he actual revolution involved by the brushless system consists in permitting the execution of an electrical motor, very powerful at low speed, which is in gear without any reducer. We have entered the age of the “torque motors”.
One of the most interesting possibility offered by the ULTRACT – RULLACT and LYRA range by R.C.V. consists in developing a motor fitted with a special winding expressly studied for applications at low speed, in gear without reducer.
The ability to eliminate a reduction stage from the driving kinematic chain has always been considered as a very positive aspect since it allows to restrict the plays and the elasticity of the system. However, this goal becomes more difficult due to the necessity to keep a high torque with steady movement at low speed and some stiffness of the axis. The new brushless system allows to pilot high torque motors, whose cost per Nm is fairly held down, since it is possible to utilise small drives which can be compared to the ones used for the motors with reducer.
The torque motors, or low speed motors, are standard motors developed with particular windings having steady high Ke and Kt. In order to fully understand the potentiality, take as an example an “ideal” brushless motor with yield equal to 1 and cos = 1 (practically, good approximation). Under these conditions, as the motor is fitted with permanent magnets and therefore in constant field, the tension to the head of the motor is proportional to the speed through the Ke constant:
whereas the torque of the motor is proportional to the current through the Kt constant:
But if we consider that the electric power entering the motor must be equal to the power delivered to the axis, we will obtain:
Therefore, if we replace 1 and 2 in 3, we will obtain:
which, simplifying, will become:
As a consequence, the voltage and the torque constant of the motor are intrinsically bound. The choice of Ke during the planning stage of the motor, is always unchanged at the highest due speed so that
As a consequence, if for instance a motor is limited to 30 rad/sec (~300 r.p.m.), instead of the traditional 314 rad/sec (3000 r.p.m.), it will be possible to develop it with a proportionally higher Ke which will be approximately 10 times higher than the Ke of the standard motor: the same proportion is intrinsically applied to the torque constant, so such a motor, with special winding, can attain exceptionally high performances.
As an example, a UL1007 motor, limited to 300 r.p.m., will have a torque constant of ~17NmA and it is therefore able to develop 100Nm using 6A only. The use of torque motors allows to pilot big motors with small drives; in conclusion:
The elimination of an eventual reducer involves the adoption of a motor capable of the torque required by the slow shaft (therefore a bigger motor) but it does not contemplate an over dimensioning of the electronics.
In order to eliminate the reducers and benefit from the advantages, at first it will be necessary to verify if the motor suitable to the torque required by the slow shaft is competitive in price in comparison to the application you want to renounce to. Such a condition occurs in reduction ratio up to 1:10.
Once again, we strongly recommend to verify accurately the following two parameters:
UNIFORMITY OF ROTATION AND MINIMUM SPEED
A brushless motor can properly operate at very low speed, too. The lowest speed which can be achieved is exclusively defined by the resolution of the seek sensor you are using: with an std encoder at 4096 impulses per revolution, 16000 position per revolution can be resolved and the rotation is uniform even widely under 1 r.p.m. Generally speaking, the lowest speed allowed for the rotation to be kept perfectly uniform, is the one where the frequency of the encoder exceeds the passing band of the system, usually 30-50Hz.
INERTIA AND STIFFNESS OF THE SYSTEM
Every system fitted with reducer will reflect to the load the inertia of the motor multiplied for the square of the gear ratio. Consequently, when we decide to eliminate the reducer, we will drastically reduce the inertia of the system. Such a condition can be extremely advantageous for all those cases where the inertial component of the load is prevailing.
The same phenomenon can at a time become a limit in case the inertia of the system was utilised to absorb impelling loads. Without inertia, such variations in load must be compensated by the feedback speed of the electronic drive. Therefore it is indispensable for the drive to be operated at the highest possible passband and the load to be connected to the motor without play, by means of conic fittings or with interference (not with cotter key).
In general, the stiffness of the motor is quite high up to the cutting frequency of the system, usually 30-50Hz, and it will then drop up to the limit which can determined by the inertia only at higher frequencies.