Design of motion control card for parallel machine tools based on PCI bus

introduction

Parallel Machine Tool, also known as Virtual Axis Machine Tool or parallel kinematic machine, is a new type of CNC machining equipment based on parallel mechanism. It is essentially a combination of robot technology, machine tool structure technology, CNC technology, etc. It takes into account many characteristics of machine tools and robots at the same time. It can be regarded as a robotized machine tool and a machineized robot. It can provide the flexibility and flexibility of a robot, and has the rigidity and precision of a machine tool. It is a new type of electromechanical equipment integrating multiple functions.

The structure of parallel machine tools

The schematic diagram of the parallel machine tool structure is shown in Figure 1. It consists of a servo (stepper) motor 1, a fixed platform 2, a bracket 3, a telescopic rod 4, a moving platform 5, a milling head 6, a workbench 7, etc. The telescopic rod installed between the fixed platform and the moving platform is driven by a servo motor (or stepper motor), which drives the ball screw and nut in the telescopic rod to move, so that the length of the telescopic rod changes. As the length of each telescopic rod (usually 6 rods or 3 rods) changes, the position of the moving platform changes, and multiple telescopic rods move together, driving the moving platform to achieve movements such as rising, falling, and flipping, and driving the milling head to achieve various movements required for work. Each telescopic rod is driven by a mutually independent servo drive device.

Open control system architecture

The biggest feature of parallel kinematic machine tools is that the mechanical structure is very simple, but the control is extremely complex. The control system of parallel kinematic machine tools must adopt an open structure. At present, there are mainly three types of open CNC systems at home and abroad.

1 Dedicated CNC + PC type is to embed the machine in the traditional non-open dedicated CNC system so that the entire system can share some computer software and hardware resources.

The CNC part completes non-real-time control tasks such as system management, and realizes functions such as auxiliary programming, analysis, monitoring and process arrangement; the CNC part is responsible for completing some real-time control tasks such as interpolation calculation, servo control and I/O control. The openness of this type of CNC system is only in the machine part, and its professional CNC part is still in a closed state, and the CNC kernel cannot be opened. This type of structure is generally adopted by mainstream CNC system manufacturers.

2 PC + motion control card This system structure based on open motion control card is based on general microcomputer as the platform and open motion control card in the form of standard plug-in as the control core. The general PC realizes the functions of CNC program editing, human-machine interface management, external communication, etc. The motion control card is responsible for the motion control and logic control of the machine tool. It supports the secondary development and independent expansion of users. It has both the openness of PC and the openness of dedicated CNC module. It can be said that it has both upper and lower levels of openness.

3 Pure PC type is a PC CNC system in full software form. This system refers to a system in which all functions are processed by the PC and the servo drive is controlled through the servo interface card installed in the expansion slot. Its software has good versatility and flexible programming, but there are problems such as the real-time performance of the operating system, standard uniformity, and system stability. At present, this type of system is in the exploratory stage and has not yet formed a product, but it represents the development direction of CNC systems.

If you are looking at the design of an economical CNC parallel machine tool control system, the open CNC system of PC+motion control card is the most ideal one at present. With PC and motion control card as the core of the control system, the system platform is directly built on the hardware and software foundation of the PC. The driving element is a stepper motor. The system is implemented in VC++ language under Windows environment, and the software design adopts object-oriented method.

Design of motion control card for parallel machine tool based on PCI busParallel machine tool motion control card realizes the movement of main axis by controlling each driving rod of machine tool, controls a large number of axes, and obtains the required tool motion trajectory at the same time.To realize high-speed and high-precision trajectory control of tool, there is a large amount of data interaction between motion control card and PC. The traditional way is to use ISA bus. Due to the bandwidth limitation of ISA bus, the data transmission speed is slow, so that PC spends more time on data transmission, thus reducing the processing time in other aspects.This system realizes the communication between motion control card and PC, which can achieve a very high data transmission rate, greatly enhance the processing capacity of PC, thus ensuring the real-time performance of CNC system, and making it easier to develop corresponding platform-based CNC software, and more convenient to interface with other supporting software.

The hardware composition of the control system is shown in Figure 2. The system uses the company's 32-bit fixed-point DSP chip TMS320VC33. The DSP chip is responsible for receiving commands and parameters from the PCI local bus, and then completing the precise motion control of the stepper motor through the motion control algorithm. At the same time, the chip is also responsible for transmitting feedback information to the PCI local bus and controlling the system's peripheral I/O modules. The communication between the DSP and the PC is realized through the bridge chip CY7C09449PV.

1 The connection bus between CY7C09449PV and PC is the most widely used and popular high-speed synchronous bus with a 64-bit bus width, an operating frequency of 33MHz, and a maximum transmission rate of 132Mb/s. It can directly connect peripherals to the local bus of the host CPU and run at the CPU speed, which can greatly improve the operating speed of peripherals. The communication between DSP and PC is connected. CY7C09449PV is a dual-port RAM interface chip with complete functions and low price provided by Cypress Company in accordance with the PCI2-2 bus specification. Among them, one port is a PCI interface, which is suitable for communication with a PC; the other port is a local interface. It provides a master/target interface and can be connected to any general-purpose microprocessor. There are 128 dual-port memories on the chip for data sharing between the local processor and the PCI bus. The connection between CY7C09449PV and PCI bus is shown in Figure 3.

2 Implementation of PCI bus development solution The dedicated chip CY7C09449PV is powered by a single 3.3V power supply and is compatible with 3V and 5V PCI signal environments. The power supply can be drawn from the PCI slot, which can provide 3.3V and 5V voltages. In the PCI slot, special attention should be paid to the two pins PRST1 and. The four combinations of grounding and floating of these two pins directly determine the power of the PCI card. When using CY7C09449PV, some pins need to be added with pull-down or pull-up resistors with a resistance value of 1kf1 to 10kf1. According to the specific situation, in this design, pull-up resistors are added to the SCI, SDA, AIE, /BE[2], RDY pins, and pull-down resistors are added to the TEST-MODE pins.

It is a new generation of floating-point DSP newly launched by TI. It is a lower-priced version developed on the basis of the original floating-point DSP. The product is characterized by high speed, low power consumption, low cost and easy development. Because it uses internal 1.8 V and external .3 V power supply, the power consumption is reduced by about one order of magnitude compared with the original model, and it can support an operating speed of up to 150 × 10 times/s. It is an ideal DSP device for portable products that require floating-point operations.
As the core of the control system, TMS320VC33 mainly completes the servo control program of real-time interrupts, including the operation of various underlying hardware. It uses a 32-bit local bus interface. The clock signal can be a separately provided clock source or any one of the PCI clock signals PCI KOUT[2 0] provided by CY7C09449PV.

Conclusion Parallel machine tools need to control a large number of axes, and involve the forward and inverse calculations of parallel mechanisms, which requires a large amount of calculation. By using a bus-based motion control card, the entire CNC system task is shared by the PC and the motion control card, which can give full play to the ideal motion control performance in an open CNC system. With the popularization of CNC systems and the improvement of product grades, this type of motion control card will have a wide range of application prospects.