By Machine Design
Stepmotors are one of the most robust yet intricate motion-control products available. Although these motors are an engineering marvel, there are times when their performance just can nott cut it.
Take Vibration Out of Step MotorsFortunately, for those times, innovation and evolving problem-solving techniques can provide many options. Gearboxes can boost speed or torque. Special motors rated for IP65 or IPX7 handle applications needing protection from dust or liquids. But what if the stepper motor suffers from resonance? Again, several different techniques are available to damp stepper-motor resonance. But before exploring those methods and how they cut resonance, it helps to understand how resonance originates.
Every stepmotor has a resonance spot. For each step that the motor takes, the rotating part, or rotor, oscillates around the new position before coming to a stop. The amount of time the motor oscillates is called its settling time. The rate of oscillation matches the resonant frequency with every step that the motor takes. The result is a motor that vibrates and has jitter. Its resonant frequency comes from the relationship between torque stiffness and inertia; changing either parameter changes the resonant frequency. For smooth operation, engineers try to design motors with resonant frequencies higher or lower than the operational envelope.
Most troubleshooting guides recommend modifying several factors to fit the application. For example, raising or lowering operating voltage or current, adding inertial load, or using a higher step resolution shifts the resonant frequency of the motor.
However, it may not be possible to change operating parameters enough to improve performance without jeopardizing the overall design. In such situations it might be best to go with an inertial dampening device or damper. Each damper type has its own pros and cons, but most effectively improve stepmotor performance in certain situations. The most common dampers are the rear-mounted inertial damper and the internal design damper.
Rear-mounted dampers essentially add an extra inertial load to the motor shaft which, in turn, changes the resonant frequency. As resonant frequency changes, the resonance spot shifts to a lower speed range. This effectively reduces vibration or jittery movement at mid to high speeds. Furthermore, it appears that rear-mounted dampers also decrease resonant spikes throughout the operational speed range of a motor.
A rear-mounted damper can significantly reduce the amount of resonance. Although these dampers are designed to be sleek and small, some systems lack the space for this type of damper. But even in designs with adequate space they tend to be an unattractive option because of the larger size and added cost of the dmper. Those situations might call for an internal damper.
One internal technique developed by Lin Engineering is to let the motor act as its own damper. This is done by placing an electromagnetic drag on the rotor through a special winding and hookup design called an R-winding and T-connection. Stepmotors rotate whenever the motor control receives a step pulse. However, the pulse that steps the motor never actually reaches the motor. Instead it controls power applied to two phase windings in the motor called Phase 1 and Phase 2. If both phases are powered, then the direction of rotation is controlled by which phase is turned off. Each step corresponds to a motor rotation of 0.45°, 0.9°, or 1.8°. The off phase is powered with opposite polarity for the third step, while the other phase is deenergized for the fourth step. The sequence the motor steps through is such that half of the motor is energized for one step, then both halves of the motor energize for the next. The one-on, two-on sequence sets up an imbalance in the motor that creates noise, vibration, and resonance.
Settling times for the one-phase-on period are shorter than the times with two phases powered. A motor with only one phase does not rotate. However, by keeping both phases powered, settling times for both parts of the sequence remain the same and are easily damped. The Tconnection and R-winding were created to eliminate the one-phase on position.
The T-connection forces the motor to always energize both phases. It uses a ratio of the two phases so the system needs no extra energy nor creates additional heat, yet the motor remains in a two-phases-on state at all times.
Furthermore, the T-connection changes the motor’s electrical characteristics such that the inductance level falls between a series and parallel connection. This connection uses less current and produces more torque at midrange speeds. Under normal wiring, series connections work best at low speeds and parallel connections at high speeds. However, the midrange speeds were always poorly served by both connection types. The T-connection fills that gap.
The R-winding also forces the motor to always step in a two-phase-on state. This is an internal design built into the windings rather than made at the outer connections as with the T-connection. Specific ratios of both phases are turned on as well, but still result in a two-phase-on state during each step. Internally, both phases are wound and connected such that electrically, there is a 22.5° shift.
All in all, stepmotors will always have resonance. The key to eliminate its effects lies in either controlling where the resonant point falls or in reducing its severity.
Figure A – Series connections work well at low speeds, while parallel connections provide good high-speed operation. However, neither provides proper operation at midrange speeds. The T-connection with R-winding by Lin Engineering optimizes midrange operations while reducing noise and jitter by maintaining current thourgh both phase windings at all times.