Apparatus for driving a spindle of an electroerosive machine

The driving apparatus for the rotation of the spindle or the work piece pallet of an electroerosive machining device comprises two electric motors which drive e.g. the spindle to a rotational movement by means of two gear boxes and a plurality of gear wheels. In order to avoid any backlash in the gear box and any clearance between the gear wheels and to compensate the elasticity of the gear transmission, the two electric motors are always operated to rotate in opposite directions and/or to yield different values of torque. Thus, the entire transmission assembly is always subjected to a certain bias force so that a precise positioning of the spindle is possible by means of simple and inexpensive driving elements.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a driving apparatus for driving the 
spindle or the workpiece pallet of an electroerosive machining apparatus 
to a rotational movement. 
It is well known in the art that an electroerosive machining apparatus 
ensures an extremely precise machining of workpieces by removing workpiece 
material under the effect of electro erosion. In order to achieve an 
accurate shaping of a workpiece, the parts and elements of the 
electroerosive machining apparatus have to be manufactured and must 
operate with at least an equal precision, if possible with an even higher 
degree of precision than the desired accuracy of shaping of the workpiece. 
In the case of e.g. a spark erosion machining apparatus, an electrode is 
used to machine the workpiece, said electrode being received in a spindle 
of the apparatus and can be raised and lowered as well as rotated. A 
driving means is provided to rotate the spindle and thereby the electrode, 
said driving means usually comprising an electric motor. In order to 
displace the electrode into a desired angular orientation, the spindle is 
rotated by the electric motor via a reduction gearbox. 
2. Prior Art 
One of the main problems in connection with known driving devices of the 
kind referred to hereinabove is the transmission backlash between motor 
shaft, gear box and spindle. Thus, one was forced to use extremely 
precisely manufactured reduction gear boxes and power transmission 
elements in order to achieve the desired high positional accuracy of the 
angular position of the spindle. Such a driving apparatus, however, is 
very expensive, requires a frequent maintenance and is subject to wear out 
comparatively quickly; thus, the precision to be achieved is impaired. The 
same facts are true for a driving apparatus for the rotation of workpiece 
pallets. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide a driving apparatus of the kind 
referred to hereinabove which avoids the disadvantages discussed 
hereinbefore. Particularly, it is an object of the invention to provide a 
driving apparatus which can be manufactured with low expense and which 
offers a reliable operation during an extended period of time with a high 
degree of precision. 
It is a still further object of the invention to provide a driving 
apparatus which renders possible a still increased degree of positional 
precision with respect to the angle of rotation of the spindle or the 
workpiece pallet, and which allows a quick rotation of the spindle or the 
workpiece pallet accurately into a desired angular position. 
SUMMARY OF THE INVENTION 
The present invention provides a driving apparatus for driving the spindle 
or the workpiece pallet of an electroerosive machining device. The 
apparatus comprises at least two electric motors adapted to be operated to 
rotate in either rotating direction. Reduction gear means are associated 
with each of said electric motors each including an input member 
operatively connected to the associated electric motor and driven by said 
electric motor as well as an output member operatively connected to and 
driving the spindle or the workpiece pallet of the electroerosive 
machining device. 
Further, the apparatus comprises a plurality of revolution sensing means 
each associated with one of said electric motors and operatively connected 
thereto. 
Finally, there is provided a control means operatively connected to said at 
least two electric motors as well as to said revolution sensing means. The 
control means operates at least two of said electric motors to rotate in 
opposite directions and/or to yield different values of torque in any 
operating condition of the driving apparatus. 
Thus, the entire transmission assembly, i.e. the reduction gear box as well 
as the associated transmission elements, are maintained always under a 
certain bias torque which renders every backlash in the gear box and any 
clearance between the gear wheels ineffective and compensates the 
elasticity which is always present in a mechanical transmission system. 
The main advantage is that the reduction gear box and the transmission 
elements can be of much simpler design and therefore are less expensive. 
Even if the transmission backlash should increase after a long period of 
operation, e.g. due to wear, this effect has no influence on the 
positional precision. 
In most cases, two electric motors are provided; however, depending on the 
individual situation, more than two electric motors may be used. 
