Preface

 

 

 

The drive and servo positioning system are important parts of the entire chip placement machine control system. Both the mechanical system and the visual system work under the command of the control system. The control system can be functionally divided into two blocks, namely motion control and switch control (commonly called I/O control). Motion control mainly performs servo control on the four motion axes of X, Y, Z and angle, including speed. and acceleration control motion trajectory control, etc.; I/O control is mainly to obtain sensor signals and control various switching quantities, such as position sensor signal collection, control of various valves, vacuum control, light flash control and CCD image acquisition trigger signal control, etc. Motion control and I/O control are coordinated and completed under the unified command of the software system according to specific process requirements. The software system is actually the front end of the electrical appliance control system. It is also the main interface for actual dialogue. When the software system receives the operator's instructions, it will mobilize the electrical appliance control system to perform a series of operations.

 

 

 

1. Overview of drive and servo positioning

 

 

 

1. Drive system

 

 

 

  Take Universal's single-beam placement machine GSM1 with a FLEXJET head as an example. It includes six axes: X, Y, Z, angle Theta, board width control PWC, and belt transfer, which together form a drive system. Among them, the X and Y axes are high power axes and are controlled by the corresponding servo amplifiers in the high power box; the angle Theta, plate width control PWC and belt transmission Belt Transfer are high power axes and are controlled by the corresponding servo amplifiers in the low power box. If the model is different and the number or type of heads on the machine is different, the number and type of axes in the drive system will also be different. For example, the single-beam placement machine GSM2 with a FLEXJET head has ten axes: X1, Y1, Z1, angle Theta1, X2, Y2, Z2, angle Theta2, board width control PWC and belt transmission Belt Transfer.

 

 

 

The accuracy and speed of the placement machine are mainly determined by the X-axis and Y-axis. Open-loop position control placement machines use chain or belt transmission mechanisms, which have low control accuracy; closed-loop position control systems generally use screw transmission mechanisms. The GENESIS high-end placement machines produced by Universal Company use advanced VRM (Variable Reluctant Motor) technology. Make control accuracy higher. A linear grating ruler is used to feedback the position of the placement head, which has high reliability and no mechanical wear.

 

 

 

2. Servo positioning system

 

 

 

The positioning system of high-precision placement machines generally adopts servo closed-loop control method, which can ensure accurate positioning function and has a great impact on placement accuracy and placement rate. At present, general placement machines are driven by DC servo motors and driven by toothed belts or ball screw screws; high-speed placement machines use frictionless linear motor drives and air bearing guide rail transmission. The linear speed of movement is 300~2000mm/s, the acceleration is 7~10 m/s2, and the positioning accuracy is ±0.01mm. There are corresponding grating rulers on the X and Y axes of the X and Y axis servo mechanisms to sense the actual coordinates of the placement head after movement.

 

 

 

3. Composition of the placement machine control system

 

 

 

A complete motion control system mainly consists of a host computer, motion controller, motor servo driver, actuator and position detection device.

 

 

 

The host computer is generally a terminal device such as an industrial computer that directly faces users. It is mainly responsible for human-computer interaction, writing programs to issue instructions to the controller and receiving feedback signals.

 

 

 

Motion controllers are usually motion control cards, PLCs with motion control functions, numerical control systems (CNC) or microcontroller systems, etc. Its function is to receive signals from the host computer and analyze and calculate specific motion commands, and then sends them to the motor driver in the form of digital pulse signals or analog quantities.

 

 

 

The function of the driver is to perform power conversion and drive the motor to rotate according to the upper control signal.

 

 

 

The actuators in the placement machine include execution motors, transmission mechanisms, and placement devices. They are the final controlled objects and achieve precise movement and positioning through the control of upper-level equipment. Commonly used execution motors in motion control systems are generally stepper motors, digital AC servo motors, and DC servo motors.

 

 

 

Position feedback devices include pulse encoders, rotary transformers, induction synchronizers, gratings, magnetic rulers and laser interferometers. Its function is to feed back the detected position to the controller or driver to form semi-closed loop or fully closed-loop control.

 

 

 

The figure below shows the schematic diagram of the placement machine control system.

 

 

 

 

 

2. X-Y servo positioning

 

 

 

1. X-Y position system classification

 

 

 

There are three types of X-Y positioning systems of placement machines: Overhead Gantry-Style system for X-Y motion positioning of the placement head, Turret System for X-Y motion positioning of printed circuit boards, and Massively Parallel System (Massively Parallel System). Each position system has advantages and disadvantages. Depending on its application or process used, there is usually a trade-off between speed and accuracy, as shown in Figure 1, Figure 2 and Figure 3 below.

