38 n PRECISION ENGINEERING AND MOTION CONTROL October 2025 www.drivesncontrols.com Understanding the Bode Plot in servo controls When controlling servo systems containing servomotors and servodrives, critical factors include the gain plot and the phase shift plot. The former shows how much the system amplifies or reduces an input signal at different frequencies; the latter explains how much the output lags or leads the input signal. These plots are named after their inventor Hendrik Bode, an engineer at Bell Labs in the US, who made his discovery in the 1930s. The purpose of a Bode Plot in servo controls is to check accuracy at low speed and ensure stability at high speed. Crucially, the plot is also used to prevent oscillations and instability, while optimising the speed of response to the input signal. To achieve this, a Bode Plot can tell us how far gain can be increased, and phase shifted, before instability occurs. It can also tell us the crossover frequency, where the system transitions from stable to unstable behaviour. To achieve a Bode Plot, measurements of amplitude (dB) and phase (degrees) are plotted on a logarithmic scale across a tested frequency range. While powerful microprocessors in digital drives can now replace the need for calculations, an understanding what a Bode Plot shows can still be crucial for designing, tuning and troubleshooting a servo system. This tried-and-tested approach can help an engineer to understand the performance they need, and what is possible, including the response rate balanced against instability, as well as the level of movement precision. A Bode Plot can also help with the diagnosis of discrepancies in motion performance, and can be particularly useful when customising servo systems for applications. What a Bode Plot reveals From the key factors revealed by the Bode Plot, it’s possible to gauge a system’s bandwidth, relating directly to its productivity potential. The bandwidth indicates the settling time of the mechanism, meaning the higher the bandwidth, the faster the time to settle to a commanded speed or position. The Bode Plot also confirms system stability, showing whether it can operate smoothly during motion or at rest. Phase and gain margins are useful indicators of system stability, where the larger the margins, the more stable the system. When designing motion systems, the resonant frequencies linked to the mechanical compliance throughout the system must also be considered. Each mechanical element of a system, such as the motor or gears, has its own natural resonance frequency, including both an anti-resonance and a resonance point. The system element with the lowest frequency is the most critical because a bandwidth higher than this level cannot be achieved. Using a Bode Plot helps to understand how the system can be optimised through tuning. The Bode Plot can also show the load-toinertia ratio magnitude. The difference in frequency between the first point of antiresonance and the first point of resonance, is proportional to the inertia ratio. The greater the difference, the greater the load-to-motor inertia ratio. The stiffness and load-to-motor inertia relationship is critical because it impacts the system’s ability to respond quickly and maintain stability to balance precision and vibration. The results of the Bode Plot can inform where and how a servo system can be optimised with tuning techniques. Applying a combination of filters, the amplitude and phase values can be changed to improve phase and gain margins, expand bandwidth, or address resonance issues. Intelligent servodrives – such as Kollmorgen’s AKD or AKD2G, used in combination with AKM or AKM2G servomotors – now include auto-tuning capabilities that can adjust key parameters, while balancing compensating factors, to optimise performance. As a result, the drive can adjust control loops automatically to achieve a balance between speed, accuracy, and stability by measuring how the motor and load respond to commands. While auto-tuning can simplify the process of balancing gain and phase shift across frequencies, understanding the Bode Plot remains important to fine-tune a system, especially for complex applications, and it can also speed up diagnostic challenges. n Optimising servo factors such as gain and phase shift is critical to precision control. While microprocessors can help to tune servo systems, traditional approaches to understanding system behaviour can still be vital. Inmoco engineer, Gerard Bush, explains how Bode Plots allow fine-tuning of complex servo systems and can enhance diagnostics. In a perfect system the amplitude plot will have a straight negative slope at –20dB/ decade. The phase should start at –90 degrees and drop at a negative slope from the point the amplitude crosses zero dB.
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