver or rotary encoder?

A resolver supplies one absolute signal per revolution and tolerates vibration and high temperature well. At low speed, however, supplying one signal/revolution hurts performance. Standard resolver-to-digital converters with 12-bit resolution and a 500 musecond sampling time can achieve a minimum speed of about 30 rpm. If a feedback sensor is also used for position control, its accuracy per revolution is important. In the case of resolvers, the R/D converter's interpolation error must also be considered.

With rotary encoders, however, the incremental signal can be used directly. Speed control range possible with resolvers is 1:200; with rotary encoders it is 1:6(10⁵). This latter value applies for a maximum speed of 6,000 rpm and a sampling time of 500 musecond, which are representative values for modern machine tools.

Selecting the proper resolver or rotary encoder therefore depends on the application. If it requires high control quality and a wide speed-control range, only rotary encoders will work well.

To contact a Heidenhain applications engineer, call 800-233-0388 or fax 847-490-3931.