The expression ". . . to rotate in opposite directions and/or to yield 
different values of torque . . ." has to be interpreted as follows, 
depending on the operating conditions of the apparatus: 
1. If the spindle or the workpiece pallet is in its stopped condition, both 
electric motors are operated to yield the same value of torque, but to 
rotate in opposite directions. 
2. If the spindle or the workpiece pallet is driven to a first, lower 
rotational speed, the two electric motors rotate in opposite senses and 
are operated to yield different values of torque. 
3. If the spindle or the workpiece pallet is driven to a second, higher 
rotational speed, the two electric motors rotate in the same sense but are 
operated to yield different values of torque. 
This means that the two electric motors never act identically on the 
spindle or the workpiece pallet, but always generate a certain bias torque 
in the transmission assembly. 
Preferably, the two electric motors are operated to rotate in opposite 
directions and to yield the same value of torque when the spindle or the 
workpiece pallet is in a stopped condition. Thereby, it may be achieved 
that no undefined state can occur in the transmission assembly in the 
moment when the spindle or the workpiece pallet starts its rotational 
movement, because one of the electric motors always yields a certain 
torque in a first direction at standstill as well as during the rotation 
of the spindle or the workpiece pallet, while the other electric motor 
yields a higher torque than the said one electric motor in the moment of 
start and, thus, rotates the spindle or the workpiece pallet. 
According to a further embodiment of the apparatus of the invention, during 
the transistion from the stopped condition of the spindle or the workpiece 
pallet to said second, higher speed of rotation, one of said electric 
motors continues to rotate in the same direction of rotation and is 
operated to yield a higher value of torque, while the direction of 
rotation of said other electric motor is reversed with a predetermined 
time lag. Also this characteristic, which is not absolutely necessary due 
to the inertia of the transmission assembly, helps to avoid that no 
undefined or unloaded state can occur in the transmission assembly.

As can be seen in FIGS. 1 and 2, the driving apparatus comprises a bearing 
housing 1 which receives a spindle 2 of an electroerosive machining 
device. The spindle 2 is rotatably mounted in the bearing housing 1 by 
means of not shown bearing elements known per se in the art. A revolution 
sensor 3 is arranged above the spindle 2 in coaxial relationship and 
torsionally fixedly connected to the spindle 2. The sensor 3 serves to 
monitor the angular position of the spindle 2 and is operatively connected 
to a control unit as will be explained in more detail in connection with 
FIG. 5. 
The spindle 2 is provided with a gear wheel 4 serving to drive the spindle 
2 to a rotational movement. Two intermediate gear wheels 5a and 5b mesh 
with the gear wheel 4 as well as with two gear wheels 7a and 7b which are 
the output members of two reduction gear boxes 6a and 6b. Two electric 
motors 8a and 8b located in the vicinity of the gear boxes 6a and 6b each 
bear a pulley 10a and 10b fixed to their respective output shafts 9a and 
9b. The pulleys 10a and 10b are operatively connected, by means of toothed 
belts 11a and 11b, to further pulleys 12a and 12b serving as input members 
of the gear boxes 6a and 6b. 
It is quite evident for every person skilled in the art that such or a 
similar gear transmission has a certain elasticity which cannot be 
neglected, and further that a certain backlash is present in the gear 
boxes 6a and 6b as well as a clearance between the meshing gear wheels 4, 
5a, 7a and 4, 5b, 7b, respectively. Such backlash and clearance are very 
difficult to minimize by constructional measures; anyway, this would 
require expensive measures. 
According to the present invention, these disadvantages are avoided by 
operating the two electric motors 8a and 8b never equally. With other 
words, the spindle 2 being in its stopped condition, the electric motor 8a 
is operated to rotate in e.g. clockwise direction and the other electric 
motor 8b in counter-clockwise direction; however, the control unit 
operates the two electric motors such that they yield the same amount of 
torque which is transferred to the driving gear mechanism and thereby to 
the gear wheel 4 of the spindle 2. Thus, the driving gear mechanism is 
subjected to a certain bias with the effect that all backlash and 
clearance is removed and the elasticity is compensated. If it is intended 
to rotate the spindle 2 slowly, e.g. into a nearby located angular 
position, one of the two electric motors, depending on the desired sense 
of rotation e.g. the electric motor 8a, is operated such that it yields a 
higher torque while the operating conditions of the other electric motor 
8b remain unchanged. If the spindle 2 has to be rotated quickly into a 
distantly located angular position, one of the electric motors, depending 
on the desired sense of rotation e.g. the electric motor 8a, is operated 
such that it yields a higher torque while the sense of rotation of the 
other electric motor 8b is reversed and the latter is operated to yield a 
lower torque than the electric motor 8a. 