 

 

 

(1) X-Y motion positioning system of placement head

 

 

 

Overhead gantry-type systems offer greater flexibility and precision over a wide range of components, but cannot match the speed of turret-type or massively parallel systems. As the component range becomes more concentrated, in active devices (Active Device), such as pin-type (Quad Flat Pack, QFP), area array (Area-Array) components, such as BGA (Ball Grid Array), the placement accuracy is very important. Achieving higher pass rates becomes even more critical. Turret-type and massively parallel systems are generally not used for these types of components. The overhead arch positioning system uses a beam to move the placement head (mounted on the X-axis beam) to a specific position on the PCB, and the PCB is clamped in a fixed position during placement. The placement head moves along the X- and Y-axis directions to pick up components from the feeder, and then moves to the photographing or placement position.

 

 

 

The placement heads installed on the overhead arch system can be of various types and modes. For details, please refer to the placement head section of this chapter. At the same time, the placement heads also have two types: fixed and replaceable. Each type, mode and method has its own characteristics. Characteristics, no system has absolute advantages.

 

 

 

(2) Printed circuit board X-Y motion positioning system

 

 

 

In the turret-type placement machine, the placement head is mounted at a fixed position, and the PCB is positioned at the placement position by the X-Y motion system. In order to maintain high speed, the movement speed of the printed circuit board in the X-Y direction must be high enough, so the positioning accuracy will be affected. For details, see the patch head section of this chapter.

 

 

 

(3) Large parallel system

 

 

 

Strictly speaking, the X-Y motion system of a large parallel system is not an independent type, but due to the characteristics of this system, for example, in an X-Y motion system, the X-direction movement distance is generally relatively small, and the working mode is different from traditional machines. So here it is listed as a type. Large parallel systems use a series of small individual patch units (or modules). Each unit has its own X-Y motion positioning system with cameras and placement heads installed. Each placement head can take materials from a limited belt feeder, and the PCB is advanced step by step in the machine at fixed intervals. Each unit machine operates slowly individually, however, they operate continuously or in parallel to achieve high output.

 

 

 

2. X-Y servo positioning system resolution

 

 

 

(1) X-axis and Y-axis resolution of the placement machine

 

 

 

The X-axis and Y-axis use a grating ruler to provide feedback to the controller. The scale on the ruler is very precise, but it does not necessarily provide the final resolution to accurately position the X-axis and Y-axis. Differentiation and multiplication of resolution are improved, thereby increasing the number of counts that the axis controller can see.

 

 

 

(2) Grating ruler resolution

 

 

 

The X and Y axes are rated to be positioned 100 µm apart from each other with a scale (40 µm apart for linear motor machines), and the read head is able to read these lines when the axis is passing through at maximum speed, as shown in the figure below.

 

 

 

(3) Reading head

 

 

 

The read head of each axis reflects a beam of light back through the grating scale. When the light beam returns to the read head, a sine wave analog voltage curve is generated. When the read head passes the data lines on the scale, the read head voltage changes due to interference between the data lines and the light beam. Like other encoders, the read head has an "A" phase and a "B" phase, which are 90° out of phase. Therefore, for each data line on the grating scale, two voltage signals with different phases are obtained ( A and B), as shown in the figure below.

 

 

 

 

 

(4) Improvement of resolution

 

 

 

The A-phase and B-phase analog voltage signals are transmitted to the differential box of the machine. The differential circuit generates a digital square wave signal that changes the direction state at different points of the sine wave input. This square wave signal consists of two phases, A and B, with a phase difference of 90°, and the operating frequency is 10 times that of the original sinusoidal curve. Because 10 digital outputs are generated from each analog sine wave input, the electronic resolution at this point is increased from 100μm to 10μm.

 

 

 

In many cases, a square wave is actually a direct current DC signal, more or less high or low. The signal generated by the read head itself is an analog wave, not digital. The signal from the read head is a sinusoidal curve. There are many ways in electronic technology to differentiate the sinusoidal curve in any way you want. The differential circuit divides each sinusoidal curve into the same 10 parts and converts it into a square wave. As shown below.

