Application of ARM System on Module ( SOM ) in Human Machine Control System of Flat Knitting Machine

1.1 Flat Panel Machine Control Panel Functional Requirements

The Flat knitting machine electric control system includes the man-machine control board and the main control board. Main control board real-time requirements are high, generally use front-end system or real-time operating system, mainly to complete the analysis of flower file instructions and motion control unit logic control.Man-machine control panel to complete the parameter setting, status query, pattern file management and other human-computer interaction process, the general use of WinCE or Linux operating system to achieve industrial control platform, functional requirements:

LCD interface, supports VGA display;
CAN bus interface, with the motor control panel, head control panel communications;
USB Host, through the U disk for pattern file updates;
TF card, data storage;  
Ethernet interface to upload data over the network.  
RS-485 interface, standby.

1.2 Flat Panel Design of Human Machine Control Difficulties
1. Control panel out of control and CAN bus communication failure
Flat machine operating environment not only has strong electricity, static electricity, but also a large number of motor generated interference signal. Customer CAN bus network in the original existence of serious common-mode interference, abnormal phenomenon:

CAN bus communication failure after power on;
Probability out of control of the nose control board during operation.
CAN bus circuit design not properly, prone to bad bus communication or even damage the entire CAN network system.

2. VGA display screen color distortion and water ripple
Industrial field VGA display common abnormalities, including screen color distortion, showing a louver or water ripple, etc., are related to the design of the VGA circuit. VGA is analog signal, the input includes RGB signal, line synchronization, frame synchronization signal, etc., interface connector also introduces the current loop mutual inductance crosstalk, improper design easily cause the interface display water ripple, color distortion, threatening the user experience effect.

3. CAN Bus application programming
New platform CAN application software development how to get started quickly? How to solve the software migration between different platforms? Relative to the Ethernet, serial port and other standard application function interface, different CAN controller, different manufacturers to provide the CAN driver operating interface is different, the user application software development, cross-platform software transplantation made relatively harsh requirements.

1.3 Our Design Implementation
Based on the customer’s functional requirements and product design problems, Graperain electronic modular design ideas, based on M3352 android SOM quickly completed the development of human-computer control panel, the product shown in Figure 2.
Man-machine control panel to achieve block diagram shown in Figure 3.
Human Machine Control Board Design Difficulties Analysis:

1. CAN bus interface circuit CAN bus design using Graperain electronic mature circuit, while external CAN bus isolated transceiver, with power isolation, signal isolation and bus protection, the circuit shown in Figure 4.
During start-up, this design can ensure that the man-machine control panel CAN bus is forced to pull high (recessive level), to avoid the system for a long time the output of the low-level signal (dominant level) Failure or network paralysis. At the same time, this design has isolation and bus protection, to avoid the surge, interference caused by bus errors and equipment out of control.

 CAN Circuit
CAN Circuit
2. VGA interface circuit
VGA circuit using a dedicated 10-bit video D / A conversion device SDA7123, RGB signal to complete the conversion. The matching resistors R58 and R64 are used to reduce the signal reflection to ensure the transmission quality of the line synchronization signal (HSYNC) and the frame synchronization signal (VSYNC). Interface filter circuit is used to solve the current loop mutual inductance caused by crosstalk. From the signal conversion, data transmission to filter the overall professional design, will effectively avoid water ripples and color distortion and other anomalies.

3. CAN bus interface application programming
M3352 board all the external interface driver, free software API interface and development of sample source code, user-friendly fast CAN bus and other complex communication programming methods. As shown in Listing 1, a few lines of code can be implemented by the CAN device.

Program List 1 CAN interface programming example

#include “ControlCan.h”
int nDeviceType = 6; /* Device Type CAN*/
int nDeviceInd = 0; /* Channel 1 CAN*/
int nReserved =500; /*Baud rate 500K*/
DWORD dwRel;
dwRel = VCI_OpenDevice(nDeviceType, nDeviceInd, nReserved); /*Open CAN Device*/

1.4 Experience Sharing
In the flat knitting machine project, the customer often uses the CAN main line communication way, but the communication cable is composed by the multi-strand thin copper wire, the twisted pair between the cable also lacks shield, causes the common mode interference seriously in the practical application, through CANScope analyzer FFT statistics Analysis, interference frequency is about 15KHz, amplitude up to 5V, coupling energy is extremely strong, resulting in CAN data frame error rate of 72%, can not be normal transmission 

Suggestions for Improvement:
Replace the CAN communication cable with a tight twisted pair (33 strands per meter) with shielding. L CAN nodes are designed with CTM series isolated transceivers to isolate the CAN bus and enhance the bus protection function.

Media Contact
Company Name: Shenzhen Graperain Technology Co., Ltd.
Contact Person: Cherry Wang
Phone: +86-755-2302 5312
City: Shenzhen
Country: China