Abstract: This paper analyzes the components of a servo system and studies the technology and principles of inkjet printers, designing a servo control system for an inkjet printer. The system uses a microcontroller to track and control the printing speed and position, and employs speed control to control the servo motor's motion. By utilizing the servo motor system's built-in control model, the system achieves smooth variable and uniform speed motion during printing, meeting the inkjet printer's requirements for position control accuracy. The high-performance servo control system ensures the printhead's smooth and accurate movement, while also enhancing the inkjet printer's competitiveness in the market.
0 Introduction
Inkjet printers are high-tech products that combine computer technology with image design, precision printhead control, and electromechanical automation. Traditional roller-type inkjet printers operate by moving the carriage and printhead left and right, while the roller moves the material forward, enabling the printing of vibrant patterns on advertising materials such as lightbox fabric, vehicle wraps, and uncoated rolls. Currently, the fabric feed motor and carriage motor of inkjet printers mainly use DC servo motors and AC servo motors. However, with product upgrades, customers are placing higher demands on the high-speed and overload capabilities of inkjet printers, especially with the increasing demand for inkjet printers exported to Europe and America. DC servo drive solutions can no longer meet the requirements of some customers. To adapt to this need, we have developed a new permanent magnet AC servo drive system (driver Acs806 + motor AcM602V36) to meet these higher customer requirements. This article focuses on the principle and characteristics of the AC servo drive system for a 3.2m format, 8-printhead inkjet printer.
Inkjet printers can be categorized into two types based on their printing format: large-format and extra-large-format. Large-format printers have a width of approximately one meter and use dye-based inks, offering high resolution (up to 600-1000 DPI), capable of printing on glossy photo paper to achieve photographic effects. However, due to ink limitations, they are not waterproof or UV-resistant, requiring post-printing lamination or coating, and are suitable for renderings and samples. Extra-large-format printers have a width of over 3 meters and use pigment-based inks, primarily targeting the outdoor large-format advertising market. Their resolution meets outdoor requirements (typically 24-70 DPI is sufficient). Due to differences in printing technology and inks, the requirements for the printing substrate also vary. Some printers restrict the brand and type of substrate, while others do not. Some printers require post-printing coatings, while others can be used directly outdoors. In addition, because pigment inks and dye inks have different color gamuts, with pigment inks having a much narrower color gamut than dye inks, the color reproduction accuracy of ultra-large format inkjet printers using pigment inks will not reach the level of small photo printers for a period of time.
Servo systems are a crucial component of automatic control systems. Their performance directly determines and influences the speed, stability, and accuracy of the automatic control system. The combination of mechanical, electrical, and hydraulic systems has become the main technological foundation of current industrial automation. A servo control system is a feedback control system used to accurately follow or reproduce a process. In many cases, a servo system specifically refers to a feedback control system where the controlled variable (system output) is mechanical displacement or displacement velocity/acceleration. Its function is to ensure that the output mechanical displacement (or rotation angle) accurately tracks the input displacement (or rotation angle). The structural composition of a servo system is not fundamentally different from other forms of feedback control systems. The main task of a servo system is to process signals according to control commands, including transformation, regulation, and power amplification, so that the torque, speed, and position output by the drive device can be flexibly and conveniently controlled. Due to the fast response, high speed, and high positioning accuracy of servo motors, many advertising clients ultimately choose inkjet printing. To cater to the domestic market, some manufacturers have begun to develop and produce their own brand of inkjet printers based on the introduction of advanced foreign technologies.
1. Printing Machine Process
Both computer engraving machines and inkjet printers are large-format inkjet printers. Inkjet printers can be divided into solid inkjet and liquid inkjet types according to their working principle, with liquid inkjet printers being the mainstream today. Computer engraving inkjet methods can be divided into bubble-type and piezoelectric types. Bubble technology uses a heated nozzle to create ink bubbles, which are then sprayed onto the printing medium. Ink is prone to chemical changes at high temperatures, making it unstable and affecting the color fidelity. Furthermore, because ink is ejected through bubbles, the direction and size of ink particles are difficult to control, resulting in uneven lines and affecting print quality. Piezoelectric inkjet technology uses a nozzle similar to thermal inkjet technology, but the ink droplets are formed by reducing the area from which the ink is ejected. This reduction is controlled by applying voltage to one or more piezoelectric plates within the ejection area. Micro-piezoelectric printhead technology utilizes the discharge characteristic of crystals under pressure to stably eject ink at room temperature. It features strong control over ink droplets, easily achieving high-precision printing quality of 1440dpi. Furthermore, since micro piezoelectric inkjet printing does not require heating, the ink in the printer will not undergo chemical changes due to heat, thus greatly reducing the requirements for ink.
Inkjet printers utilize a semiconductor piezoelectric crystal material called PZT, on which a series of extremely fine channels are created. The resulting mechanical effect propels ink droplets outwards. During the PZT production process, it undergoes polarization treatment: the atomic charges of the material are forcibly aligned in a specified direction. This is a crucial characteristic because when an external electric field is applied to the polarized material, it causes a physical deformation of the PZT according to its orientation, and this phenomenon grants it the ability to be ejected. The fabric feed motor must be able to rotate in both directions, maintain smooth fabric feeding and retraction speeds, and automatically lock when the machine stops to prevent fabric slippage. The carriage motor must be stable and vibration-free during movement, and maintain sufficient force when the carriage is stationary.
The main requirements for the servo performance of the nozzle axis and fabric feed axis are high dynamic response and high positioning accuracy. The all-digital AC servo drive features high-speed frequency response, resonance suppression, precise tuning, and vibration elimination; its control accuracy can reach one pulse, and the maximum input frequency can reach 250Kpps, all of which effectively guarantee the drive requirements of the fabric feed axis. For the spindle servo, rapid start-stop characteristics and stable speed control are required. The all-digital AC servo drive has an open parameter adjustment interface, allowing users to set parameters according to their specific needs.