The diagram of FIG. 3 shows these operating conditions in a simplified 
illustration. The solid line refers to the electric motor 8a and the 
dashed line refers to the electric motor 8b. 
The spindle 2 is in its stopped condition during the time interval t.sub.0 
to t.sub.1 because the electric motor 8a yields a positive torque +M.sub.1 
and the electric motor 8b yields a negative torque -M.sub.1 ; the absolute 
values of the two torques are identical but have opposite direction. At 
the time t.sub.1 the spindle 2 has to be rotated into a comparatively 
distantly located angular position. For this purpose, the motor 8a is 
operated such that the torque it yields increases from the value +M.sub.1 
to the value +M.sub.3. Simultaneously, the sense of rotation of the motor 
8b is reversed and it is operated to yield a torque with a value of 
+M.sub.2. The value +M.sub.2 is less than the value +M.sub.3 ; 
consequently the bias in the gear transmission is maintained. 
It is supposed that an angular position quite near to the desired angular 
position is reached at the time t.sub.2. Now, the sense of rotation of the 
motor 8b is reversed at the time t.sub.2 and it yields, exactly as during 
the the stopped condition, a torque with the value -M.sub.1 again. Thus, 
it operates in increased manner as a brake for the motor 8a and thereby 
increases the bias in the gear transmission. As soon as the desired 
angular position is reached, i.e. at the time t.sub.3, the motor 8a is 
operated to yield a torque with a value +M.sub.1, with the consequence 
that the spindle 2 is in its stopped condition during the time interval 
from t.sub.3 to t.sub.4. The operating conditions of the motor 8b, 
however, remained unchanged even at the time t.sub.3 ; it is operated 
further to yield a negative torque with a value of -M.sub.1. 
It is further supposed that, subsequently, the spindle 2 has to be rotated 
in the same sense of rotation into a nearby located angular position at 
the time t.sub.4. For this purpose, the motor 8a is operated to yield a 
higher torque with the value +M.sub.3 while the operating conditions of 
the motor 8b remain unchanged. At the time t.sub.5, as soon as the desired 
new angular position is reached, the motor 8a is operated to yield the 
former torque with the value +M.sub.1 again; this means that the torques 
yielded by the two motors 8a and 8b compensate each other and the spindle 
2 stops. At the time t.sub.6 the spindle has to be rotated back into its 
former nearby angular position. For this purpose, the motor 8b is operated 
such that the torque yielded by this motor increases from the value 
-M.sub.1 to the value -M.sub.3 while the operating conditions of the motor 
8a remain unchanged, i.e. it yields a torque with a value + M.sub.1. 
Consequently, the spindle 2 rotates in the opposite direction under the 
influence of the motor 8b. 
At the time t.sub.7 the new position is reached and the motor 8b is 
operated such that the amount of torque yielded by it decreases to the 
value -M.sub.1 so that the spindle will be in its stopped condition at the 
time t.sub.8. Thereafter, a greater rotation of the spindle in the 
minus-direction should take place. For this purpose, simultaneously, the 
motor 8b is operated to yield an increased torque with the value -M.sub.3 
and the sense of rotation of the motor 8a is reversed and it is operated 
to yield a torque with a value -M.sub.2 which is less than the torque with 
the value -M.sub.3 yielded by the motor 8b. Thus, the motor 8a operates as 
a weak brake an maintains the bias in the gear transmission. When the new 
desired angular position is nearly reached, i.e. at the time t.sub.9, the 
sense of the rotation of the motor 8a is reversed and it is operated to 
yield a torque with a value +M.sub.1 ; thereby the motor 8b is retarded to 
a greater extent. At the time t.sub.10 the new desired angular position is 
reached and the motor 8b is operated to yield a torque with the lower 
value -M.sub.1 while the motor 8a still yields a positive torque with the 
value +M.sub.1. Consequently, the spindle 2 is in its stopped condition up 
to the time t.sub.11. 
It can be clearly understood from the above explanations that the two 
motors 8a and 8b act always in different senses on the gear transmission 
assembly comprising the transmission belts 11a, 11b, the gear boxes 6a, 
6b, the gear wheels 5a, 5b and 7a, 7b and the gear wheel 4 and thereby 
constantly create a bias force in the entire transmission assembly. Even 
if this bias force is somewhat lower if the spindle is quickly rotated, it 
is anyway sufficient since the precision requirements are not extremely 
high during this high speed rotation. 
A critical moment could occur under certain circumstances at the times 
t.sub.1 and t.sub.8, i.e. when, simultaneously, the sense of rotation of 
one motor is reversed and the torque yielded by the other motor is 
increased. If the moment of inertia of the gear transmission should be 
insufficient to provide sufficient damping, the invention provides 
according to a further embodiment that reversing of the sense of rotation 
of the one motor and the increase of torque yielded by the other motor, 
respectively, is not effected exactly simultaneously but with a certain 
time lag. 
As can be seen from FIG. 4a, the motor 8a is operated to yield a higher 
torque exactly at the time t.sub.1 and the sense of rotation as well as 
the increase of the torque to the value +M.sub.2 of the motor 8b is 
effected with a small time lag .DELTA.t. Thus, it, is ensured that the 
sense of rotation of the motor 8b takes place only when the motor 8a 
yields its predetermined torque and thus maintains the bias in the gear 
transmission during the start of the motor 8a. The situation shown in FIG. 
4b is similar at the time t.sub.8 : The sense of rotation of the motor 8a 
is reversed only after a certain time lag .DELTA.t, i.e. as soon as the 
motor 8b yields the required torque with a value -M.sub.3. 
The absolute values of and the relations between the torques M.sub.1, 
M.sub.2 and M.sub.3 of course depend of the individual operating 
conditions of the driving apparatus. As a guide, the following figures can 
be given: The value of M.sub.1 is from about 15% to about 40%, preferably 
about 20% of the value of M.sub.3, and the value of M.sub.2 is from about 
60% to about 85%, preferably about 80% of the value of M.sub.3. 
In FIG. 5, a function diagram of the apparatus of the invention is shown. 
For simplicity's sake, the reduction gearboxes 6a and 6b as well as the 
transmission means 10a, 11a, 12a and 10b, 11b, 12b, respectively, are not 
shown in full detail. However, the spindle 2, the gear wheel 4 connected 
to the spindle 2, the revolution sensor 3 coupled to the spindle 2 and the 
two electric motors 8a and 8b with associated gear wheels 5a and 5b can be 
seen. 
The control unit of the apparatus comprises a digital/analog converter 20 
having a first input 22 connected to the output of a conventional 
NC-control apparatus known in the art and not shown in the drawing. The 
second input of the digital/analog converter is connected to the output of 
the revolution sensor 3. The output of the digital/analog converter 20 is 
connected to the input of a buffer amplifier 24 whose output is connected 
to the first input of a first differential amplifier 26 and to the first 
input of a second differential amplifier 28. Each electric motor 8a and 
8b, respectively, is provided with a tacho generator 30 and 32, 
respectively. The output of the tacho generator 30 is connected to the 
second input of the first differential amplifier 26, and the output of the 
tacho generator 32 is connected to the second input of the second 
differential amplifier 28. 
Further, a third differential amplifier 34 and a fourth differential 
amplifier 36 are provided. While the output of the first differential 
amplifier 26 is connected to the first input of the third differential 
amplifier 34, its second input is biased with a preset positive voltage 
+V. Correspondingly, the output of the second differential amplifier 28 is 
connected to the first input of the fourth differential amplifier 36, 
while its second input is biased with a preset negative voltage -V. The 
output signal of the third differential amplifier 34 is fed, via a first 
power amplifier 38, to the electric motor 8a, and the output signal of the 
fourth differential amplifier 36 is fed, via a second power amplifier 40, 
to the electric motor 8b. 
