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In control theory, input shaping is an open-loop control technique for reducing vibrations in computer-controlled machines. The method works by creating a command signal that cancels its own vibration. That is, a vibration excited by previous parts of the command signal is cancelled by vibration excited by latter parts of the command.

Input shaping is implemented by convolving a sequence of impulses, known as an input shaper, with any arbitrary command. The shaped command that results from the convolution is then used to drive the system.

If the impulses in the shaper are chosen correctly, then the shaped command will excite less residual vibration than the unshaped command. The amplitudes and time locations of the impulses are obtained from the system's natural frequencies and damping ratios. Shaping can be made very robust to errors in the system parameters.^[1]

Example

[edit]

As an example, consider a linear system with a resonant frequency of 1Hz. If a step input is applied, the system will oscillate at 1 hertz for a length of time determined by the damping associated with the resonant mode. For some systems, such as cranes, the oscillation can last for minutes, while a comparable rigid system could be maneuvered in seconds.

However, if the original step input is applied at half magnitude, and again at half magnitude 0.5 seconds later (which for 1 Hz is half the period of the resonant frequency), the resulting oscillations in the system are out of phase and cancel entirely. This applies to both acceleration and deceleration of the system.

References

[edit]

^ Rush D. Robinett; Rush D. Robinett III; John Feddema; G. Richard Eisler; Clark Dohrmann; Gordon G. Parker; David G. Wilson; Dennis Stokes (2001). Flexible Robot Dynamics and Controls. Springer. ISBN 0-306-46724-0.

External links

[edit]

Input shaping simulator demonstrates the filter principle on a gantry crane control problem.

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Input shaping is a feedforward control technique in engineering that modifies command inputs to flexible mechanical systems in order to minimize or eliminate residual vibrations caused by motion-induced oscillations.^[1] By convolving the desired input signal with a sequence of impulses—each with specific amplitudes and time delays calculated based on the system's natural frequencies and damping ratios—the shaped command ensures that the vector sum of vibration contributions cancels out, resulting in smoother and faster settling times without exciting the system's resonant modes.^[2] The origins of input shaping trace back to posicast control, introduced by O.J.M. Smith in 1957 as a method to achieve deadbeat response in lightly damped oscillatory systems by splitting the input into phased segments that counteract ringing.^[3] This approach was significantly advanced in 1990 by N.C. Singer and W.P. Seering, who developed the modern framework of input shaping, demonstrating its effectiveness in preshaping commands to reduce endpoint vibrations in flexible structures while maintaining simplicity in implementation.^[4] Subsequent refinements, including robustness enhancements to handle modeling errors and parameter uncertainties, have made the technique widely applicable across various domains. Key variants of input shaping include the zero-vibration (ZV) shaper, which uses a minimal two-impulse sequence to theoretically eliminate vibration for precisely known parameters, and the zero-vibration derivative (ZVD) shaper, which adds a third impulse to improve insensitivity to errors in natural frequency estimates by constraining the first derivative of the residual vibration curve.^[2] More advanced forms, such as specified-insensitivity (SI) shapers, limit residual vibration to a tolerable level (e.g., under 5%) across a range of uncertain frequencies through iterative optimization or frequency sampling methods.^[2] These techniques can be combined with feedback control or time-optimal bang-bang profiles to further enhance performance in dynamic environments. Input shaping finds extensive use in applications requiring precise and vibration-free motion, such as overhead cranes to prevent payload swinging, spacecraft attitude maneuvers to avoid structural flexing, industrial robots for faster point-to-point positioning, and precision manufacturing equipment like wafer steppers and coordinate measuring machines to reduce settling times and improve accuracy.^[2] Its advantages include low computational overhead, ease of integration into existing controllers, and effectiveness even in underactuated systems, making it a staple in modern vibration suppression strategies.^[4]

Overview

Definition and Principles

Input shaping is a feedforward control method designed to suppress residual vibrations in dynamically flexible systems by modifying the input command signals prior to their application to the system.^[2] Unlike feedback control approaches that react to detected vibrations, input shaping proactively alters the trajectory to prevent oscillatory responses, making it suitable for systems where precise positioning and minimal settling time are critical, such as robotics and precision machinery.^[5] The core principle involves convolving the desired reference trajectory—such as a position or velocity command—with a discrete sequence of impulses, collectively termed the input shaper.^[2] These impulses are carefully timed and scaled in amplitude so that the vibrations they induce in the flexible modes of the system interfere destructively, resulting in a net response that is free of oscillations at the end of the commanded motion.^[5] This cancellation ensures that the system's output settles quickly to the target without ringing, enhancing overall performance while maintaining robustness to minor modeling uncertainties in parameters like natural frequency or damping.^[2] A representative example illustrates this mechanism in a second-order underdamped system, which models many flexible structures with inherent oscillatory behavior due to low damping ratios (typically ζ < 1).^[5] For a step input, the unshaped command produces overshoot followed by decaying ringing as the system oscillates around the setpoint. Applying input shaping convolves the step with impulses positioned at intervals related to the system's natural period (e.g., at 0, half-period, and full period), causing the induced vibrations to cancel each other out and yielding a critically damped-like response that reaches the final position smoothly without overshoot or subsequent oscillations.^[5] This approach originated within control theory applications for computer-numerical-control (CNC) machines and similar precision equipment, where achieving zero residual vibration at motion completion is essential for accuracy and throughput.^[2] By focusing on open-loop modification of commands, input shaping provides a computationally efficient means to handle flexible dynamics without requiring real-time sensors or complex feedback loops.^[5]