 

 

 

 

 

(5) Final resolution

 

 

 

The A and B phase digital signals with different phases are sent from the differential box to an up/down counter circuit on the axis controller of the VME box. This circuit counts each time a phase changes its voltage state: so when phase A reaches a high voltage state, it counts (I); when phase A reaches a low voltage state, it counts (II); when phase B voltage rises, It counts (III); when the B-phase voltage decreases, it counts (IV). On phase A and phase B, each line must be turned on and off (here we are talking about digital on/off). This is what the encoder manufacturer calls basic differentiation. Each line therefore gives 4 counts, effectively increasing the machine's resolution to 2.5 μm. (Linear motors can reach 1μm). Note: Because A and B are in different phases, the counter circuit can automatically detect when an axis is driven in different directions and automatically increment the up and down count accordingly, as shown in the figure below.

 

 

 

The final resolution can be calculated according to the following formula (taking a linear motor as an example).

 

 

 

① X-axis and Y-axis (the scale spacing of the linear grating scale is 40 μm):

 

Rxy=N/(SMe)

 

In the formula, Rxy is the final resolution of the X-axis and Y-axis; N is the scale spacing of the linear grating ruler; S is the reading on each line; Me is the electron multiple.

 

For example, if the system uses a linear grating scale with a scale spacing of 40 μm, the reading on each line is 4, and the electron multiple is 1, then

 

Rxy=40/(4×10)=1(μm)

 

That is, the final resolution of the X-axis and Y-axis of the system is 0.003 6°.

 

 

 

② Theta angle axis final resolution:

 

Rθ=360°/(NrNMe)

 

In the formula, Rθ is the final resolution of Theta angle axis; Nr is the number of lines of the rotary encoder; N is the count of each line; Me is the electron multiple.

 

For example, if the system uses a rotary encoder with 2500 lines, the count of each line is 4, and the electronic multiple is 10, then

 

Rθ=360°/（2500×4×10）=0.0036°

 

That is, the final resolution of the Theta angle axis of this system is 0.003 6°.

 

 

 

3. Z-axis servo positioning

 

 

 

In a general-purpose machine, the base supporting the placement head is fixed on the X guide rail, and the base does not move in the Z direction. The Z-axis control system of the placement machine completes the positioning of the placement head nozzle during the movement, that is, the Z-direction positioning during the material picking and placement processes to adapt to different PCB thicknesses and component heights and meet placement requirements. Common Z-axis methods include pneumatic drive and stepper/servo motor drive. The accuracy of angular axis motion has a great influence on the placement accuracy, and the resolution requirements are high, especially for fine-pitch components. The Z-direction servo control of the nozzle on the patch head is similar to the X-Y servo positioning system, that is, an AC/DC servo motor using a circular grating encoder-ball screw or synchronous belt mechanism. Its servo motor - ball screw is installed above the suction nozzle; when using AC/DC servo motor - synchronous belt mechanism, its motor can be installed on the side, and the suction nozzle can be controlled in the Z direction through the transmission wheel. Since the Z-direction movement stroke of the suction nozzle is short and a grating encoder is used, the control accuracy can meet the requirements.

 

 

 

The Z-axis and PWC width adjustment axis, Transfer Belts belt transmission axis and Theta rotation angle axis are low-power axes in the placement machine, and their drive comes from a low-power motor; the motor drive control comes from a low-power servo box; the servo box in the servo box The corresponding servo board is controlled by the axis controller (low-end controller) in the VME box; the axis controller executes instructions from the machine main controller (high-end controller). The moving position of the motor is counted by the encoder, and then the count is multiplied through the optional differential box to improve the accuracy. The position count is fed back to the corresponding axis controller, and then the axis controller reports it to the machine main controller, so that the machine The main controller can judge the operating status of the corresponding axis (PWC width adjustment axis, Transfer Belts belt transmission axis, Theta angle and Z axis), as shown in the figure below.

 

 

 

 

 

4. Motion control

 

 

 

In the field of industrial control, since the beginning of the 20th century, closed-loop control theory and application systems based on negative feedback have been widely used in various branches. Especially after the Second World War, with the rapid development of the world's semiconductor industry, The semiconductor and placement equipment manufacturing industry that emerged to meet this emerging industry has received huge development opportunities. The closed-loop servo motion control system based on negative feedback occupies a major position in the placement equipment manufacturing motion control system. The following is The structural characteristics of motion control systems currently commonly used in the patch equipment industry are introduced.

 

1. Commonly used servo control systems for placement machines

 

 

 

The commonly used control system structure of placement machines mainly includes the following two aspects. These two aspects mainly implement point-to-point position control.

