Digitized brushless DC servo motor system, or according to the habit called digital permanent magnet AC servo motor system ( permanent magnetic AC servo system ), has undoubtedly become the contemporary field of motion control servo drive mainstream products and development trend. Major foreign servo drive manufacturers and motor manufacturers, in the late 90's have completed the general development of permanent magnet AC servo motor system products. Rated power general in the 50W ~ 20kW, speed in general 1000 ~ 3000r / min. Domestic servo system consists of Research Institute and university R & D start time is not later than abroad, into the field of industrial production is late. In a representative Zhuhai Motion Control Motor Company Limited as an example, the earliest production of their own design of servo motor in 2000 2003, preliminary form 50W ~ 5kW servo system products, the beginning of 2006 to complete, perfect products and gradually develop in the direction of mass production. Shows homebred servo products will enter a new stage of development, to better meet the needs of users widely. But in the development of domestic servo products also discovers in the process, because the operation control system involved in the field is extremely wide, the servo system request the myriads of changes, are very diverse, so in the main series of products based on the application, but also according to system requirements, continuously expand the relatively special products, to better meet the broad market need, this is our product development in one of the ways [1 ]. This article describes the is applied to NC shoe last machine in high speed servo system an example of.

China is the world's largest shoe-making powers, footwear production and exports has been ranked first in the world. The development of the footwear industry make the integral shoe last industry rapid development, accordingly also appeared NC shoe last machine, or commonly referred to as computer shoe-last machine. Shoe last machine used for processing shoe last model, its motion control system includes four coordinates, as shown in figure 1. The theta axis drive a workpiece to rotate, X, Y and theta three axis motion synthesis of shoe last model contour; R shaft drives the cutter rod to rotate, finish machining. Before the three shaft is typical of the position servo system, the basic range of suitable models can meet the requirements of. The R axis is not the same, no position control requirements, as long as the processing object of different materials, to maintain the proper rotation speed can be. As a result of shoe last model material mainly plastic, ask tool is rotated at high speed, at least 9000r / min above, otherwise it is difficult to guarantee the processing surface smooth and beautiful appearance, but also in the cutting process can not because the variation of cutting force and the speed has the obvious fluctuation. Basic series of AC servo system in high speed, no such specifications varieties, so the existing NC shoe last machine, only the three shaft adopts AC servo system [2 ], the knife rod drives using standard asynchronous motor, see is an example of using an ordinary quadrupole asynchronous motor, 1 : 5 ascending speed drives the cutter bar rotating speed, the cutter bar is about 7000r / min. The shortcomings of such systems are: the structure is not compact, low speed, and unstable, making the surface processing of poor quality. If the AC servo system, although not its position servo function, but its speed loop and current loop, can guarantee the speed is stable, even if the load changes also effect is very small, but in the long shoe last machine can fully ensure the long workpiece shape and surface quality is consistent, will bring NC shoe last machine breakthrough progress in visible range, this basic specialized development of a high speed servo system is valuable.

Fig 1 Schematic diagram of shoe last machine four axis

Design structure characteristics and the key technology

Drive NC shoe last machine knife rod to rotate the cutting processing of the servo motor, to take every single head knife bar independent drive mode, according to the practical experience, the basic technical requirements are identified as: rated speed nN = 9500r / min; PN = 1000W rated power.

The high-speed permanent magnetic motor rotor in general, on the demand for higher strength, the requirements of bearings increase, will cause noise and vibration problems, will cause the mechanical loss core loss and temperature rise, increase raise issues [3 ]. Servo motor to consider adopting high speed encoder problem. The servo drive is not principle effect.

Basic series of servo system with position sensor is a general incremental optical encoder, with 2500 main lines. From the pulse frequency limit and mechanical strength point of view, this kind of encoder maximum speed is limited to 5000r / min, can not adapt to the requirement of high speed servo system. Position sensor to select TS2620N21E11 type rotary variable, its maximum speed of 30000r / min, can fully meet the needs of. At the same time using the matching decoding chip Au6802N1, it can provide the rotation required excitation signal, the rotary variable output signal demodulation and conversion into different types of digital signal, including incremental optical encoder as a digital signal, easy of application [4 ].

In very high speed is not, in the 10000r / min left and right sides, bearing without the use of special cooling mode, can be used with the grease lubricated plain bearings can be. Just load permissible condition, the appropriate use of the size small bearing, in the assembly suitable bearing housing and bearing gear mating dimensions.

Permanent magnet motor rotor, in most cases using surface permanent magnet, permanent magnet pole are usually glued on a rotor yoke surface. High speed motor for use of non-magnetic metal sheath, or fiberglass cloth and epoxy curing after winding. The production of basic products, permanent magnet motor rotor structure, gradually has been replaced with a built-in permanent magnet ( IPM ) structure, from the mechanical point of view, fully meet the requirements of high-speed motor, high reliability.