In a first operating condition, the spindle 2 is in rest. Thus, no input 
signal reaches the input 22 of the digital/analog converter 20 and, 
consequently, the output signal thereof is zero. The output of the buffer 
amplifier 24 also yields a zero signal to the first inputs of the first 
and second differential amplifiers 26 and 28, respectively. The second 
inputs of the first and second differential amplifiers 26 and 28, 
respectively, are also at zero level since the tacho generators 30 and 32 
coupled to the electric motors 8a and 8b which are not rotating do not 
yield an output signal. Since the first and second inputs of the first 
differential amplifier 26 have the same level, no output signal is 
generated. The output of the first differential amplifier 26 being 
connected to the first input of the third differential amplifier 34 and 
having zero level and the second input of the third differential amplifier 
34 being tied to a voltage of +V, the output signal of the third 
differential amplifier 34 is positive. This positive output signal is 
amplified by the first power amplifier 38 and attempts to drive the 
electric motor 8a to a positive rotational movement. 
Simultaneously, since the first and second inputs of the second 
differential amplifier 28 have the same level, no output signal is 
generated. The output of the second differential amplifier 28 being 
connected to the first input of the fourth differential amplifier 36 and 
having zero level and the second input of the fourth differential 
amplifier 36 being tied to a voltage of -V, the output signal of the 
fourth differential amplifier 36 is negative. This negative output signal 
is amplified by the second power amplifier 40 and attempts to drive the 
electric motor 8b to a negative rotational movement. 
It may be easily understood that the spindle 2 will not rotate because the 
electric motors 8a and 8b are driven in opposite senses of rotation with 
the same value of driving signal, and that the transmission and gear box 
parts are biased thereby removing any backlash. 
In a second operating condition, the spindle 2 has to be driven to a 
rotational movement with a first, lower speed in a negative sense of 
rotation. For this purpose, an appropriate pulse train is generated in the 
NC-control of the machining apparatus and fed via the input 22 to the 
digital/analog converter 20. The latter one creates a continuous analog 
positive output signal +V.sub.1 which is delivered via the buffer 
amplifier 24 to the first input of the differential amplifier 26. The 
absolute value of the voltage of +V.sub.1 is less than the absolute value 
of the voltage +V. 
In an initial phase, the output signal of the tacho generator 30 is zero 
and then slowly increasing as soon as the electric motor 8a starts to 
rotate. In the moment when the positive signal +V.sub.1 arrives at the 
first input of the first differential amplifier 26, its second input 
connected to the output of the tacho generator 30 is still zero with the 
result, that a positive output signal appears at the output of the first 
differential amplifier 26. Thus, a positive signal is fed to the first 
input of the third differential amplifier 34 which is less in value than 
the signal +V connected to the second input of the third differential 
amplifier 34. The consequence is that a positive signal appears at the 
output of the third differential amplifier 34 which is fed via the power 
amplifier 38 to the electric motor 8a and which attempts to drive the 
electric motor 8a in a positive sense of rotation. 
Simultaneously, the positive output signal of the buffer amplifier 24 is 
fed to the first input of the second differential amplifier 28. Since the 
second input of the second differential amplifier 28 is tied to the output 
of the tacho generator 32 which still does not rotate, the second input of 
the second differential amplifier is at zero level and a positive output 
signal appears at the output of the second differential amplifier 28. This 
output signal is fed to the first input of the fourth differential 
amplifier 36. As the second input thereof is tied to -V with an absolute 
value considerable higher than the positive signal appearing at the first 
input of the fourth differential amplifier 36, a high negative output 
signal will appear at the output of the fourth differential amplifier 36. 
This output signal is fed via power amplifier 40 to the electric motor 8b 
and attempts to drive it in a negative sense of rotation. 
As the value of the negative output signal of the power amplifier 40 is 
higher than the value of the positive output signal of the power amplifier 
38, and as the two electric motors 8a and 8b are positively coupled to 
each other via gear wheels 4, 5a and 5b, the spindle will be rotated in a 
negative sense because the electric motor 8b yields a negative torque with 
an absolute value which is higher than the absolute value of positive 
torque yielded by the electric motor 8a. 
Up to now, we have supposed that the outputs of the tacho generators 30 and 
32 and, consequently, the second inputs of the first and second 
differential amplifiers 26 and 28 are at zero level. However, we have 
shown that the two electric motors 8a and 8b and thereby the two tacho 
generators 30 and 32 will rotate in a negative sense. Thus, the tacho 
generators 30 and 32 will yield a negative signal which preferably is 
limited to a selected value. 