Historical Context

The origins of input shaping trace back to the late 1950s with the development of posicast control, a feedforward technique introduced by Otto J. M. Smith to suppress oscillations in lightly damped systems.^[6] Smith's method, detailed in his 1957 paper, involved splitting and delaying command inputs to cancel vibratory responses, achieving deadbeat settling in oscillatory feedback loops without requiring feedback modifications.^[6] This approach laid foundational principles for later vibration reduction strategies, though it was initially limited to single-mode systems with precise damping knowledge. A significant milestone occurred in the 1980s when researchers at MIT, including Neil C. Singer and Warren P. Seering, advanced the concept into modern input shaping by introducing zero-vibration shapers for flexible structures.^[5] Their work, building on posicast ideas, used convolved impulses to generate commands that minimized residual vibrations in multi-degree-of-freedom systems, with early publications appearing in the late 1980s and early 1990s.^[7] This innovation shifted focus from simple delay-based methods to more versatile, open-loop techniques applicable to precision motion control. In the 1990s, input shaping evolved to address multi-mode systems and robustness against modeling uncertainties, with extensions like specified-insensitivity shapers ensuring vibration suppression despite parameter variations.^[8] These developments enabled practical implementations. By the 2000s, the technique integrated into real-time robotics applications, enhancing trajectory planning for industrial arms and coordinate measuring machines to achieve faster, vibration-free operations.^[9] Post-2010, input shaping saw adoption in consumer technologies, notably through resonance compensation features in open-source 3D printer firmware like Klipper, which implemented shaper algorithms in 2020 to mitigate print artifacts from mechanical resonances.^[10] This marked a broader democratization of the method beyond aerospace and industrial domains.

Theoretical Foundations

Modeling Vibrations in Flexible Systems

Flexible structures in dynamic systems, such as those encountered in robotics and precision machinery, are typically modeled as multi-degree-of-freedom (MDOF) systems. These systems feature multiple natural frequencies, each corresponding to a vibrational mode, along with associated damping ratios that dictate the decay rate of oscillations. The flexibility introduces underdamped behavior in many practical cases, where energy dissipation is insufficient to prevent prolonged vibrations, necessitating control strategies to mitigate unwanted motion.^[2]^[11] The foundational mathematical model for vibrations in these systems focuses on individual modes, approximated as second-order linear oscillators. For a single mode, the governing equation is

m \ddot{x} + c \dot{x} + k x = f(t),

where

m

represents the effective modal mass,

c

the modal damping coefficient,

k

the modal stiffness,

x

the modal displacement, and

f(t)

the applied input force. This differential equation captures the essential dynamics, with the natural frequency given by

\omega_n = \sqrt{k/m}

ωn=k/m

and the damping ratio by $\zeta = c / (2 \sqrt{km})$

). In MDOF systems, the overall response is a superposition of such modal contributions, obtained through modal analysis.^[11]^[12] Residual vibrations in these flexible systems primarily stem from abrupt input commands, such as step changes in force or velocity, which excite the natural modes. In underdamped configurations where $\zeta < 1$ , these excitations result in decaying sinusoidal oscillations that persist after the command, degrading positioning accuracy and efficiency. The amplitude and duration of these residual vibrations depend directly on the system's natural frequencies and damping characteristics.^[1]^[2] Central to addressing these vibrations are the mode shapes and natural frequencies, which define the spatial distribution of deformation and the temporal periodicity of oscillations, respectively. These parameters serve as essential inputs for control design, enabling targeted cancellation of modal responses. Flexible modes frequently manifest in applications like robot arms, where elastic joints allow relative motion between links, or in slender beams, where deflection under load introduces bending vibrations. For instance, experimental validation on flexible manipulators has demonstrated significant vibration reduction when models account for these characteristics.^[11]^[4]