A built-in permanent magnet structure on improving cogging torque has obvious advantages [5], the rotor mass asymmetric distribution of advantageous, beneficial to reduce the motor torque fluctuation, improvement of motor noise and vibration, it is particularly important for high speed motor.

Basic series servo motor electromagnetic design features: usually uses high performance permanent magnet material, generally for sintering NdFeB; high effective utilization rate of material, which adopts the high electric, magnetic load; mostly three poles or four on the pole body, in order to obtain the high power ( torque ) density and high response characteristics, the design is not exceptional also, basically are four pairs of pole structure. Four pairs of pole permanent magnet motor in 9500r / min, electromagnetic frequency of 633Hz, has entered the medium frequency ( > 400Hz ) range, according to the usual think core loss calculation, will power (50Hz ) of 27 times, visible standards of electromagnetic design makes the iron loss is too large, the temperature rises too high, can not afford. That is to say to reduce motor iron loss will become the high speed servo motor design characteristic and key technology.

Speed increasing retrofit design

Retrofit design main characteristic is to make full use of the basic series of basis, as little as possible to change the basic series of design components, in particular to need to change a portion of the mold, such as core punching sheet size. Some change only when there is no easy way to achieve the goal of modified when it is affected by these practical considerations, because of restrictions, the retrofit design scheme is unlikely to be the ideal design, can only be a usable, but overall was more economical solution.

Changing the rated speed design is quite simple, the other under the condition of constant change, as long as the winding can be. In the power supply voltage to remain unchanged, let the rated speed phase EMF constant can be, i.e. potential coefficient Ke1 and nN rated speed is inversely proportional to the phase windings, is also effective series turns N and nN inversely.

A modified request rated speed:

N ' N = knN ( 1)

Desirable modified motor turns:

N ' = N / K ( 2)

If you keep slot fullness rate unchanged, then the phase windings of a cross-sectional area of the conductor ( S ' Cu ) should be the prototype of the K times, S ' Cu = kScu ( 3)

If maintain winding rated current density is constant, the rated current for a prototype of the K times, power will increase K times. In fact because of the motor electromagnetic load ( magnetic flux density and line load ) are not changed, the electromagnetic torque is the same output power of TN, PN with rotational speed increased to K, T 'n = Tn ( 4) < BR > P n = kPn ( 5)

The above analysis actually reveals the motor design a basic principle, namely the motor volume determines the motor torque, power and speed are directly related to. But this rule only when the speed in a certain range is applicable, speed exceeds a certain range, the motor power loss increases, especially the core losses increase, so that the motor temperature rise beyond the allowable values and not easy to achieve.

In fact in the current density is still unchanged, with copper quantity invariable situation, winding copper loss is the same. Iron loss is different, the magnetic flux density is constant, at high speed magnetic alternating frequency increased, iron loss increasing PFE, wherein the hysteresis loss and frequency is proportional to F increase, eddy current loss and frequency of the two is proportional to, usually think of total iron and frequency ( 1.3) power in proportion, so:

P k = k1.3Pk ( 6)

Visible, although from the view of electromagnetic design, certain geometrical size motor can obtain a certain torque, so that the output power of the motor with a speed proportional to the increased, but it can be affected by the heating limit. When the speed reaches a certain value, total motor loss exceeds the allowable value, will reduce the current density J, so that power could not directly proportional to the increased, even may be reduced. In extreme cases, that the rotation speed is high, the motor iron loss ( plus mechanical loss has exceeded the allowable value of fever ), motor no load capacity, only changing winding retrofit design to achieve.

It is necessary to consider reducing the iron loss measures, main method should reduce the core: within the magnetic density, lower field alternating frequency; the lower loss of silicon steel sheet designation or the thickness of a thin silicon steel sheet. Combined with the modified design example, choice is the prototype of 92BL ( 3) D100-30H ( ST ) type motor, this is a PN = 1000W, nN = 3000r / min, 8 pole motor. Due to the basic series motors have been used for cold rolled non oriented low loss and the thickness of 0.35mm silicon steel sheet, so changing the core material drop approach the reality is not reduced, pole number, such as the original 8 pole motor is changed to the 4 pole motor, should be a more promising approach, but should take into account the multi-pole motor yoke height of small, to 4 very if not specially reducing air-gap flux density or reduced pole arc coefficient, because each pole magnetic flux increases while the yoke flux density is too high, will lead to increased iron loss, can't even see a reduction in the frequency of action. The modified design example is artificially reduced pole arc coefficient, so that the yoke flux density is increased, with the suitable number of windings to be able to achieve a goal.