CNC Driving System Components

The systems that transform the commands from NC to machine movements are

shown in Fig. 1.3. Figure 1.3a depicts the servo driving mechanism that consists

of a servo motor and power transmission device. The servo, the word originates from

”servue” in Latin, is the device that carries out faithfully the given command. The

command from NC makes the servo motor rotate, the rotation of the servo motor is

transmitted to a ball screw via a coupling, the rotation of the ball screw is transformed

into linear movement of a nut, and finally the table with the workpiece moves linearly.

In summary, the servo driving mechanism controls the velocity and torque of

the table via the servo driving device of each axis based on the velocity commands

from NC. Recently, PMSMs (Permanent Magnet Synchronous Motors) have been

used as servo motors in machine tools.

Depicts the spindle unit which consists of a spindle motor and power

transmission device. The rotation of the spindle motor is transmitted to the spindle

body via a belt and the velocity ratio is dependent on the ratio of pulley sizes between themotor and spindle body. Recently, inductionmotors have come into general use as

the spindlemotors of machine tools because the inductionmotor,which has no brush,

is better than DC motors with respect to size, weight, inertia, efficiency, maximum

speed, and maintenance.

Some machine tools use gears to transmit power instead of a belt. However, power

transmission by gears is not suitable for high-speed machining. Recently, a direct

drive, in which the spindlemotor and spindle body (headstock) are directly connected

without a power transmission device, has been used for high-speed rotation beyond

10,000 rpm.

Driving Motor and Sensor

The term “driving motor” is used to mean both the servo motor, which moves the

table, and the spindle motor, which rotates the spindle. The spindle is the device that

generates adequate cutting speed and torque by rotating the tool or workpiece. Consequently,

high torque and high speed are very important for spindle motors and an

induction motor is generally used due to the characteristics of the spindle motor. Unlike

3-phase motors, the servo motor needs characteristics such as high torque, high

acceleration, and fast response at low speed and can simultaneously control velocity

and position. Machine tools, such as turning machines and machining centers, need

high torque for heavy cutting in the low-speed range and high speed for rapid movement

in the high-speed range. Also, motors with small inertia and high responsibility

are needed for machines that frequently repeat tasks whose machining time is very

short; for example, punch presses and high-speed tapping machines.

The fundamental characteristics required for servo motors of machine tools are

the following:

1. To be able to get adequate output of power according to work load.

2. To be able to respond quickly to an instruction.

3. To have good acceleration and deceleration properties.

4. To have a broad velocity range.

5. To be able to control velocity safely in all velocity ranges.

6. To be able to be continuously operated for a long time

7. To be able to provide frequent acceleration and deceleration.

8. To have high resolution in order to generate adequate torque in the case of a small

block.

9. To be easy to rotate and have high rotation accuracy.

10. To generate adequate torque for stopping.

11. To have high reliability and long length of life.

12. To be easy to maintain.

Servo motors are designed to satisfy the above-mentioned characteristics and the

term comprises the DC Servo Motor, Synchronous Type AC Servo Motor, and Induction

Type AC Servo Motor.

DC Servo Motor

The DC Servo Motor is built as shown in Fig. 1.4a.

The stator consists of a cylindrical frame, which plays the role of the passage

for magnetic flux and mechanical supporter, and the magnet, which is attached to

the inside of the frame. The rotor consists of a shaft and brush. A commutator and

a rotor metal supporting frame (rotor core) are attached to the outside of the shaft

and an armature is coiled in the rotor metal supporting frame. A brush that supplies

current through the commutator is built with the armature coil. At the back of the

shaft, a detector for detecting rotation speed is built into the rotor. In general, an

optical encoder or tacho-generator is used as a detector.

In the DC servo motor, a controller can be easily designed by using a simple circuit

because the torque is directly proportional to the amount of current. The factor

that limits the output of the power is the heat from the inside of the motor due to current.

Therefore, efficient removal of the heat is essential to generate high torque. The

velocity range of DC servo motors is very broad and the price is very low. However,

friction with the brushes results in mechanical loss and noise and it is necessary to

maintain the brushes.

Synchronous-type AC Servo Motor

The stator consists of a cylindrical frame and a stator core. The stator core is located

in the frame and an armature coil is wound around the stator core. The end of the coil

is connected with a lead wire and current is provided from the lead wire. The rotor

consists of a shaft and a permanent magnet and the permanent magnet is attached

to the outside of the shaft. In a synchronous-type AC servo motor, the magnet is

attached to a rotor and an armature coil is wound around the stator unlike the DC

servo motor. Therefore, the supply of current is possible from the outside without a

stator and a synchronous-type AC servo motor is called a “brushless servo motor”

because of this structural characteristic. Because this structure makes it possible to

cool down a stator core directly from the outside, it is possible to resist an increase

in temperature. Also, because a synchronous-type AC servo motor does not have

the limitation of maximum velocity due to rectification spark, a good characteristic

of torque in the high-speed range can be obtained. In addition, because this type of

motor has no brush, it can be operated for a long time without maintenance.

