Milling machine stepper motor(Part 1)

A stepper motor is a motor that converts a pulse signal into an angular displacement and is often used in the feed drive of an economical milling machine. As the stepper motor drive without speed and position detection and feedback, so the speed and position control accuracy by the stepper motor and feed the mechanical chain to ensure the accuracy.

1.Composition structure

Stepper motor stator consists of core and winding. The stator core is made of silicon steel sheet and is made into a pole shoe. Pole pieces of uniform surface of the tooth groove, pole with a winding coil, a pair of pole pieces to form a phase winding. The rotor has a reactive type and a mixed type. The reactive rotor core is made of silicon steel sheet, and the circumferential surface is arranged in the tooth groove. The hybrid rotor is equipped with permanent magnet in the rotor core. Stator per phase winding in accordance with the phase sequence after the pulse power generated stator magnetic field, the rotor under the stator magnetic field rotation. The picture shows the five-phase hybrid stepping motor structure diagram.

2. Control

(1) Step angle

Stepping motor each phase of the winding once every step, stepper motor rotor that is turned through an angle, the angle is called the step angle. The magnitude of the step angle determines the amount of linear displacement that the ball screw drives the milling machine table. For example, a milling machine stepper motor and ball screw through the coupling directly connected to the stepper motor step angle of 0.36°, screw pitch of 12mm, then each step angle corresponding to the table of the linear displacement of (12mm/360°) x 0.36°= 0.012mm. Therefore, the position control can be realized by controlling the number of energizing pulses. Stepping motor running process, the number of steps the rotor does not follow the pulse signal is called out of step, out of step will affect the position control accuracy.

(2) Speed and steering control

The speed and direction of the milling machine stepper motor are controlled by the frequency and phase sequence of the energized pulse power. Changing the frequency of the pulse power can change the speed, but the speed range is not wide, the maximum input frequency is generally not greater than 25kHz; change the phase sequence of the pulse power supply change the turn. There are two types of single-shot and double-shot, double-step angle is half of the single shot. The picture shows the three phase stepper motor power phase sequence diagram.

Milling machine servo driver

AC servo driver is a common type of milling machine servo drivers. The servo motor needs to cooperate with the servo driver to complete the corresponding function.

1.AC servo motor vector control

AC servo motor frequency control is a self-control variable frequency synchronous motor system. The frequency and phase of the output current (voltage) of the frequency conversion device (servo drive) are controlled by the position of the rotor pole, which is a closed-loop control mode for stator frequency and phase of the stator winding. In the vector control process, the rotor magnetic pole position of the rotor pole axis of the U-phase stator winding axis angle θ, varies with the rotation of the rotor. The angle can be obtained by direct detection of the encoder or rotary transformer connected by the milling machine servo motor, as shown.

The AC servo drive is essentially a frequency converter for driving an AC servo motor. The AC servo drive controls the AC servo motor by the vector control strategy for the AC servo motor, as well as the SPWM and the inverter, as shown in the figure. Milling machine AC servo motor with vector control can get very good static and dynamic characteristics.

2. Control signal

AC servo drive control signals are analog and digital. The encoder in the servo motor feeds the speed detection signal to the milling machine servo drive on the one hand, carries on the speed control, and on the other hand, sends the position detection signal to the CNC system through the servo drive to carry on the position control. Simulation control, the wiring between the devices, the signal susceptible to interference, affecting the stability of the servo drive. With the development of digital bus control technology, the current milling machine CNC system and servo driver between the use of digital bus communication.

Milling machine electromagnetic brake

The milling machine servo motor which drives the vertical ball screw is equipped with an electromagnetic brake, and the electromagnetic brake is an integral part of the servo motor. The structure is shown in figure.

When the ball screw is mounted vertically (such as the vertical axis of the milling machine or the Z axis of the machining center), the ball screw has no self-locking function. When the machine is powered off and there is no locking device, spindle box or turret will fall on their own. In order to prevent this phenomenon, the electromagnetic brake is mounted on the servo motor, and the motor rotor is locked by the electromagnetic brake when the servo motor is de-energized so that the screw can not be rotated.

In the electromagnetic brake, the brake disc is connected with the rotor shaft by splines, rotates with the rotor, and moves axially. When the servo motor is running normally, the brake coil is energized (+24V). The armature is overcome by the force of the electromagnetic force against the brake spring. The brake disc is released under the action of the return spring. The servo motor is in release state. When brake, the brake coil power loss, the armature in the brake spring under the action of rapid axial movement and push the brake disc, so that the brake disc pressed on the chassis, friction torque generated by the friction plate to lock the rotor shaft, so as to achieve the purpose of braking.

The above is about the milling machine electromagnetic brake introduction, if there are problems, welcome to discuss.

CNC milling machine servo motor (Part 2)

In the last article, we introduced the development process of CNC milling machine servo motor and its composition structure. Today, let's talk about its working principle.

AC servo motor stator winding three-phase alternating current, the stator and rotor air gap generated between the rotating magnetic field, the rotor and the permanent magnet on the rotor magnetic field to attract each other, rotating the magnetic field to drive the rotor rotation, the rotor speed and rotating magnetic field speed consistent. Because the stator rotation magnetic field speed is proportional to the frequency of the power supply, so in the rated torque range, no matter how the load changes, as long as the power frequency unchanged, the motor speed remains unchanged, and the rotor speed is proportional to the power frequency.