 

 

 

(1) Position and speed feedback come from the servo motor itself

 

 

 

The advantage of this control method is that it has lower requirements on the entire closed-loop servo system control loop, and is suitable for occasions with low requirements for high-speed and high-precision position control, such as PCB transfer mechanisms, as shown in the figure below. Because the position feedback sensor is installed on the motor body, it cannot reflect the precise position of the driven load very accurately and in real time, especially when the system is required to be fast and highly accurate.

 

 

 

 

 

(2) Position and speed feedback come from the actuator (driven component)Position and speed feedback comes from the load's servo control system, as shown in the figure below.

 

 

 

 

 

This control method has high requirements for the entire closed-loop servo system control loop, and is suitable for high-speed, high-repetition positioning accuracy applications, such as the X-Y motion system of the placement machine and the Z-axis and rotation axis of the placement head. This structure can overcome the defects described in Figure 1 below, so it is widely used in placement machines. Figures 1, 2 and 3 below show the position control closed-loop and Z-axis closed-loop servo system control loop diagrams of the X-Y servo positioning system of a typical placement machine.

 

 

 

Figure 1 High power servo system control loop diagram of a typical placement machine

 

 

 

 

 

Figure 2 Control loop diagram of the Z-axis press induction closed-loop servo system of a typical placement machine

 

 

 

 

 

Figure 3 Control loop diagram of a low-power servo closed-loop servo system of a typical placement machine

 

 

 

2. Linear drive structure commonly used in placement machines

 

 

 

The linear drive structure used in the placement machine typically includes the X-Y motion system, board width adjustment and Z-axis structure of the placement head, which are mainly divided into the following two types.

 

 

 

(1) The structure using a rotating motor ball screw and a sliding guide rail uses a rotating motor ball screw and a sliding guide rail as shown in the figure below. The characteristic of this structure is that the rotary motion of the rotary motor is converted into linear motion through the ball screw and the linear sliding guide rail, thereby achieving point-to-point position control in the X-Y plane or Z direction. The elements that make up this structure also include synchronous belts/pulleys or couplings, which transmit the torque of the rotating motor to the ball screw, thereby pushing the nut (Nut) of the ball screw, which is fixed with the load-bearing block through screws. The screw material bearing block is fixed on the slide block of the linear guide rail), and under the guidance of the linear sliding guide rail, the conversion from rotary motion to linear motion is realized. The transmission efficiency of this system with good coordination can reach more than 95%. This structure has been widely used by the equipment industry in the past few decades. With the improvement of modern linear motor technology, it has become increasingly mature, resulting in a structure of linear motors and sliding guides. .

 

 

 

 

 

The picture below is a physical picture of the Universal Instruments AC30 using this structure.

 

 

 

 

 

(2) A structure using linear motors and sliding guide rails

 

 

 

The linear system composed of a rotary motor uses fewer components, has a low failure rate, and basically requires no maintenance. Currently, in new chip placement machines, this structure is gradually replacing the structure of a rotary motor ball screw and a sliding guide rail. For actual pictures, please refer to the Universal Instruments Genesis series, as shown below.

 

 

 

 

 

5. Commonly used motors and control/transmission components of placement machines

 

 

 

Commonly used servo motors for patch machines are mainly divided into two categories, namely, rotary servo motors and linear servo motors. There are many types of control/transmission components. Due to space limitations, this section only introduces a few main components. For further understanding, you can refer to relevant materials. and monographs.

 

 

 

1. The classification of rotary motors and rotary servo motors is not consistent in various regions around the world. The following is a brief introduction to the two commonly used ones in the placement machine industry.

 

 

 

(1) DC brushed servo motor (BDC)

 

The picture below is a typical structural picture of a DC brushed servo motor.

 

 

 

 

 

As can be seen from the picture above, this type of motor has the following basic characteristics:

 

① The stator permanent magnet is fixed on the motor body to form the stator.

 

② The rotor coil and rotor silicon steel laminations are fixed on the motor shaft.

 

③ The commutation brush is fixed on the end of the motor shaft (coaxial). The commutation brush electrode is connected to the motor driver through two leads. The motor commutation is achieved by the mechanical rotation of the commutation brush.