Open-Loop Feedforward Control

Open-loop feedforward control preemptively modifies input commands using a precise model of the system dynamics to achieve the desired output, operating without any feedback from the process. This approach generates control actions offline based on anticipated system behavior, ensuring that the shaped input directly counters unwanted effects like oscillations in flexible structures. Pioneered in vibration reduction techniques, it alters the reference signal before it reaches the actuator, relying solely on the accuracy of the predictive model for effectiveness.^[1] In contrast to closed-loop control, which incorporates real-time sensor feedback to correct errors and maintain stability against disturbances, open-loop feedforward methods do not perform ongoing adjustments and thus depend entirely on the fidelity of the system model for reliable performance. Without feedback, stability is inherently guaranteed if the model is exact, but the system becomes vulnerable to unmodeled dynamics or external perturbations that could amplify errors. This paradigm is particularly advantageous in environments where feedback sensors would introduce noise or complexity, prioritizing predictive accuracy over adaptive correction.^[2]^[13] Within this framework, input shaping serves as a specialized feedforward technique that pre-filters command profiles to cancel predicted vibrations in systems prone to oscillatory responses, such as flexible mechanical linkages or lightweight manipulators. By designing the input to avoid exciting natural frequencies, it ensures minimal residual motion at the end of repeatable maneuvers, like point-to-point positioning, without requiring online computation or sensing. This method is especially suited to deterministic tasks where system parameters remain consistent, allowing offline optimization of the shaping process.^[1]^[7] The practical benefits of input shaping as an open-loop feedforward strategy include its inherent simplicity, which eliminates the need for additional hardware like feedback sensors and mitigates issues such as noise amplification or latency in high-bandwidth loops. It also provides computational efficiency, as the shaping can be precomputed and applied rapidly, enabling faster overall system response in high-speed applications without compromising precision. These attributes make it a lightweight enhancement for control architectures in domains requiring quick settling times.^[2]^[13] Input shaping constitutes a focused subset of broader command generation methods in control engineering, emphasizing vibration mitigation through input modification rather than holistic trajectory optimization, which typically addresses path smoothness without targeting specific dynamic resonances. This distinction positions it as a complementary tool for enhancing feedforward performance in vibration-sensitive systems.^[7]

Core Techniques

Impulse-Based Shaping

Impulse-based shaping forms the core of input shaping techniques, where the shaper is constructed as a finite sequence of impulses, each modeled as a scaled Dirac delta function

\delta(t - t_i)

with amplitude

A_i

at time

t_i

. This approach, originally developed to preshape command inputs for flexible systems, ensures that the convolved output cancels residual vibrations by strategically timing and scaling these impulses. The timing of the impulses is determined by the natural period

T = \frac{2\pi}{\omega_n}

of the system's dominant vibrational mode, where

\omega_n

represents the natural frequency; impulses are typically spaced at multiples or fractions of this period to achieve phase opposition for cancellation. For effective vibration suppression in a single mode, the amplitudes

A_i

must satisfy two key constraints: they sum to unity (

\sum A_i = 1

) to preserve the steady-state command value, and they are computed using trigonometric expressions that account for the system's damping ratio

\zeta

, such as factors involving

\cos(\omega_d t_i)

and exponential decay terms

e^{-\zeta \omega_n t_i}

, where

\omega_d = \omega_n \sqrt{1 - \zeta^2}

ωd=ωn1−ζ2

. A minimum of two impulses is required to cancel vibrations in a single-mode system, as a single impulse cannot produce the necessary phase shift for destructive interference without altering the overall command magnitude. To facilitate the design of these impulse sequences, the concept of impulse vectors has been introduced as a mathematical tool, representing each impulse as a vector in a complex parameter space (with magnitude $A_i$ and phase related to $t_i$ ); the shaper is then designed by ensuring the vector sum closes to the origin for zero residual vibration. This vector representation enables graphical analysis and optimization of shaper parameters for various system characteristics.^[14]

Convolution Process

The convolution process in input shaping modifies the original command signal

v(t)

to produce a shaped input

u(t)

that suppresses residual vibrations in flexible systems. This is achieved by convolving

v(t)

with an impulse train

h(t)

, defined as

u(t) = v(t) * h(t)

, where

h(t) = \sum_{i=1}^{n} A_i \delta(t - t_i)

and

\delta

denotes the Dirac delta function.^[1] The resulting shaped input leverages the superposition principle to cancel oscillatory responses at the system's natural frequencies.^[4] Due to the finite nature of the impulse sequence, the convolution simplifies to a weighted sum of time-shifted versions of the original command:

u(t) = \sum_{i=1}^{n} A_i v(t - t_i),

where

A_i

are the impulse amplitudes (summing to 1 to preserve the steady-state value) and

t_i

are the corresponding impulse times.^[1] In practice, this process is implemented digitally by sampling the command signal at discrete time steps

\Delta t

, approximating the continuous convolution as

u(k \Delta t) = \sum_{i=1}^{n} A_i v((k - m_i) \Delta t)

