R+W Coupling Technology recently proposed a system on how to properly select your servo couplings. They state that servo systems require mechanical components with high torsional stiffness in order to perform properly in applications requiring rapid acceleration and deceleration of high inertia loads. Flexible couplings usually have the lowest torsional stiffness of any component in a motion system. Couplings are often selected based on factors other than torsional stiffness, often to the detriment of system performance. Proper servo coupling selection can pay off when considering the overall picture.
R+W also say engineers go to great lengths to ensure that inertia mismatch between the load and the servo motor is compensated for. Motors and gearheads must be selected in order to ensure the ability of the drive to be able to accelerate the load with ease. The mechanical connection between the drive and the load can however unvaryingly compromise the efforts of the drive system. The most compliant component in the mechanical system (e.g. the coupling) will be twisted back and forth by the settling motion of the load at any major velocity change. The formula provided by R+W for calculating torsional deflection based on load and stiffness is as follows:
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f | = | torsional deflection (degrees) |
| TAS | = | peak torque (Nm) | |
| CT | = | torsional stiffness of coupling (Nm/rad) |
Depending on the inertia of the load and its effect on peak torque, this can happen to varying degrees. In any case, more power is required in order to accelerate the load at the desired rate when a less rigid component is installed between the drive and the load. According to R+W, this may or may not pose a concern depending on the application, and tends to be of higher importance in cases with a high inertia load that must be rapidly indexed.
When tuning servo drives, R+W claims that velocity and position feedback loops must be set to a low enough frequency so as not to excite the most torsionally compliant component in the system by reaching its natural frequency. Higher coupling stiffness leads to a higher natural frequency of the entire system, which means that feedback loops can be set to a higher frequency. This leads to a faster moving, more accurate machine, and ultimately higher throughput and higher quality.
A commonly used calculation by R+W for determining required coupling stiffness, and / or maximum drive frequency, utilizes what is called the “two-mass system.” In practice, if the calculation is carried out based on coupling stiffness alone, the calculated resonant frequency of the load has to be at least twice as high as the excitation frequency of the drive.
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fe | = | Resonant frequency of the system (Hz) |
| CT | = | torsional stiffness of coupling (Nm/rad) | |
| JL | = | Moment of inertia, load (kgm^2) | |
| JA | = | Moment of inertia, drive (kgm^2) |
Bellows couplings quite simply posses the highest torsional stiffness of commercially available flexible couplings, and are considered by many to be the standard for servo applications. Hydroformed from a continuous tube of stainless steel, bellows can easily flex laterally, angularly, and axially with only very gentle restoring forces, while remaining highly rigid in rotation. This, paired with a low moment of inertia, according to R+W, makes bellows couplings appropriate for almost any application requiring optimum efficiency and performance in acceleration and positioning.
The exception lies, says R+W, in cases where neither a high level of dynamic positioning accuracy, nor the ability to optimize servo loop gains is critical. A vibration damping coupling can be a very dependable and a low cost alternative in these cases. When pushing the limits of efficiency, accuracy, and speed, the most torsionally rigid coupling possible should be used in order to design the best servo system possible.
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