 

 

 

(2) AC servo motor

 

 

 

There are two typical representatives of AC servo motors: one is the AC brushless servo motor (BLDC), and the other is the permanent magnet sine AC servo motor (PMSM). They are commonly used in patch equipment to convert electrical energy into mechanical energy. The typical structure of the components is as shown in the figure below. The difference from the DC brush servo motor is that the structure of its rotor (motor) and stator is opposite to that of the DC brush servo motor, as can be seen from the figure below:

 

① The stator coil and stator silicon steel sheet are fixed on the motor body.

 

②The rotor permanent magnet is fixed on the motor shaft, and the bearings at both ends of the motor shaft form the rotor.

 

③ Since the coil is on the motor body, there is no need for a commutation brush. Instead, an electronic auxiliary commutation sensor is installed on the motor shaft end. At present, the more common electronic commutator is mainly composed of Hall devices, and the phase angle of the motor is fed back to the control system through the Hall sensor.

 

 

 

 

 

There is a difference between AC brushless servo motor (BLDC) and permanent magnet sine AC servo motor (PMSM). The main difference lies in the different winding methods of the stator coil. The ultimate goal is to make the AC brushless servo motor (BLDC) generate square waves. Back EMF, while a permanent magnet sinusoidal AC servo motor (PMSM) produces a sine wave back EMF.

 

 

 

At present, many motor manufacturers have also developed a variety of high-efficiency combined AC servo motors. The picture below shows one of them developed by a well-known motor manufacturer.

 

 

 

This kind of electric motor can provide higher torque while maintaining a small size.

 

 

 

(3) VRM rotating motor

 

 

 

The picture below is a 3D picture and a physical picture of the Universal Instruments high-speed lightning head composed of a VRM rotary motor. As the name suggests, this is a rotary motor using a VRM structure. Its principles and characteristics are similar to Universal Instruments' VRM linear motor and will not be discussed here. Application representative: Universal Instruments Lightning Head.

 

 

 

With the further miniaturization of patch components, higher requirements have been placed on the positioning accuracy and speed of the X-Y motion mechanism of the patch machine. In this case, the traditional rotating motor plus a set of conversion mechanisms (ball screw The linear motion drive device composed of (plus sliding guide rail) has gradually been unable to meet the requirements of modern patch control systems. Therefore, in recent years, the world's major patch equipment manufacturers have been researching, developing and applying linear motors, making the application of linear motors Getting wider.

 

 

 

 

 

 

 

2. Linear motor

 

 

 

A linear motor is a driving device that directly converts electrical energy into mechanical energy for linear motion without any intermediate conversion transmission. It has the characteristics of high efficiency, energy saving and high precision that traditional motor-driven electromechanical equipment cannot achieve. It can effectively overcome the large size, low efficiency, high energy consumption and poor precision of the transmission chain inherent in the mechanical conversion mechanism when using traditional rotating motors. Disadvantages such as environmental pollution.

 

 

 

The linear servo motor can be considered as a structural deformation of the rotary motor. It cuts and straightens the stator of the traditional cylindrical motor, changes the closed magnetic field of the stator permanent magnet into an open magnetic field, and splices it according to the needs of different strokes. The winding method of the mover (moving part) coil has changed. After the three-phase symmetrical sinusoidal current is passed into the three-phase winding of the electric motor, an air gap magnetic field is generated. The distribution of the air gap magnetic field is similar to that of the rotating motor, and it is distributed in a sinusoidal wave along the unfolded straight line direction. When the three-phase current changes with time, the air gap magnetic field moves along a straight line and translates according to the directional phase sequence, which is called a traveling wave magnetic field. When the current of the mover (moving part) coil interacts with the air gap magnetic field to generate electromagnetic thrust, since the stator is stationary, the mover moves linearly in the direction of the traveling magnetic field.

 

 

 

Linear motors are mainly used in three aspects: First, they are used in automatic control systems, and there are many such applications; second, they are used as drive motors for long-term continuous operation; third, they are used in applications that require huge linear motion in a short time and short distance. in a capable device.

 

 

 

The following is a brief introduction to the two linear motors commonly used in the chip industry.

 

 

 

(1) Stator permanent magnet linear motor

 

 

 

The simplified structure is shown in Figure 1 below, and the actual picture is shown in Figure 2 below.

 

 

 

Figure 1 Simplified structure diagram

 

 

 

 

 

Figure 2 Actual picture

 

 

 

Features:

 

① High cost;

 

② Provide continuous thrust without being affected by the position of the mover;

 

③ High efficiency, low noise;

 

④ High push-to-weight ratio. Application representatives: Universal Instruments (5588), Siemens, Panasonic and HITACH, etc.