, where

m_i = t_i / \Delta t

are integer delays. This discrete form allows real-time computation using finite impulse response (FIR) filters, enabling straightforward integration into motion control systems without requiring feedback modifications.^[2] The step-by-step application begins with generating the impulse train based on system parameters, followed by applying the time shifts and amplitude scalings to segments of

v(t)

, and finally superimposing these scaled and delayed signals to form

u(t)

. Each impulse response contributes to the total system output, and their vector sum at the end-point is designed to yield zero vibration amplitude.^[4] This convolution extends the trajectory duration by the time span of the last impulse

t_n

, typically on the order of half to one period of the dominant vibration mode, thereby increasing rise time compared to the unshaped command. However, it eliminates end-point residual vibrations, often reducing them by factors of 10 or more in experimental validations.^[1]^[2] Exact vibration cancellation via convolution relies on the assumption that the underlying system is linear and time-invariant, ensuring the principle of superposition holds for the impulse responses.^[4] Deviations from linearity, such as nonlinear friction or varying payloads, can degrade performance, though robust shaper designs mitigate modeling errors to some extent.^[2]

Types of Input Shapers

Zero-Vibration Shaper

The Zero-Vibration (ZV) shaper is the foundational input shaping technique designed to eliminate residual oscillations in lightly damped flexible systems by ensuring that the commanded input excites no vibration at the modeled natural frequency and damping ratio. Introduced in the seminal work on preshaping commands, it uses a minimal set of two impulses to achieve perfect cancellation under exact modeling conditions, making it optimal for systems with precisely known parameters.^[4]^[15] The shaper is applied through convolution with the desired command profile, resulting in a smoothed input that reaches the setpoint without overshoot or ringing.^[2] The design of the ZV shaper places impulses at times

t_1 = 0

and

t_2 = \Delta t = \pi / \omega_d

, where

\omega_d = \omega_n \sqrt{1 - \zeta^2}

ωd=ωn1−ζ2

is the damped natural frequency, $\omega_n$ is the undamped natural frequency, and $\zeta$ is the damping ratio. The corresponding amplitudes are given by $A_1 = \frac{1}{1 + K}, \quad A_2 = \frac{K}{1 + K},$ where $K = e^{-\zeta \omega_n \Delta t}$ . This configuration ensures unity gain for steady-state commands while canceling dynamic response at the targeted mode. For undamped systems where $\zeta = 0$ , $K = 1$ , yielding equal amplitudes $A_1 = A_2 = 0.5$ separated by half the natural period $\pi / \omega_n$ .^[2]^[5] The derivation relies on two key constraint equations derived from the system's second-order dynamics. First, the sum of amplitudes must equal unity for accurate setpoint tracking: $A_1 + A_2 = 1$ . Second, the residual vibration amplitude must be zero, which requires the phasor sum at the complex pole to vanish. This leads to the condition on the real part (with the imaginary part inherently zero due to the $\pi$ -phase shift at $\Delta t$ ): $A_1 + A_2 K \cos(\omega_d \Delta t) = A_1 - A_2 K = 0,$ since $\cos(\pi) = -1$ . Solving these simultaneously yields the amplitudes above. Graphically, this can be analyzed in the time-amplitude plane, where the first impulse at $(0, A_1)$ defines a constraint curve—typically a locus ensuring phase opposition and amplitude balance—for the second impulse's position $(t_2, A_2)$ , with the intersection providing the unique solution for minimal duration.^[2]^[4]^[15] Despite its effectiveness under ideal conditions, the ZV shaper has notable limitations. It exhibits high sensitivity to modeling errors, particularly in $\omega_n$ and $\zeta$ ; even small deviations (e.g., 5% in frequency) can cause residual vibrations to exceed 10% of unshaped levels, as shown in sensitivity curves plotting vibration amplitude against parameter uncertainty. Additionally, the shaper extends the command duration by $\Delta t$ , increasing the system's rise time by approximately 50% compared to unshaped inputs, which can limit throughput in time-critical applications. These drawbacks motivate more robust variants for practical deployment.^[2]^[5]

