July-August 2021

HYDRAULICS 40 HYDRAULICS & PNEUMATICS July/August 2021 www.hpmag.co.uk affect it: holding torque and the total inertia of the system. Unwanted effects Holding torque depends on the rated current of the motor. Using a higher current to increase the holding torque has the unwanted effects of increasing joule losses, leading to increased coil temperature. However, it is possible to use a lower current to shift the natural frequency down if the lower torque still meets the application requirements. Looking at inertia, the mechanical system’s moment of inertia is the sum of the motor’s rotor inertia plus the load inertia. The design engineer could select a motor with different specifications to change to the rotor inertia. Or it may be possible to adjust the load inertia to shift the natural frequency up or down, if this doesn’t impact on the motor’s performance in the application. Another measure to prevent resonance is to operate the motor in microstepping mode rather than driving the motor with full steps. The smaller step angle requires less energy to move from one stable position to the next, so the resulting overshoot and magnitude of oscillation are smaller. In addition, microstepping generally offers lower noise, less vibration and a smoother operation. Preventing resonance with damping As discussed previously, the motor system’s inherent damping will steadily decrease the magnitude of oscillation in many applications and prevent resonance occurring. So, where resonance is a problem, can we increase damping to eliminate the problem? The answer is yes, and there are a number of mechanisms available to do so. Mechanical friction provides a braking torque that is constant and independent of speed. Increasing load friction or motor bearing friction might be an option to increase damping in some applications, although because it acts on the motor at all speeds it is important to ensure that the motor’s performance will not be compromised. Adding viscous friction A better option is generally to add viscous friction; this also provides a braking torque, but its magnitude is dependent on motor speed (being higher at higher speeds). Thus, it provides strong braking while the oscillation amplitude is great and only very light braking once the oscillation is smaller. There are a number of different phenomena that can bring viscous friction to a system. Eddy-currents generated in the iron of the stator act as a braking torque, but these levels of these iron losses vary between different motor technologies. Disk magnet motors usually have only limited iron losses; this enables them to achieve high speeds but means eddy-currents alone should not be relied upon to prevent resonance. The back-EMF (voltage) induced in the coil provides a current that also creates a braking torque that will dampen oscillation and can also be considered viscous friction. The effectiveness depends on the motor drive – chopper drivers typically don’t enable this type of damping since the current is kept constant despite back-EMF variations. The design engineer might also want to consider other electronic damping solutions, driving the motor in a particular way without changing any mechanical parameters within the system. Or there may be the option to add an external mechanical damper to absorb some of the vibration energy to prevent resonance. In any stepper motor application where resonance is a problem, it can often be attributed to several conditions. Sometimes acting on just one of these conditions can be sufficient to eliminate resonance. In any application, it is always worth engaging with a knowledgeable supplier to determine what frequency ranges are likely to cause a resonance problem, and to look at the possible solutions to eliminate it. www.portescap.com In any application, it is always worth engaging with a knowledgeable supplier to determine what frequency ranges are likely to cause a resonance problem.