In addition, the milling machine AC servo motor with a start, stop responding fast, wide speed range, overload capacity and so on. AC servo motor according to the speed, output torque, inertia and accuracy indicators such as series of products, such as FANUC AC servo motor αi and βi series. The former has a good acceleration and deceleration performance, the maximum speed of up to 6000r/min, the maximum output torque up to 500N∙m, for high-speed, high-precision CNC milling machine servo feed drive. The latter acceleration and deceleration and high, low speed and other performance less than αi series, but the price is high, for the economy of CNC milling machine servo feed drive.

The above is the basic introduction on the CNC milling machine servo motor working principle, welcome to continue to pay attention.

CNC milling machine servo motor (Part 1)

In the CNC milling machine feed servo system, the feed motor drives the ball screw to realize the feed drive, and the drive which matches the feed motor receives the control signal of the numerical control system, and supplies the power supply to the feed motor by the power amplification so that the feed motor output torque, speed and angular displacement. At present, CNC milling machine closed-loop or semi-closed-loop servo system used in the feed motor and drive is usually AC servo motor and servo drive, open-loop servo system used to feed the motor and drive into the stepper motor and stepper drive.

CNC milling machine servo motor through the early DC servo motor, brushless DC motor, the development of the current three-phase AC sine wave permanent magnet synchronous motor. The current use of CNC milling machine AC servo motor is usually referred to as three-phase AC sine wave permanent magnet synchronous motor. The picture shows the appearance and setting of the FANUC AC servo motor.

AC servo motor stator without frame. Stator core is made of silicon steel sheet, the stator shape is polygonal; stator core coil groove embedded with three symmetrical winding, through three-phase AC power; in addition, the stator winding embedded in the thermistor, the motor temperature detection, to achieve overload and over current protection. The AC servo motor rotor consists of a rotor core and a permanent magnet. The rotor core is made of silicon steel sheet, the permanent magnet is NdFeB permanent magnet material, or laying on the rotor core surface (surface type or convex type), or embedded in the rotor core slot (built-in type). In addition, the AC servo motor rotor coaxially coupled encoder or rotary transformer, used to detect the servo motor speed and angular displacement.

Above is the basic introduction of the CNC milling machine servo motor development process and the composition, you are welcome to continue to pay attention. 

CNC milling machine spindle inverter

The inverter is essentially a power conversion device. The inverter used in the CNC milling machine is usually a voltage type inverter. According to the frequency reference control signal, the input sine wave AC power through the internal control circuit and power switch, the output voltage and frequency variable, and sine wave equivalent three-phase square wave voltage, in order to achieve the milling machine spindle motor frequency speed, as shown.

Frequency given a variety of signal forms, such as voltage, current analog frequency reference, switch digital fixed frequency reference, and bus and network frequency given. Among them, the voltage 0~+10V and the inverter output power frequency linear relationship, change the size of a given voltage can change the output frequency of high and low, in order to achieve frequency control. For example, set the given voltage 10V corresponding to the output frequency of 50Hz, if the inverter connected to 4 pole (pole pairs p=2) three-phase asynchronous motor, the motor speed is 1470r/min; if the given voltage is 5V, the inverter output Power frequency of 25Hz, at this time, the motor speed is about 735 r/min. CNC milling machine analog spindle control, the CNC system through the implementation of the S command, the arithmetic processing output 0~+10 V frequency given voltage to the inverter. The input switch signal in the inverter includes the control signals of the inverter running, such as forward rotation, reverse rotation, stop, and the rise and deceleration switching frequency reference and the switch digital fixed frequency reference signal; the output switch signal includes the frequency converter Alarm, running status and other signals. CNC milling machine, the inverter switching signal from the CNC system PLC control, CNC system through the implementation of instructions, the PLC program on the inverter input and output switch signal control.

The inverter mainly consists of main circuit board and control circuit board. The main circuit board has power module, drive circuit and so on. The control circuit board includes a control circuit composed of a microprocessor, a detection circuit, an input / output circuit, and the like. The other includes an auxiliary power supply, an operation panel and a display unit, as shown in the figure.

CNC milling machine motorized spindle

CNC milling machine motorized spindle is a device that combines the motor with a high precision spindle. In the electric spindle, the rotor of the motor is directly used as the main shaft of the milling machine, which saves the mechanical drive link of the main shaft, improves the drive accuracy of the main shaft, and reduces noise and vibration. It is widely used in high speed and high precision CNC milling machine. As shown in the diagram, the application of motorized spindle in 5 axis CNC milling machine.

Motorized spindle drive by the inverter, the maximum speed of more than 100000r/min. The main shaft of the motor has a cooling pipe, and the motor is recirculated cooled through an external heat exchanger. The main shaft bearings are lubricated by aerosol, and the lubricating oil is regularly and quantitatively pressed into the main shaft bearing by compressed air. The milling machine motorized spindle is equipped with an encoder for spindle speed and position detection. The electric spindle is also provided with a temperature sensor to detect the temperature of the motor. Some electric spindles are also equipped with axial displacement sensors to detect the axial displacement caused by the thermal deformation of the spindle and compensate by the numerical control system. The rear end of the milling machine motorized spindle is provided with a loose and tight cylinder, and the handle is clamped and loosened by a pull rod, a disc spring and a claw.