| There are many kinds of methods to measure the speed of the cars, such as radar, laser and video etc, while the speedometers based on the principles of ground coils take over 90% of all the motor vehicle speedometers, now many calibration devices based on ground coils have many problems. The calibration method of signal analog and direct digital frequency synthesis technology have been systematically studied. The following several jobs has been completed in this paper.Firstly, the measurement principles of the coils speedometers were analyzed, and the advantages and disadvantages of real car test method and simulation signal test method were compared, chose simulation signal test method, which is putting one stimulating sensor on each ground coil, let the control output module of the verification part control the high-precision signal source to input a signal into the two stimulating sensors, which will cause the sensor of speedometer change, the speedometer take this change as the signal of a car passing and measure the speed of the car. Comparing the speed generated by the standard test device with the speed measured by the speedometer to calibrate the speedometer. Secondly, a terraced coils speedometer calibration device has been developed according to this method. Finally, many tests on the verification device were doing and test data were analyzed.The hardware design of land coils speedometer verification device mainly includes two parts, it is respectively verification part and the signal source part. Verification part includes core control module,control output module,man-machine interactive module etc. Setting imitate parameters such as the distance,speed etc through human-computer interaction module, such as the keyboard and liquid crystal. Then these parameters are sent to the core control module to produce a precise time interval signal to turn on or turn off the signal source. The signal of signal source is generated by frequency synthesizer. In introducing the design of the signal part, firstly, introduced the current frequency synthesis methods such as direct frequency synthesis,indirect frequency synthesis,direct digital frequency synthesis etc, then compared their advantages and disadvantages, chose directly Digital Synthesis (DDS) method as the method used in this paper. Finally, designed the core control module of the signal source, interactive modules, signal generation module, filter module, the operational amplifier module etc according to this method.Software parts are mainly written in C language and applied modular structure to maintain and transplant program. The programming tool is JTAG MKII which is produced by ATMEL company and supports the function of simulating on-line. The compile environment is Studio4 which can accomplish source programming, compilation, debugging, download, etc. