Can-based intelligent ultrasonic level transmitter design (1)

O introduction

Fieldbus is used for field instrumentation and control systems and control room between a decentralized, digital, intelligent, two-way, interconnection, multivariate, multipoint, multi-point communications network.

CAN (Controller Area Network, controller area network) is Germany Bosch company in the early 1980s to address modern vehicle within the large number of control and measuring instruments and the exchange of data between a serial data communications protocol. CAN-bus is the most widely used one fieldbus, because a lot of new technology and unique design, and general communication bus, CAN bus data communication has outstanding performance, and reliability, real-time performance and flexibility.

In recent years, with the study of ultrasonic technology, coupled with its high accuracy, non-destructive, non-contact, etc., ultrasound applications become more and more popular.

Ultrasonic level measurement has many advantages: detection components can not contact with the medium, wide range of measurement; measuring the level of low-temperature medium; long life; to point and continuous level measurement; convenient installation and maintenance. Ultrasonic measurement has been successfully applied to the river water, chemical and pharmaceutical industry, food processing, filling level, and many other areas.

1 measuring method

Ultrasonic pulse echo method is level measurement applications broader a method by measurement of ultrasonic propagation time measurement distance.

Ultrasonic pulse echo detection method of the basic principle is: launch wave transducer excitation by pulse signals issued by ultrasonic sound media to be logging level, the formation of the reflected wave; then reflection through the audio media returns to receive transducer, sensors make sound signal conversion to electrical signals, by ultrasonic instrument is calculated from the launch to the receiving the transmission time, and then under ultrasound at medium speed, use-(1) determine level height

Type: H represents the probe and the distance from the bottom of the container; l ultrasonic transmission distance of 50 per cent; that of ultrasonic velocity v; t h ultrasonic propagation time; that is, the actual height measuring liquid level.

System uses ultrasonic software filtering modes determine the echo of the starting point, use the AD converter will echo signals into digital signals to the ARM processor, using the ARM processor strong signal processing power on the echo signals for digital filtering, numerical processing, determining ultrasonic propagation time.

The system selects the Philips company LPC2119 chip as control operations processor.

LPC2119 is based on a support real-time simulation and keep track of 16/32-bit CPU ARM7TDMI-S with 128 KB FLASH memory embedded high-speed, high-performance, low power consumption characteristics. LPC2119 chip integrated 2 CAN controller, complies with the specification for CAN2.0B, CAN ISO11898-1, you can access 32-bit registers and RAM, a single bus data baud rate up to 1 Mb/s, global acceptance filters identify nearly all bus 11 and 29-bit identifiers, acceptance filters Rx for select standard identifier provides FullCAN-style automatically receive.

2 system hardware design

2.1 measuring circuit design

Measuring circuit is ultrasonic transmitting and receiving circuits, design for transceiver type, as shown in Figure 1.

Use LPC2119 chip internal timer 0 produces 40 kHz pulse, the value of the incentive to NMOS FET gate of Q1. When the control-side PO.22 OUT for low level, NMOS FET Q1, Q2 PMOS FET near zero voltage of Ugs, Q2 due and output low level to high; PO.22 OUT level, Q1, Q2 of gate on-voltage from the resistor divider, 2 the Ugs is less than the threshold voltage V left the Q2 breakover, output. So by controlling the end so that the transmitter produces positive high-voltage pulses to stimulate ultrasonic probe. + 50 V high voltage from the AD company's switching DC-DC converter ADPllll provides.

In receiving circuit using diode clamp pulse of high voltage transmission function to prevent entry into the receiving circuit.

Circuit two Schottky diodes and current limiting resistors to limit the input voltage V, but O.3 on smaller echo signals do not work for ultrasonic transmitter, receiver circuit integration. Circuit requires a faint echo to zoom, use the OP27 operational amplifier will echo signal amplification 200 times. Capacitance C20 filtered out echo signal DC components.