2. Principle of inkjet printer
Figure 1 shows the working principle diagram of an inkjet printer.
Figure 1. Schematic diagram of an inkjet printer
2.1 Driver
Servo drives typically employ three control methods: position, speed, and torque. They are primarily used in high-precision positioning systems and represent the pinnacle of transmission technology. An encoder is a device that encodes signals or data, converting them into a signal format suitable for communication, transmission, and storage.
A driver is a drive amplification element that amplifies signals sent from a host computer (such as a motion control card) to enable the motor to run. The MAC series motion control card is a bus-based motor motion control card. It uses a dedicated control chip as its core component, with both input and output signals opto-isolated. It can connect to various types of stepper motor drivers to drive stepper motors, forming a high-precision position control system or speed control system. It can form a master-slave control structure with a PC: the PC is responsible for the human-machine interface management and other management tasks; while the control card handles all the details of motion control. Users can easily and quickly develop their own required motion control functions using the dynamic link library we provide. Figure 2 shows the servo driver structure diagram.
Figure 2 Servo driver structure diagram
2.2PC Bus
The core of an open-loop dynamic controller is a DSP, which features high computing speed and support for complex motion algorithms, meeting the requirements of high-precision motion control. Therefore, multi-axis dynamic control cards based on DSPs are increasingly widely used in motion control systems. By inserting the multi-axis dynamic control card into the expansion slot of a PC, a high-precision motion control system can be formed. The acquisition of position feedback signals, closed-loop control calculations, and output of control quantities are all completed by the dynamic control card, which greatly improves the computing speed and control response speed. This frees up the resources of the industrial control computer from tedious data acquisition and calculation, thereby enabling better management of the entire control system.
2.3 Motion Control Card
A motion control card is a host control unit that controls servo motors. Based on a PC bus, it utilizes high-performance microprocessors (such as DSPs) and large-scale programmable devices to achieve multi-axis coordinated control of multiple servo motors. A high-performance stepper/servo motor motion control card includes functions such as pulse output, pulse counting, digital input, digital output, and D/A output. It can emit continuous, high-frequency pulse trains. By changing the frequency of the emitted pulses, the motor speed is controlled; by changing the number of emitted pulses, the motor position is controlled. Its pulse output modes include pulse/direction and pulse/pulse. The motion control card not only sends pulses to the motor driver but also receives pulse count feedback from the servo motor encoder and feedback signals from the linear encoder, thereby controlling the servo motor's speed. The servo driver must be connected to the motion control card via a data cable and must also be connected to a power socket.
2.4 Encoder
An encoder is a device that converts angular or linear displacement into electrical signals. The former is called a code disk, and the latter a code scale. According to the reading method, encoders can be divided into two types: contact and non-contact. Contact encoders use brushes for output, with one brush contacting a conductive or insulating area to indicate whether the code state is "1" or "0". Non-contact encoders use photosensitive or magnetic sensitive elements to receive signals. When using photosensitive elements, the light-transmitting and opaque areas are used to indicate whether the code state is "1" or "0".
3 Servo Control System Design
Basic requirements for servo motors in automatic control systems:
(1) No “self-rotation” phenomenon. That is, the control motor is required to rotate rapidly when there is a control signal, and stop rotating immediately when the control signal disappears. The phenomenon that the motor continues to rotate after the control signal disappears is called self-rotation. Automatic control systems are not allowed to have the phenomenon of “self-rotation”.
(2) Low no-load starting voltage. When the motor is unloaded, the minimum control voltage required for the rotor to rotate continuously from rest is called the starting voltage. The lower the starting voltage, the higher the sensitivity of the motor.
(3) It has linear mechanical and regulating characteristics. Linear mechanical and regulating characteristics are beneficial to improving the control accuracy of the system and can smoothly and stably adjust the speed over a wide range.
(4) Good fast response. That is, the motor should have a small electromechanical time constant, a large stall torque, a small moment of inertia, and the speed should be able to change rapidly with the control voltage.
The structure and types of mechatronic servo control systems are diverse, but from the perspective of automatic control theory, a servo control system generally includes five parts: controller, controlled object, execution link, detection link, and comparison link.
Comparison phase
The comparison stage is a stage that compares the input command signal with the system feedback signal to obtain the deviation signal between the output and the input. It is usually implemented by a dedicated circuit or computer.
controller
The controller is usually a computer or a PID control circuit. Its main task is to transform and process the deviation signal output by the comparator in order to control the actuator to act as required.
Execution phase
The function of the actuator is to convert various forms of energy into mechanical energy according to the requirements of the control signal, thereby driving the controlled object to work. In mechatronics systems, actuators generally refer to various motors or hydraulic/pneumatic servo mechanisms, etc.
Controlled object
Mechanical parameters, including displacement, velocity, acceleration, force, and torque, are the controlled objects.
Testing process
The detection stage refers to the device that can measure the output and convert it into the dimensions required by the comparison stage. It generally includes sensors and conversion circuits.
4. Conclusion
The servo control system designed in this paper boasts superior performance, ensuring the smoothness and accuracy of printhead movement. Its operational performance is comparable to imported servo systems, offering a superior cost-performance ratio. This inkjet printer fully integrates the advantages of servo systems, such as fast response, precise positioning, and stable operation. After customer trials, the printed images met all customer requirements, with all indicators fulfilling the necessary standards. Furthermore, it improved production efficiency, maximizing customer satisfaction. This equipment fully demonstrates the advantages of servo control systems to customers and enhances the competitiveness of inkjet printers in the market.