Zero-Vibration-Derivative Shaper

The Zero-Vibration-Derivative (ZVD) shaper improves upon the Zero-Vibration (ZV) shaper by incorporating an additional constraint that sets the derivative of the residual vibration amplitude with respect to frequency to zero at the nominal modeling parameters, thereby enhancing robustness to modeling errors while reducing jerk at the end of motion for smoother settling. This design achieves zero residual vibration while setting the first derivative of the residual vibration amplitude with respect to the natural frequency to zero at the nominal parameters, thereby enhancing robustness to small modeling errors, making it suitable for applications requiring precise positioning in flexible systems. Introduced by Singer and Seering in 1990, the ZVD shaper addresses limitations in the ZV method by providing better tolerance to parameter variations without significantly compromising vibration suppression.^[4] The ZVD shaper employs a three-impulse sequence with impulses at times

t_1 = 0

t_2 = \Delta t

, and

t_3 = 2 \Delta t

, where

\Delta t = \frac{\pi}{\omega_n \sqrt{1 - \zeta^2}}

Δt=ωn1−ζ2

π and $\omega_n$ is the natural frequency. The amplitudes $A_1$ , $A_2$ , and $A_3$ are determined by solving the system of equations that enforce the zero residual conditions: $\begin{align} A_1 &= \frac{1}{1 + 2K + K^2}, \\ A_2 &= \frac{2K}{1 + 2K + K^2}, \\ A_3 &= \frac{K^2}{1 + 2K + K^2}, \end{align}$ where $K = e^{-\frac{\zeta \pi}{\sqrt{1 - \zeta^2}}}$

ζπ and $\zeta$ is the damping ratio. These amplitudes ensure the convolved input cancels oscillatory components up to the first derivative, with the sequence summing to unity for amplitude preservation.^[4] By distributing the impulses more evenly than the two-impulse ZV shaper, the ZVD produces a smoother command profile that mitigates high jerk values at motion completion, facilitating faster and more stable settling in low-damping systems. It also reduces sensitivity to frequency uncertainties by approximately a factor of five compared to the ZV shaper, allowing up to ±14% parameter variation with less than 5% residual vibration.^[2] A primary drawback of the ZVD shaper is its longer duration—one full damped period versus half a period for the ZV shaper—which increases the overall rise time compared to the ZV shaper due to its longer duration.^[4]

Multi-Mode and Robust Shapers

Multi-mode input shapers address systems exhibiting multiple natural frequencies of vibration, such as flexible structures with coupled modes, by designing impulse sequences that suppress oscillations across several modes simultaneously. These shapers often employ multi-hump configurations, where the robustness curve features multiple peaks (or "humps") to accommodate parameter variations while maintaining a specified duration. For coupled modes, the design leverages mode superposition to ensure the shaper cancels vibrations from interacting frequencies without requiring separate shapers for each mode.^[16] Robust input shapers extend this capability by incorporating greater tolerance to modeling uncertainties, such as errors in natural frequency or damping ratio. The extra-insensitive (EI) shaper, for instance, relaxes the zero-vibration constraints of earlier designs like the zero-vibration-derivative (ZVD) shaper to achieve wider insensitivity regions, allowing up to 20% error in frequency modeling compared to the ZV shaper's 5% tolerance.^[17] These robust shapers are typically designed using minimax optimization, which minimizes the maximum residual vibration over an ellipsoid of parameter uncertainties, such as variations in frequency

\omega

and damping

\zeta

. The key robustness metric is defined as

R = \max_{\omega, \zeta} |V(\omega, \zeta)|,

where

V(\omega, \zeta)

represents the magnitude of the vector diagram for the system's residual vibration amplitude, and the optimization adjusts shaper parameters to achieve the desired insensitivity level. This approach ensures high performance even under significant modeling errors, prioritizing seminal methods that balance speed and reliability in flexible systems.^[18]

Design and Analysis

Parameter Identification

Parameter identification in input shaping involves estimating key system parameters, primarily the natural frequency (

\omega_n

) and damping ratio (

\zeta

), which are crucial for designing effective shapers that cancel residual vibrations in flexible systems. These parameters define the oscillatory behavior modeled in prior sections, such as the underdamped second-order response where vibrations decay exponentially while oscillating at

\omega_n \sqrt{1 - \zeta^2}

ωn1−ζ2

. Accurate estimation ensures the shaper impulses are timed and scaled to counteract mode excitations, but inaccuracies can lead to incomplete vibration suppression or overcompensation.^[19] Common techniques include frequency response testing, such as sine sweep excitation, where a sinusoidal input sweeps across a frequency range to reveal resonance peaks corresponding to $\omega_n$ . This method identifies modal frequencies by observing amplitude amplification at natural modes, often using accelerometers or encoders to capture the system's response. Step response analysis complements this by applying a sudden input change and examining the transient output: the oscillation period yields $\omega_n$ , while the logarithmic decrement of successive peaks provides $\zeta$ via $\zeta = \frac{\delta}{2\pi}$ , where $\delta = \ln\left(\frac{A_i}{A_{i+1}}\right)$ for peak amplitudes $A_i$ and $A_{i+1}$ . These approaches are typically performed offline to avoid real-time disruptions, requiring controlled test setups like fixed payloads on robotic arms or beams.^[5]^[20] Autotuning methods enhance automation through adaptive algorithms that process sensor data, such as accelerometer measurements of ringing frequency and damping during motion tests. For instance, the fast Fourier transform (FFT) applied to step or impulse responses extracts $\omega_n$ by identifying dominant spectral peaks, filtering noise to isolate modal components. Least-squares fitting refines $\zeta$ by minimizing the error between the observed decay envelope and a fitted exponential model, often solving for poles in the system's transfer function. In advanced cases, extended Kalman filters (EKF) recursively estimate parameters by fusing measurement noise models with predicted dynamics, improving robustness to variations.^[20]^[5]^[21] Challenges arise from nonlinear effects in real systems, such as varying stiffness due to payload changes or friction, which distort linear assumptions and complicate parameter extraction—leading to detuned shapers that amplify vibrations at off-nominal conditions. Offline calibration mitigates this by isolating the system for precise testing but demands periodic recalibration as parameters drift with wear or environmental factors. In 3D printing applications, tools like Klipper's ring analyzer automate identification using printed test patterns, such as ringing towers, where oscillation counts and distances at varying accelerations compute resonant frequencies without hardware sensors.^[20]^[22]^[10]

Performance Metrics and Evaluation

The primary metric for evaluating the effectiveness of input shapers is the percent residual vibration, defined as

\%V = 100 \times |V(\omega_d, \zeta)|

, where

V(\omega_d, \zeta)

represents the normalized amplitude of the residual vibration resulting from the shaped input applied to the system,

\omega_d = \omega \sqrt{1 - \zeta^2}

ωd=ω1−ζ2

is the damped natural frequency, $\omega$ is the natural frequency, and $\zeta$ is the damping ratio.^[2] This measure quantifies the reduction in oscillatory amplitude relative to an unshaped command, with ideal shapers targeting $\%V < 5\%$ under nominal conditions, though robust variants maintain levels below 1% across parameter uncertainties.^[5] Another key metric is the rise time increase, which accounts for the added duration of the shaper—typically 50% of the natural period for zero-vibration (ZV) shapers and 100% for zero-vibration-derivative (ZVD) shapers—extending the overall command execution time to suppress vibrations.^[1] Evaluation of input shapers combines simulation and experimental approaches to verify performance. In simulations, often conducted in the Laplace domain, the residual vibration amplitude $V$ is computed analytically by evaluating the shaper's impulse response across a range of frequencies and damping ratios, enabling rapid assessment of vibration suppression without physical hardware.^[2] Experimentally, ringing tests measure actual system responses using accelerometers mounted on the flexible structure to capture acceleration profiles during commanded motions; these tests compare shaped and unshaped inputs, revealing residual oscillations through time-domain plots or frequency spectra, with effective shapers showing near-complete elimination of post-motion ringing.^[5] Robustness is assessed by the maximum residual vibration over a specified range of parameter variations, such as ±20% in natural frequency or damping, often visualized through sensitivity curves that plot $\%V$ against modeling errors.^[2] These curves highlight trade-offs, where more robust shapers tolerate larger uncertainties before exceeding acceptable vibration thresholds (e.g., 5%). Comparisons between variants demonstrate that ZVD shapers reduce sensitivity to parameter errors by approximately 4 times compared to ZV shapers, achieving robustness to ±20% frequency variations versus ±5%, though this comes at the cost of doubled duration.^[1] Overall, well-designed shapers, such as ZVD, can achieve less than 1% residual vibration while incurring less than 10% increase in effective duration for many underdamped systems with moderate modeling accuracy.^[5]

Applications

Robotics and Manipulators

Input shaping has been widely applied in robotics to suppress vibrations in lightweight robotic arms, particularly during high-speed pick-and-place operations where flexible links can lead to payload oscillations that degrade precision and efficiency. By convolving the desired joint trajectories with an input shaper, residual vibrations are minimized, allowing for faster motion without compromising accuracy. In experimental studies on flexible manipulators, this technique has demonstrated reductions in settling time by 50-75%, enabling the system to reach and maintain the target position more quickly after command initiation.^[7] For instance, in a planar flexible manipulator model, settling times were reduced from approximately 5 seconds to under 1.5 seconds for small angular maneuvers.^[7] Implementation of input shaping in robotic manipulators typically involves real-time convolution of the shaper impulses with the reference joint trajectories generated by the motion planner. This feedforward approach is often hybridized with feedback controllers, such as proportional-derivative (PD) or linear quadratic regulator (LQR), to enhance tracking performance and robustness against modeling errors. The convolution process modifies the command profile to cancel oscillatory modes, with the shaper parameters derived from the system's natural frequencies and damping ratios identified through modal analysis or experimental tests. In industrial settings, this integration allows for seamless operation on standard robot controllers without requiring hardware modifications. A notable case study from the 1990s at MIT involved experiments on flexible beam-like manipulators simulating the Space Shuttle Remote Manipulator System (SRMS), where input shaping was tested on models with varying payloads to evaluate vibration suppression under changing dynamics. Zero-vibration (ZV) and zero-vibration-derivative (ZVD) shapers reduced settling times by up to 76% in scenarios with midsize payloads, from 38 seconds to 9 seconds for small slews, highlighting the technique's effectiveness in flexible structures.^[7] More recently, input shaping has been implemented in industrial SCARA robots for high-speed pick-and-place tasks, achieving up to 86% reduction in end-effector residual vibrations while minimally increasing motion duration by about 0.06 seconds per cycle.^[23] The primary benefits of input shaping in robotics include enabling higher operational speeds without inducing payload oscillations, which is critical for maintaining precision in tasks involving delicate components. This results in improved throughput and reduced wear on mechanical joints. Since the early 2000s, the technique has been adopted in automotive assembly lines, such as for wheel installation on moving vehicles, to achieve faster cycle times by damping oscillations during rapid maneuvers.^[24] For manipulators with multiple flexible modes, robust shapers can be briefly referenced to handle parameter uncertainties, ensuring reliable performance across varying configurations.^[25]

Additive Manufacturing

Input shaping has been adapted for additive manufacturing, particularly in fused deposition modeling (FDM) 3D printers, to compensate for mechanical resonances that cause print artifacts such as ghosting and ringing. These vibrations arise from rapid accelerations and decelerations during extrusion head movements, degrading surface quality on features like corners and curves. Firmware implementations like Klipper, introduced in 2020, and Marlin, which added support in version 2.1.2 (2022), apply input shaping to modify acceleration profiles, preemptively canceling oscillatory responses in the printer frame. This technique draws from established vibration control methods, enabling higher speeds while preserving dimensional accuracy and aesthetic finish.^[10]^[26] Implementation in 3D printing firmware is axis-specific, targeting X and Y movements where resonances are most pronounced, with tuning achieved through accelerometers like the ADXL345 or printed test patterns such as ringing towers. Klipper supports zero-vibration (ZV), multi-zero-vibration (MZV), and extra-insensitive (EI) shapers, selected based on measured resonance frequencies and desired robustness to modeling errors. Marlin primarily uses ZV shapers but extends to ZVD, EI, and others in later versions via fixed-time motion algorithms. Configuration involves embedding shaper parameters in the firmware's motion planner, which convolves command signals to produce vibration-free trajectories without hardware modifications. For example, in Klipper, the [input_shaper] section in the configuration file specifies shaper type and frequency per axis, applied during real-time step generation. Autotuning processes measure frame resonances, typically in the 20-60 Hz range for Cartesian printers but up to 200 Hz for lighter cores, by exciting the system with sinusoidal accelerations and analyzing response peaks. These frequencies inform shaper design, ensuring the modified input nullifies residual vibrations when applied to trapezoidal acceleration profiles.^[10]^[26]^[27] The impact of input shaping in additive manufacturing is significant, allowing 2-3 times higher accelerations—such as from 2000 mm/s² to 5000 mm/s²—without introducing artifacts, effectively enabling print times reduced by up to 50% for complex geometries while maintaining sub-0.1 mm accuracy. Experimental validation on cantilever-style printers demonstrates reduced corner bulging and ringing, with surface deviations minimized to 0.06 mm on test cubes. Printers from manufacturers like Prusa and Voron have integrated input shaping as a standard feature post-2020; Prusa's firmware 5.0.0 (2023) for the MK4 series includes it with PrusaSlicer profiles for speed-optimized printing, while Voron designs, reliant on Klipper, incorporate it via default tuning guides for coreXY kinematics. This adoption has become commonplace in open-source and commercial FDM systems, enhancing throughput in prototyping and small-batch production.^[10]^[28]^[29]^[30]