 

 

 

(2) VRM linear motor

 

 

 

The picture below is a physical picture of the Universal Instruments VRM motor.

 

 

 

 

 

The stator of the VRM motor uses silicon steel laminations (as shown in the figure below), and the mover consists of two "C" shaped magnets.

 

 

 

 

 

Features:

 

① Low cost;

 

② Provide continuous thrust without being affected by the position of the mover;

 

③ High efficiency, fast heat dissipation, low noise;

 

④ Large volume, high thrust-to-weight ratio, capable of driving large loads. Application representative: Universal Instruments Genesis series.

 

 

 

(3) Characteristics of linear motors

 

Generally speaking, compared with rotary motors, linear motors mainly have the following characteristics.

 

① Simple structure: Since linear motors do not require additional devices (such as ball screws, synchronous belts/pulleys and couplings) to convert rotational motion into linear motion, the structure of the system itself is greatly simplified, and the weight and volume are large. For drop, such as stator permanent magnet linear motor.

 

② High positioning accuracy: Where linear motion is required, linear motors can achieve direct transmission, thus eliminating various positioning errors caused by intermediate links. Therefore, the positioning accuracy is high. When servo control is used, the entire The positioning accuracy of the system.

 

③ Fast response, high sensitivity, and good follow-up: It is easy for linear motors to support the mover with magnetic levitation, so that a certain air gap is always maintained between the mover and the stator without contact, which eliminates the need for the stator and the stator to be in contact. The contact friction resistance between the elements greatly improves the sensitivity, rapidity and follow-up of the system.

 

④ Safe and reliable work, long life: Linear motors can achieve contactless transmission of force, and mechanical friction loss is almost zero, so there are few faults, maintenance-free, safe and reliable work, and long life.

 

 

 

3. Other components of motion control/transmission system

 

 

 

In the motion control system that constitutes the placement machine, in addition to the motor, servo controller and drive amplifier, there are also the following main components.

 

 

 

(1) Encoder An encoder is a device that converts mechanical displacement (including rotation angle and linear displacement) into electrical pulse signals. During the working process, it will transmit electrical pulse signals or position data to the control system, and then feed back the mechanical displacement of the controlled component to the control system, thereby realizing closed-loop position control. Judging from the current application situation, there are two main categories, namely, rotary encoders for angular position feedback and magnetic/grating rulers for linear position feedback, which will be briefly introduced below.

 

 

1) Rotary encoder

 

Rotary encoders are divided into incremental type and absolute type according to the signal principle. The incremental encoder only outputs pulse signals to the servo controller. It only provides relative position and increase/decrease information. The position increase/decrease information is controlled based on the occurrence of phase A and phase B (whose phase is leading). The absolute encoder outputs a digital number to the servo controller, which contains a microprocessor inside. It defines an absolute origin, and the pulse signal it generates is processed by the processor and then converted into a digital code and sent to the servo controller. For servo control, these position data are absolute positions relative to the origin. Rotary encoders can be mainly divided into two categories according to the induction principle. One is a rotary encoder composed of magnetic induction principle; the other is a rotary encoder composed of photoelectric conversion principle. The following is an introduction to the basic structure of the currently popular rotary encoder based on the principle of photoelectric conversion.

 

 

 

2) Incremental rotary encoder

 

The picture below is a physical picture of a certain brand of incremental rotary encoder.

 

 

 

 

 

The figure below is the basic structure of an incremental rotary encoder. The output signal of the encoder in the figure is a TTL pulse signal.

 

 

 

 

 

An enlarged view of the encoder disc is shown below.

 

 

 

 

 

① Working principle:

 

The code disk with a shaft in the center is shown in Figure 3.41. There are radially distributed slots on the code disk (usually there are 3 groups, A, B and Z). There are photoelectric transmitting and receiving devices (photoelectric transmitters) distributed on both sides of the code disk. Sensor, as shown in Figure 3.41), when a slot passes the corresponding photoelectric sensor, the photoelectric sensor is turned on, and its angle code is converted through the three internal photosensitive (A-phase, B-phase and Z-phase) receiving tubes. The timing and phase relationship of the disk generate output signals, and finally after being processed by the encoder, 3 types of pulse signals will be obtained and combined into A, B, and Z. The B phase lags the A phase by 90° (360° relative to one cycle), while the Z phase pulse is output every 360° mechanical angle. Phase A and phase B will be used for position feedback. Through them, the angular displacement of the code plate can be increased (positive direction) or reduced (negative direction). At the same time, since the A and B phases are 90° different, the A phase can be compared by comparing the Whether the encoder is forward or B-phase is forward, determine the forward and reverse rotation of the encoder, and the Z-phase pulse can be used as a zero reference.

