To correct oscillations in a machine,
mechanical design improvements and electronic filters are typically employed.
However, these solutions often deliver limited success. Engineers should consider
adding anti-resonance control and vibration suppression to their quiver of
solutions to correct such oscillation problems via the servo controller and
amplifier system.
Limitations
of Low-pass and Notch Filters
Because vibrations most often occur at
high frequencies, traditional vibration-filtering methods use low-pass and
notch filters. The low-pass filters attenuate the high-frequency response of
the servo-control system and thus limit the bandwidth of the servo system.
Low-pass filters are necessary, but end results would be better if they could remove
as little of the upper bandwidth as possible. The filter cutoff frequency can
range from 1 kHz to as high as 5 kHz.
Modern servos set the low-pass filter to automatically remove as little
as possible of the high-frequency response during execution of an auto-tuning
sequence.
A notch filter and a band-stop filter
control oscillations between 500-1000 Hz-a problematic range because it
contains much audible noise, has a long settling time and generates high levels
of vibration. This frequency range also exists within the usable portion of the
machine's bandwidth. A notch filter has an advantage over a low-pass filter in
that it preserves the high-frequency response of the servo system, which means
engineers can safely increase tuning gains. Using one or two notch filters
along with a low-pass filter can result in a servo system with a stable
response because the filters remove problem frequencies in the 500-1000Hz
range.
But determining the notch-filter
frequency and bandwidth creates a challenge. Engineers must set tuning gains in
stable proportions until they detect the machine's natural resonance, but
without causing extreme vibrations. Software tools with fast Fourier transform
(FFT) analysis capabilities make it easier to determine the machine's resonant
frequency.
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Servo-control systems now take this
process one step further through inclusion of the notch-filter frequency-determination
process in the amplifier. Just as a digital oscilloscope can measure the
frequency of an input signal, the amplifier circuit measures the resonant
frequency and sets the frequency and bandwidth of the notch filters
accordingly. The automatic setting of the notch filter serves as an additional
tool to reduce vibration. The notch filter works in conjunction with the
servo's low-pass filter. This new capability, however, is not found in all controllers
or servo amplifiers.
A low-pass filter and notch filter
suffice in many servo-control systems, but they have little effect when a
machine vibrates at frequencies between about 10 to 500 Hz. To counter this
problem, engineers must use advanced technologies such as anti-resonance
control and vibration suppression.
Anti-resonance
Control and Vibration Suppression
Using Yaskawa's Sigma-5 servo amplifier
as an example, the "Anti-resonance Control" graphic illustrates how the
amplifier's anti-resonance control reduces audible and visible (as seen on the
graph) noise. The vibration-suppression function reduces oscillation at the end
of a positioning move, thereby improving position settling time, as noted in "Vibration
Suppression" graphic.
Anti-resonance control uses speed and
torque signals to identify a resonant frequency as the motor moves. The control
circuit then applies a correction to the speed-feedback signal, measures the
resonant frequency and automatically applies that information to the amplifier.
Alternately, an operator can override this measured parameter and set the
frequency manually. If desired, the user also can manually adjust parameters
such as gain compensation, damping gain and filter time constants to further
enhance servo-control performance.
The vibration-suppression function
implements a position-reference filter as well as an active position
compensator based on feedback of frequency parameters. This function calculates
the correct compensation command and applies it at the right time based on the
frequency parameters and according to a proprietary algorithm. Four parameters
are automatically detected by the servo-amplifier algorithm and set during
advanced auto-tuning, or an engineer can set the parameters manually. This
function produces a compensated position reference command at the end of the
move, which compensates for the previously detected vibration and brings the
load to a clean stop.
By reducing vibrations when movement
stops, many machines experience a shorter settling time. This benefit, though,
becomes most useful in systems with complex and non-rigid loads. Gantries with
overhanging loads are good examples of such systems.
Three other applications where these
techniques have proved particularly useful are in retrofits, in dealing with
unexpected machine mechanical performance issues and in OEM design optimization.
Bill
Leang is the manager of motion engineering at Yaskawa and has more than 20
years of experience in the design and application of servo systems. Matt
Pelletier, product training engineer at Yaskawa, has more than 10 years of
experience in servo systems.
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