Overhead Cranes and Transport Systems

Input shaping is widely applied in overhead cranes, particularly bridge and gantry types, to suppress payload swing during point-to-point transportation maneuvers. By convolving the reference command with a series of impulses, the technique generates shaped velocity profiles that cancel oscillatory modes, ensuring residual sway is minimized without requiring feedback sensors. This approach is especially valuable in industrial settings where precise load positioning is critical to prevent collisions and maintain operational efficiency.^[31]^[32] Implementation typically involves combining input shapers with smooth acceleration profiles, such as S-curves, to limit jerk and enhance robustness. The zero-vibration-derivative (ZVD) shaper, which specifies zero vibration and zero vibration derivative at the modeled natural frequency, is commonly used to filter commands for trolley motion, effectively reducing peak sway amplitudes by approximately 84% in experimental setups. For container handling in ports, input shaping integrates with modern control architectures, including wireless systems for remote operator commands and automated transfer operations, allowing seamless sway-free movements across large spans.^[32]^[2]^[33] Notable case studies demonstrate significant performance gains. In port crane applications, shaped commands have enabled up to 50% faster container transfers by allowing full-speed operations without sway-induced delays, boosting overall productivity while maintaining precision. Similarly, input shaping was applied to the NASA Space Shuttle Remote Manipulator System (RMS) for arm extension maneuvers, where a robust three-impulse shaper reduced residual vibrations by a factor of 25 compared to unshaped inputs, improving energy efficiency by 20% during joint and Cartesian trajectory executions.^[34]^[35] A key challenge in these systems is handling variable payloads, which alter the pendulum natural frequency and can degrade shaper performance if not accounted for. Robust variants, such as multi-mode shapers, address this by designing for a range of frequencies, though they may slightly increase command duration. Input shaping has been adopted in shipyards and ports since the 1990s, following its initial development, leading to substantial safety improvements by minimizing sway-related accidents and incidental load contacts.^[36]^[37]^[34]

Advantages and Limitations

Key Benefits

Input shaping significantly reduces residual vibrations in flexible systems by convolving the command input with a series of impulses that cancel out oscillatory modes, achieving near-zero vibration residuals when system parameters are accurately modeled.^[4] This enables substantially faster settling times, with reductions up to 76% observed in simulations and experiments on flexible manipulators, allowing operations at higher speeds without compromising precision.^[38] In flexible systems, such as robotic arms, input shaping can cut overall cycle times by 25-50% in simulations by minimizing post-maneuver damping periods.^[5] A primary advantage of input shaping lies in its simplicity as a feedforward technique, requiring no additional sensors or hardware modifications—implementation occurs entirely through software by altering the input signal based on basic system parameters like natural frequency and damping ratio.^[39] This open-loop approach avoids the complexity and potential instability associated with high-gain feedback controllers, making it suitable for real-time applications with minimal computational overhead.^[5] Input shaping integrates seamlessly with existing feedback controllers, such as PID, to form hybrid systems that leverage the strengths of both: feedforward shaping preempts vibrations while feedback handles disturbances and model inaccuracies.^[40] This compatibility enhances robustness without requiring a complete overhaul of established control architectures.^[39] By effectively suppressing vibrations, input shaping reduces the need for overly stiff or heavily damped materials in system design, enabling the use of lighter, more compliant structures that lower overall mass and manufacturing costs.^[5] Such design flexibility can lead to energy savings and improved efficiency in applications like aerospace and robotics.^[4]

Challenges and Mitigation Strategies

One major challenge in input shaping is its sensitivity to inaccuracies in system parameters, such as natural frequency and damping ratio, where even small errors can lead to significant residual vibrations.^[2] This performance degradation arises because standard shapers like the Zero Vibration (ZV) type rely on precise modeling, resulting in rapid increases in vibration amplitude with parameter deviations.^[39] To mitigate this, robust shapers such as the Extra-Insensitive (EI) variant are employed, which constrain residual vibration to below 5% over a wider range of uncertainties by solving constrained optimization problems during design.^[2] For instance, EI shapers can tolerate up to 40% uncertainty in system parameters, though they typically increase command duration by approximately 100% compared to basic ZV shapers.^[39] Non-linear effects, including payload variations and friction, further complicate input shaping by altering system dynamics during operation, potentially invalidating the pre-designed shaper impulses.^[41] These issues can cause incomplete vibration suppression, especially in systems like manipulators where payload changes shift resonant frequencies.^[41] Mitigation strategies include adaptive input shaping, which re-tunes shaper parameters in real-time based on ongoing system identification, and hybrid approaches combining feedforward shaping with feedback control to handle dynamic non-linearities.^[41] Recent advances as of 2025, such as data-driven interpolation-based methods and optimized Zero Vibration Derivative-Derivative (ZVDD) shapers, further enhance adaptability for multi-axis and flexible systems.^[42]^[43] For example, integrating input shaping with proportional-integral-derivative (PID) feedback has been shown to enhance robustness in gantry cranes under varying loads and friction.^[44] The computational demands of input shaping pose another practical hurdle, particularly for real-time implementation where convolving the command signal with multi-impulse shapers must occur without introducing delays.^[2] Designing robust shapers requires solving non-linear constraint equations, which can be intensive for complex systems.^[2] Solutions involve pre-computing shaper impulses for known trajectories to offload processing, or leveraging hardware accelerators like field-programmable gate arrays (FPGAs) for efficient convolution in embedded applications.^[7] Despite these mitigations, input shaping has inherent limitations: as a feedforward technique, it is ineffective against unknown external disturbances that cannot be anticipated in the shaper design.^[39] Additionally, it cannot stabilize inherently unstable systems on its own and must be paired with stabilizing feedback controllers.^[2]

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