Every brushless drive is made up by one electronic amplifier, a motor and at least one sensor of rotation. The motor operates exclusively as force generator; the effect produced by this force is measured by the sensor; the electronics compares the effect with the desired result and it modifies the force developed by the motor in order to attain the final result.
For instance, in appliances requiring a steady speed, the electronics will gradually increase the torque developed by the motor until the sensor detects a speed equivalent to the requested value. If the load suddenly increases, the speed will fall, the sensor will detect this decrease and the electronics will increase the delivered torque in order to bring the motor back to the speed originally preset.
As a consequence:
- the precision in speed is almost independent from the load and it is fully independent from the motor but it only depends on the quality of the sensor and on the settings of the electronics;
- the time occurred to react to the variations in load depends in a critical way on the detection speed of the signal of the sensor and on the settings of the electronics.
The modern brushless systems can attain reaction times of some milliseconds, and, therefore, they offer top performances; despite this, very frequently these performances are limited by the mechanical response time of the system; in order to exploit the new performances, a further evolution of the mechanical design of the appliances is necessary.
As example, take into consideration a steady speed drive as the one mentioned in the previous example. If the motor is coupled to the load by means of toothed belt, there will be elasticity between the driving and the load axle. If we attribute a significant inertia to the load, and by analysing the first instants of the motion, we can imagine the following sequence:
- the electronics delivers a current and the motor starts rotating, by loading the elasticity of the system and therefore without moving the load inertia;
- if the electronics is quick, at this step already it detects that the motor has reached a speed which is higher than the preset one and it reduces the torque;
- at same time, the belt will stretch and slow down the motor thus reducing the speed;
- the combined effect of the torque reduction and of the load speed acceleration through the belt will cause the tension of the belt to fall;
- the electronics detects the reduction in speed and it increases the torque of the motor by re-starting up a cycle.
An oscillating phenomenon has thus been originated, where both the motor and the load continue accelerating and decelerating. In practice, both a vibration and a high noise level are observed. A superficial exam will ascribe this phenomenon to a noisy motor; such a conviction could then be strengthened when it is found out that thanks to the replacement of the existing motor with a motor offering lower performances and therefore slower response, the noise is sometime eliminated.
The intuitive analysis of the previous process will allow us to easily understand that:
- the phenomenon must be ascribed to the discrepancy between the elasticity of the system and the settings of the electronics; in practice, the motor reacts with a speed compared to the reaction time, or to the load take-off of the mechanics;
- the possible solutions consist in:
- reducing the elasticity of the system thus accelerating the load take-off time of the mechanics, for instance by replacing the belt with gears;
- slowing down the response time of the motor-electronics system and consequently renouncing a part of the possible performances.
Obviously, the second solution will inevitably down-grade quality of the machine, because it increases the time necessary to reach the desired position or speed; in one word, it reduces the ability of the motor to react to sudden troubles and loads. We wish to point out that the motors featuring a less advanced technology, namely bigger and slower, compensate the lack of speed with a considerable inertia; on the contrary, the brushless motor, which features a very low inertia, must be operated in some cases with sufficient speed, since otherwise it will result in a significant drop in performances.
According to the above example, we can easily imagine the behaviour of a brushless system under mechanical clearance, for instance a key; for this reason, the top quality brushless motors are manufactured with smooth shaft and they must be coupled under allowance by means of a conical fitting. In addition to this, the only flexible joints suitable to the dynamics are the ones with metallic bellows.
The above considerations lead to a very important remark:
whilst the traditional motorization (C.C. and P.M. motors) generally represented, due to their inertia, a limit to the dynamic performances of the drive system, the superior performances assured by the brushless motors very frequently allow for the dynamic limit of the system to be determined by the mechanics itself.
As a consequence, it is much more important to fully understand and rationalise the mechanics of the system in order to carry out effective applications.
With reference to the above example, we can draw some comments:
- precision does not depend on the motor but on the sensor;
- the response speed and therefore the ability to follow the reference under the highest precision, depend in a critical way on the stiffness of the driving gear.
The problem related to the noise developed by the system neither depends on the motor nor on the electronics, but very often it is originated by a rough mechanics which is not suitable to the performances offered by the brushless motor; indeed, the mechanics itself would not have originated any problem if fitted with a less advanced and slower motor; the noise of the motor is originated by the continuous accelerations and brakings; under these conditions, an overheating of the motor can indeed be a consequence which can not be ascribed to an insufficient sizing.
As the dynamics of the system is fundamental for the sizing of the motors, it is of utmost importance to determine it in detail.
The dynamics is made up by two elements:
- the ability to exert more or less high accelerations to the load, which exclusively depends on the ratio torque/moment of inertia of the motor; this feature is sometime called “high signal pass-band”.
- Control pass-band, the higher it is, the shorter will be the time the drive reaction ring needs to stabilise at the desired value. This parameter depends in a critical way on the mechanics. To set up a stable system, it is not possible to stabilise the electronics before a period of time equivalent to 2-3 times the softening period of all the oscillations which are typical of the mechanics of the system in use.
As example, let us suppose to carry out the axle of a nibbling machine operating at 10 strokes per second in different positions which are continuously updated by a fast numerical control.
If the drive shaft between the motor and the piece (coupling, screw, supports, etc.) has a frequency of mechanical resonance equal to 50Hz, therefore oscillating within a period of 20 milliseconds, it will not be possible to stabilise the system in less than 3×20 msec, namely 60 msec thus leaving 40 msec. only of the complete cycle for the stroke and for the entire movement. The application is most likely impossibile, despite the type of the motor. If, on the contrary, the mechanics is improved by stiffening the couplings, by uprating the screw etc. until 100Hz of the resonance frequency of the mechanics is attained, we can expect a stabilisation time of the drive in 30 msec, leaving 70 for both the stroke and the motion. Now the application is more likely to be realised.