Machine manufacturers are increasingly called upon to design systems for optimal time utilization. However, there are often conflicts between shortening the acceleration and positioning time and the limits of the mechanical system. The resulting torsional resonances represent greater mechanical force and must be avoided. This article attempts to clarify the influence of servo motor design on the dynamic behaviour of the drive, by comparing disc motors and DC conventional electric motors.

The following parameters are of interest in this study:

• Constructed length
• Shaft diameter
• Moment of inertia
• Electrical time constant

Key data on the motors compared are listed in the table. Figure 1 shows the simplified structure of a position controller circuit via current cascade adjustment and speed control. Controller theory states that the maximum step frequency of the loop cascade controller is w = w. Therefore, the step frequency of the internal controller loop must be maximized. This is also necessary when the exterior controller loop has less stringent requirements, as this will reduce the force exerted on all components, provide better suppression of interference and increase the stability - all of which make the system easier to handle. As the internal controller loop, the current controller circuit should have the widest possible bandwidth. Modern servo amplifiers have values of 1-2 kHz, which depend not only on the switching frequency used (fs preferably ³ 8 kHz), but also on the road. A comparison of the time constants clearly shows lower values (see table) for the disc motors (ironless rotor). This allows current controller circuits with larger bandwidths to be designed.

The speed control circuit is the second controller loop, and the high bandwidth of the current controller can be utilized in this case. Figure 2 shows the simplified mechanical structure of a servomotor. The moment of inertia of the motor, JM, is flexibly coupled to

the moment of inertia of the tachometer, JT, creating a vibrating system. The natural frequency of this system must be taken into account, as it has significant influence on dynamic behaviour. For the natural frequency, Conventional electric motors Two options for maximizing the natural frequency can be deduced from this relationship. This has the following design-related advantages for the disc motor: The reduced length (e.g., Factor 2) allows a 2 increase in shaft diameter with the same mass inertia. Since the polar moment of inertia increases with the 4th power of the diameter, torsional stability is increased four-fold. The natural frequency of the system is higher, resulting in improved system properties. This is shown in Figures 3 and 4. The same considerations should be made for the motor-load system. This allows relevant mechanical drive requirements to be taken into account. A direct relationship between the minimum positioning time achievable and the natural frequency can be established for each system.

Mavilor Motor MA

ω = √ C (JM + JT)

JM · JT

ω

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Higher accelerations lead to resonance, increasing the positioning times. The resonance that appears is generally offset with the corresponding specification for the rated values (ramps). The measurements also showed that conventional electric motors tend to experience vibration in a wider margin of amplification, P, of the speed control circuit, unlike disc motors. Moreover, sudden interruption of this vibration was also not observed when the amplification, P, typical in disc motors, is reduced. This behaviour makes it hard to adjust the speed controller circuit and may cause unwanted vibration to appear during operation. In this context, therefore, the following selection criteria can be established for servo motors:

• Elevated torsional stability (short design, larger shaft diameter)
• Low mass inertia
• Low inductivity (and therefore larger bandwidth for the current controller circuit)
• Lower mass of the tachometer generator (measurement system in general)
• Stablest possible coupling of the measurement system to the motor shaft (hollow-bored shaft mounting)

These requirements were taken into account in the design of the disc motors of the MO, MT, MS (DC), SE and AM (AC) series (Fig. 5). Therefore, these motors meet the necessary dynamic requirements for modern drive systems, achieving lower acceleration and positioning times, greater precision and good system stability and handling

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