The presence of the negative tacho generator output signal at the second 
input of the first differential amplifier 26 causes that the positive 
output signal thereof somewhat increases. Thereby, the difference between 
this positive output signal and +V decreases such that the output signal 
of the third differential amplifier also decreases. Thus, the electric 
motor 8a yields a smaller positive torque. Correspondingly, the presence 
of the negative tacho generator output signal at the second input of the 
second differential amplifier 28 causes that the positive output signal 
thereof somewhat increases. Thereby, the difference between this positive 
output signal and -V decreases such that the output signal of the fourth 
differential amplifier also decreases. After a short time, the operating 
conditions are stabilized and the spindle 2 will rotate in negative sense 
with said first lower speed. 
In a third operating condition, the spindle 2 has to be driven to a 
rotational movement with a second, higher speed in a negative sense of 
rotation. For this purpose, an appropriate pulse train is generated in the 
NC-control of the machining apparatus and fed via the input 22 to the 
digital/analog converter 20. The latter one creates a continuous analog 
positive output signal +V.sub.2 which is delivered via the buffer 
amplifier 24 to the first input of the differential amplifier 26. The 
absolute value of the voltage of +V.sub.2 is higher than the absolute 
value of the voltage +V. 
In an initial phase, the output signal of the tacho generator 30 is zero 
and then slowly increasing as soon as the electric motor 8a starts to 
rotate. In the moment when the positive signal +V.sub.2 arrives at the 
first input of the first differential amplifier 26, its second input 
connected to the output of the tacho generator 30 is still zero with the 
result, that a positive output signal appears at the output of the first 
differential amplifier 26. Thus, a positive signal is fed to the first 
input of the third differential amplifier 34 which is higher in value than 
the signal +V connected to the second input of the third differential 
amplifier 34. The consequence is that a small negative signal appears at 
the output of the third differential amplifier 34 which is fed via the 
power amplifier 38 to the electric motor 8a and which attempts to drive 
the electric motor 8a in a negative sense of rotation with small torque. 
Simultaneously, the positive output signal +V.sub.2 of the buffer amplifier 
24 is fed to the first input of the second differential amplifier 28. 
Since the second input of the second differential amplifier 28 is tied to 
the output of the tacho generator 32 which still does not rotate, the 
second input of the second differential amplifier is at zero level and a 
positive output signal appears at the output of the second differential 
amplifier 28. This output signal is fed to the first input of the fourth 
differential amplifier 36. As the second input thereof is tied to -V with 
an absolute value somewhat lower than the positive signal +V.sub.2 
appearing at the first input of the fourth differential amplifier 36, a 
very high negative output signal will appear at the output of the fourth 
differential amplifier 36. This output signal is fed via power amplifier 
40 to the electric motor 8b and attempts to drive it in a negative sense 
of rotation with high torque. 
As the value of the negative output signal of the power amplifier 40 is 
higher than the value of the negative output signal of the power amplifier 
38, and as the two electric motors 8a and 8b are positively coupled to 
each other via gear wheels 4, 5a and 5b, the spindle will be rotated in a 
negative sense with high speed. However, the gearing and transmission 
members are still biased because the electric motor 8b yields a negative 
torque with an absolute value which is higher than the absolute value of 
negative torque yielded by the electric motor 8a. 
Up to now, we have supposed that the outputs of the tacho generators 30 and 
32 and, consequently, the second inputs of the first and second 
differential amplifiers 26 and 28 are at zero level. However, we have 
shown that the two electric motors 8a and 8b and thereby the two tacho 
generators 30 and 32 will rotate in a negative sense. Thus, the tacho 
generators 30 and 32 will yield a negative signal which preferably is 
limited to a selected value. 
The presence of the negative tacho generator output signal at the second 
input of the first differential amplifier 26 causes that the positive 
output signal thereof somewhat increases. However, it will be still higher 
than +V. Thereby, the difference between this positive output signal and 
+V decreases, but remains negative, such that the output signal of the 
third differential amplifier also decreases. Thus, the electric motor 8a 
yields a smaller negative torque. Correspondingly, the presence of the 
negative tacho generator output signal at the second input of the second 
differential amplifier 28 causes that the positive output signal thereof 
somewhat increases. Thereby, the difference between this positive output 
signal and -V decreases such that the output signal of the fourth 
differential amplifier also decreases. After a short time, the operating 
conditions are stabilized and the spindle 2 will rotate in negative sense 
with said second higher speed. 
In a fourth operating condition, the spindle 2 has to be driven to a 
rotational movement with a first, lower speed in a positive sense of 
rotation. For this purpose, an appropriate pulse train is generated in the 
NC-control of the machining apparatus and fed via the input 22 to the 
digital/analog converter 20. The latter one creates a continuous analog 
negative output signal -V.sub.1 which is delivered via the buffer 
amplifier 24 to the first input of the second differential amplifier 28. 
The absolute value of the voltage of -V.sub.1 is less than the absolute 
value of the voltage -V. 
In an initial phase, the output signal of the tacho generator 32 is zero 
and then slowly increasing as soon as the electric motor 8a starts to 
rotate. In the moment when the negative signal -V.sub.1 arrives at the 
first input of the second differential amplifier 28, its second input 
connected to the output of the tacho generator 32 is still zero with the 
result, that a negative output signal appears at the output of the second 
differential amplifier 28. Thus, a negative signal is fed to the first 
input of the fourth differential amplifier 36 which is less in value than 
the signal -V connected to the second input of the third differential 
amplifier 34. The consequence is that a negative signal appears at the 
output of the fourth differential amplifier 36 which is fed via the power 
amplifier 40 to the electric motor 8b and which attempts to drive the 
electric motor 8b in a negative sense of rotation. 
Simultaneously, the negative output signal of the buffer amplifier 24 is 
fed to the first input of the first differential amplifier 26. Since the 
second input of the first differential amplifier 26 is tied to the output 
of the tacho generator 30 which still does not rotate, the second input of 
the first differential amplifier is at zero level and a negative output 
signal appears at the output of the first differential amplifier 26. This 
output signal is fed to the first input of the third differential 
amplifier 34. As the second input thereof is tied to +V with an absolute 
value considerable higher than the negative signal appearing at the first 
input of the third differential amplifier 36, a high positive output 
signal will appear at the output of third differential amplifier 34. This 
output signal is fed via power amplifier 38 to the electric motor 8a and 
attempts to drive it in a positive sense of rotation. 
As the value of the positive output signal of the power amplifier 38 is 
higher than the value of the negative output signal of the power amplifier 
40, and as the two electric motors 8a and 8b are positively coupled to 
each other via gear wheels 4, 5a and 5b, the spindle will be rotated in a 
positive sense because the electric motor 8a yields a positive torque with 
an absolute value which is higher than the absolute value of negative 
torque yielded by the electric motor 8b. 
Up to now, we have supposed that the outputs of the tacho generators 30 and 
32 and, consequently, the second inputs of the first and second 
differential amplifiers 26 and 28 are at zero level. However, we have 
shown that the two electric motors 8a and 8b and thereby the two tacho 
generators 30 and 32 will rotate in a positive sense. Thus, the tacho 
generators 30 and 32 will yield a positive signal which preferably is 
limited to a selected value. 
The presence of the positive tacho generator output signal at the second 
input of the first differential amplifier 26 causes that the negative 
output signal thereof somewhat decreases. Thereby, the difference between 
this negative output signal and +V decreases such that the output signal 
of the third differential amplifier also decreases. Thus, the electric 
motor 8a yields a smaller negative torque. Correspondingly, the presence 
of the positive tacho generator output signal at the second input of the 
second differential amplifier 28 causes that the negative output signal 
thereof somewhat decreases. Thereby, the difference between this negative 
output signal and -V decreases such that the output signal of the fourth 
differential amplifier also decreases. After a short time, the operating 
conditions are stabilized and the spindle 2 will rotate in positive sense 
with said first lower speed in positive direction of rotation. 
In a fifth operating condition, the spindle 2 has to be driven to a 
rotational movement with a second, higher speed in a positive sense of 
rotation. For this purpose, an appropriate pulse train is generated in the 
NC-control of the machining apparatus and fed via the input 22 to the 
digital/analog converter 20. The latter one creates a continuous analog 
positive output signal -V.sub.2 which is delivered via the buffer 
amplifier 24 to the first input of the second differential amplifier 28. 
The absolute value of the voltage of -V.sub.2 is higher than the absolute 
value of the voltage -V. 