Like a DC servo motor, this type of AC servo motor uses an optical encoder or

a resolver as a detector of rotation velocity. Also, a ferrite magnet or a rare earth

magnet is used for the magnet which is built into the rotor and plays the role of a

field system.

In this type of AC Servo Motor, because an armature contribution is linearly proportional

to torque, Stop is easy and a dynamic brake works during emergency stop.

However, because a permanent magnet is used, the structure is very complex and the

detection of position of the rotor is needed. The current from the armature includes

high-frequency current and the high-frequency current is the source of torque ripple

and vibration.

Induction-type AC Servo Motor

The structure of an induction-type AC servo motor is identical with that of a general

induction motor. If multi-phase alternating current flows through the coil of a stator,

a current is induced in the coil of rotor and the induction current generates torque. In

this type of AC servo motor, the stator consists of a frame, a stator core, an armature

coil, and lead wire. The rotor consists of a shaft and the rotor core that is built with a

conductor.

An induction-type AC servo motor has a simple structure and does not need the

detector of relative position between the rotor and stator. However, because the field

current should flow continuously during stopping, a loss of heating occurs and dynamic

braking is impossible, unlike the AC servo motor.

Encoder

The device that detects the current position for position control is called an encoder

and, generally, is built into the end of the power-transmission shaft. In order to control

velocity, the velocity is detected by a sensor or is calculated by position control

data detected from the encoder. The method for detecting velocity uses the encoder,

a way of counting pulses generated in unit time and a means of detecting the interval

between pulses together.

The detection part of a magnetic-type encoder is different from that of an opticaltype

encoder but the two kinds of encoder generate an output signal in the same

manner. Therefore, in this book, only the optical-type encoder will be addressed in

detail.

An optical-type encoder can be classified as an incremental type or an absolute

type with respect to function.

1. Incremental-type encoder

Figure 1.6 shows the structure of the incremental-type encoderwith three kinds of

slit - A, B, and Z; Slits A and B generate an output waveform, the Z slit generates

the zero phase. The light emitted from an LED is detected by a photo-detector

after passing one slit of the rotation disk and one of the slits A, B, or Z on a fixed

slit panel. Slits A and B are arranged for a phase difference of 90 degrees and the

electric signal of the output is generated as a square wave whose phase difference is 90 degrees. The Z slit generates a square wave, indicating one revolution of the

encoder.

An incremental-type encoder has a simple structure and is cheap. It is also easy to

transmit a signal because the number of wires needed for sending output signals is

small. The number of output pulses from the encoder does not indicate the absolute

rotation position of a shaft but indicates the rotation angle of the shaft. If we

want to know the absolute rotation angle, the number of output pulses should be

summed and the rotation angle is calculated based on the number of accumulated

pulses. Because the rotation angle is detected continuously, the noise that occurs

during signal transmission can be accumulated in a counter. Therefore, some measure

for preventing noise should exist as a basic requirement. If the power is off

then this type of encoder cannot indicate a position. Because this type of encoder

only generates pulses, the number of output pulses should be transformed into an

analog signal that is proportional to the pulse frequency in an F/V converter in

order to detect rotation velocity.

Absolute-type encoder

The structure and the signal generation method of an absolute-type encoder are

identical with those of an incremental-type encoder. However, the disk slit of an

absolute-type encoder and the arrangement of photo-detectors are different from

those of an absolute-type encoder as shown in Fig. 1.7. In this type of encoder,

the slit on a disk slot provides a binary bit; so that, the outermost part of a disk

is set to the lowest bit and as many slits and photo-detectors exist as the number

of bits. The slits are arranged along concentric circles towards the interior of the

disk. Based on these components, the rotation position data is output in binary or

decimal form. In this way, the method where absolute position data is used is a

graycode method.

Resolver

A resolver is a detector of rotation angle and position and is used as the sensor of a

motor. Unlike an encoder that generates an output signal in digital format, a resolver

generates an output in analog format. A resolver consists of a stator, a rotor, and

a rotation transformer. The coils of the stator and rotor are arranged to make the

distribution of magnetic flux a sine wave with respect to the angle. A resolver has

a similar structure to a motor and is insensitive to vibration and mechanical shock.

In addition, because the output is an analog signal, the long-distance transmission of signals and the miniaturization of the device are possible. However, the signalprocessing

circuit is complex and the device is more expensive than a rotary encoder.

Speed Sensor

Although an encoder and a resolver are typical position sensors, they can be used as

speed sensors because speed can be calculated based on positional information from

them. A tacho-generator is one of the typical speed sensors. In general, this is called

a tacho-sensor and can be classified into brush-built-in types and brushless types. A

brush-built-in type has a similar structure to a direct current dynamo. It comprises

a stator, which is made from a permanent magnet, and a rotor, which is coiled. As

a coil emits a magnetic flux with rotation of the rotor, a voltage is generated and is

transmitted to the outside via the brush. The brushless-type comprises a rotor, being

a permanent magnet, a coiled stator, and a single device that detects the position

of the rotor. According to the rotational position of the rotor, the smoothed voltage

induced from each coil is output sequentially. These two types generate a voltage

that is proportional to the rotation speed. However, because the brush-built-in type

has a limitation on life length, it is not used as the speed sensor of a servo motor.