 

 

 

② Encoder disc material:

 

The encoder code disc is an important part of the encoder, and its materials include glass, metal and plastic. The glass code disk is a thin groove deposited on the glass, which has good thermal stability and high accuracy. The metal code disk is directly grooved with pass and non-pass, and is not easy to break. However, due to the certain thickness of the metal, the accuracy is limited. Its thermal stability is an order of magnitude worse than that of glass; plastic code discs are economical and have low cost, but their accuracy, thermal stability and lifespan are poorer.

 

 

 

③ Resolution:

 

The number of grooves provided by the encoder per 360° rotation is called resolution, also called analytical graduation or directly called the number of lines. Generally, the graduation per revolution is 5 to 10,000 lines.

 

 

 

④ Signal output:

 

Signal outputs include sine wave (current or voltage), square wave (TTL and HTL), open collector (PNP and NPN) and push-pull. Among them, TTL is a long-line differential drive (symmetrical A, A-; B, B -; Z, Z-), HTL is also called push-pull and push-pull output. The signal receiving device interface of the encoder should correspond to the encoder.

 

 

 

⑤ Problems with incremental encoders:

 

Incremental encoders have zero-point cumulative errors and poor anti-interference. The receiving equipment needs to be powered off and memorized when it is stopped. Problems such as change or reference position should be solved when starting up. These problems can be solved by using absolute encoders. The general applications of incremental encoders are: measuring speed, measuring direction of rotation, and measuring movement angle and distance (relative).

 

 

 

⑥ Example of calculating the theoretical feedback accuracy of the system:

 

The position feedback system composed of an incremental rotary encoder. Assuming that the encoder is directly connected to the motor shaft and has 1000 lines, then every time the motor rotates 360°, 4 times the frequency is used (taking the rising and falling edges of A phase and B phase edge) the servo controller will record the number of pulses 1000×4=4000. Assuming that the transmission ratio of this system is 1, then for:

 

Angular position system, such as theta axis, the theoretical feedback accuracy of this angular position system is

 

360/1/4000=0.09°/pulse

 

For a linear system, assuming that the pitch of the ball screw is 10mm per revolution (360°), the theoretical feedback accuracy of the linear system is

 

10/1/4000=0.002 5mm/pulse=2.5 μm/pulse

 

 

 

3) Absolute rotary encoder

 

 

 

There are many optical channel slots on the absolute encoder optical code disk, and eachThe slots are arranged in order of 2 lines, 4 lines, 8 lines, 16 lines... In this way, at each position of the encoder, by reading the pass and dark of each slot, a set of zero times from 2 can be obtained The only binary code (usually using Gray code) to the n-1 power of 2, this is called an n-bit absolute encoder.

 

 

 

Each position of an absolute encoder determined by the mechanical position is unique. It does not need to be memorized (and therefore not affected by power outages), does not need to find a reference point, and does not need to keep counting. Whenever you need to know the position, you can read it. position, thus the anti-interference characteristics of the encoder and the reliability of data are greatly improved.

 

 

 

Absolute encoders are divided into: single-turn absolute encoders and multi-turn absolute encoders.

 

 

 

① Rotating single-turn absolute encoder:

 

Measure each slot of the photoelectric code wheel during rotation to obtain a unique code. When the rotation exceeds 360°, the code returns to the origin, which does not comply with the principle of absolute code uniqueness. Such a code can only be used in the rotation range 360 Measurements within ° are called single-turn absolute encoders.

 

If you want to measure rotation beyond 360°, a multi-turn absolute encoder is used.

 

 

 

② Multi-turn absolute encoder:

 

Encoder manufacturers use a principle similar to that of combined gear machinery (such as mechanical watches). When the central code disc rotates, another set of code discs (or multiple sets of gears, multiple sets of code discs) are transmitted through the gears. Based on the single-turn encoding The number of turns is added to the encoder to expand the measurement range of the encoder. Such an absolute encoder is called a multi-turn absolute encoder. It is also encoded by mechanical position determination. Each position code is unique and non-repetitive, and does not require memory.