In an initial phase, the output signal of the tacho generator 32 is zero 
and then slowly increasing as soon as the electric motor 8b starts to 
rotate. In the moment when the negative signal -V.sub.2 arrives at the 
first input of the second differential amplifier 28, its second input 
connected to the output of the tacho generator 32 is still zero with the 
result, that a negative output signal appears at the output of the second 
differential amplifier 28. Thus, a negative signal is fed to the first 
input of the fourth differential amplifier 36 which is higher in value 
than the signal -V connected to the second input of the fourth 
differential amplifier 36. The consequence is that a small positive signal 
appears at the output of the fourth differential amplifier 36 which is fed 
via the power amplifier 40 to the electric motor 8b and which attempts to 
drive the electric motor 8a in a positive sense of rotation with small 
torque. 
Simultaneously, the negative output signal -V.sub.2 of the buffer amplifier 
24 is fed to the first input of the first differential amplifier 26. Since 
the second input of the first differential amplifier 26 is tied to the 
output of the tacho generator 30 which still does not rotate, the second 
input of the first differential amplifier is at zero level and a negative 
output signal appears at the output of the first differential amplifier 
26. This output signal is fed to the first input of the third differential 
amplifier 34. As the second input thereof is tied to +V with an absolute 
value somewhat lower than the negative signal -V.sub.2 appearing at the 
first input of the third differential amplifier 34, a very high positive 
output signal will appear at the output of the third differential 
amplifier 34. This output signal is fed via power amplifier 38 to the 
electric motor 8a and attempts to drive it in a positive sense of rotation 
with high torque. 
As the value of the positive output signal of the power amplifier 38 is 
higher than the value of the positive output signal of the power amplifier 
40, and as the two electric motors 8a and 8b are positively coupled to 
each other via gear wheels 4, 5a and 5b, the spindle will be rotated in a 
positive sense with high speed. However, the gearing and transmission 
members are still biased because the electric motor 8a yields a positive 
torque with an absolute value which is higher than the absolute value of 
positive torque yielded by the electric motor 8b. 
Up to now, we have supposed that the outputs of the tacho generators 30 and 
32 and, consequently, the second inputs of the first and second 
differential amplifiers 26 and 28 are at zero level. However, we have 
shown that the two electric motors 8a and 8b and thereby the two tacho 
generators 30 and 32 will rotate in a positive sense. Thus, the tacho 
generators 30 and 32 will yield a positive signal which preferably is 
limited to a selected value. 
The presence of the positive tacho generator output signal at the second 
input of the first differential amplifier 26 causes that the negative 
output signal thereof somewhat decreases. However, it will be still higher 
than +V. Thereby, the difference between this negative output signal and 
+V decreases, but remains negative, such that the output signal of the 
third differential amplifier also decreases. Thus, the electric motor 8a 
yields a smaller negative torque. Correspondingly, the presence of the 
positive tacho generator output signal at the second input of the second 
differential amplifier 28 causes that the negative output signal thereof 
somewhat decreases. Thereby, the difference between this negative output 
signal and -V decreases such that the output signal of the fourth 
differential amplifier also decreases. After a short time, the operating 
conditions are stabilized and the spindle 2 will rotate in positive sense 
with said second higher speed. 
It is understood that the control unit schematically shown in FIG. 5 and 
hereinbefore described represents just one example to control the electric 
motors 8a and 8b as required by the present invention. It should be 
apparent to any person skilled in the art that many other designs of 
control units could be realized which would be in a position to drive the 
two electric motors in opposite senses of revolutions with the same 
torque, in opposite senses of revolution with different values of torque 
and in the same sense of revolution with different values of torque. 
The driving apparatus according to the present invention ensures that every 
backlash in the gear box and every clearance between the gear wheels is 
eliminated or rendered ineffective and that the elasticity in the entire 
transmission assembly is compensated. Thus, an extraordinarily high 
positional accuracy with regard to the rotation angle may be realized 
which is in the range of 1/1000 angular degrees without the need to use 
sophisticated, precise and expensive driving and gear elements which are 
highly subjected to wear. The spindle or the workpiece pallet may be 
rotated very quickly into a desired angular position and the rotational 
movement can be stopped immediately without overshoot. The latter is 
particularly important in the field of spark erosion machining as a spark 
suddenly appears when the electrode is moved near to the work piece; in 
this moment, the movement of the electrode must be stopped immediately, 
but the exact position where this happens cannot be predicted reliably.