 

 

 

In addition, due to the large measurable range of multi-turn absolute encoders, actual use is often more flexible. In this way, there is no need to bother to find the zero point during installation. Just use a certain intermediate position as the starting point, which greatly simplifies the difficulty of installation and debugging. , of course its cost is higher than that of incremental rotary encoders.

 

 

 

The theoretical feedback accuracy of the position system composed of an absolute encoder is similar to that of an incremental encoder. Please refer to the previous explanation.

 

 

 

(2) Magnetic scale

 

 

 

The magnetic scale is composed of a magnetic scale and a magnetic head detection circuit. It uses electromagnetic characteristics and magnetic recording principles to measure displacement. The magnetic scale is made by depositing a layer of magnetic film (usually 10-20 μm) on the non-magnetic linear scale substrate using chemical coating or electroplating process. The representative is recorded on the magnetic film. A square wave or sine wave magnetic trajectory signal with a certain length and wavelength. The magnetic head moves on the magnetic scale to read the magnetic signal, converts it into an electrical signal, and then outputs it to the servo controller to form a closed-loop position feedback. The picture below is its basic schematic diagram.

 

 

 

 

 

 

 

At present, its resolution can reach up to 1 μm, and the position accuracy of the X-Y system is usually 20 μm.

 

 

 

Features: The magnetic scale is simple to manufacture, easy to install, has high stability, and has a large measuring range. It can effectively resist dust, oil, moisture, and other pollution, impact, and vibration in industrial environments.

 

 

 

The theoretical feedback accuracy of the linear system composed of magnetic scale is the same as the calculation method of the rotary encoder and will not be discussed again.

 

 

 

(3) Grating ruler

 

 

 

The grating scale system is similar to the magnetic scale system and consists of a grating scale and a read head. The grating ruler is a vacuum-deposited coating on a transparent glass or metal mirror surface, and uses photolithography technology to produce uniform and dense stripes (100 to 300 stripes per millimeter). The stripes are equally spaced and parallel.

 

 

 

① Basic principles:

 

The working principle of the grating displacement sensor is that when the main grating (scale grating) and the auxiliary grating (indicating grating) in a pair of gratings are relatively displaced, the interference and diffraction of light jointly produce alternating black and white (or alternating light and dark) images. Regular striped patterns are called moiré fringes. After the photoelectric device is converted, the black and white (or light and dark) stripes are converted into a sinusoidal electrical signal, which is then amplified by the amplifier and shaped by the shaping circuit to obtain two sine waves or square waves with a phase difference of 90°, which are finally sent to the servo The controller constitutes a closed-loop position control system.

 

 

 

The picture below is an actual picture of a certain brand of grating ruler.

 

 

 

 

 

② Resolution:

 

At present, its resolution can reach up to 0.001 μm, and the position accuracy of the formed X-Y system can reach 1 μm or higher.

 

 

 

③ Features:

 

The application environment of the equipment it constitutes has high requirements, especially in terms of dust and oil resistance. It is not as good as the magnetic scale in this regard, but the feedback accuracy of the equipment it constitutes can be one to several orders of magnitude higher than the magnetic scale system. In addition, considering the shortcomings of the grating scale structure, some grating scale manufacturers, such as Heidenhain, have introduced closed-structure grating scales to overcome these shortcomings.

 

 

 

Grating scales are also divided into two types according to the signal principle, namely incremental grating scales and absolute grating scales.

 

 

 

(4) Coupling

 

 

 

The picture below is a physical picture of a certain type of coupling. It can realize the torque transmission from the rotating motor to the ball screw and overcome minor non-axial problems.

 

 

 

 

 

(5) Synchronous toothed pulley and synchronous belt

 

 

 

The picture below is a physical picture of a certain model of synchronous pulley.

 

 

 

 

 

Figure 1 below is a physical picture of a certain model of synchronous belt, and Figure 2 below is an assembly diagram of a certain model of synchronous belt/pulley.

 

 

 

Figure 1 Physical picture of synchronous belt

 

 

 

 

 

Figure 2 Timing belt/pulley assembly diagram

 

 

 

Typical representative: Gates.

 

 

 

(6) Ball screw

 

The picture below is a physical picture of a certain model of ball screw, and the left side is an anatomy of the screw nut.

 

 

 

 

 

Typical representatives: SKF and THK.

 

 

 

(7) Sliding rail

 

 

 

The picture below is a physical picture of a certain model of sliding guide rail.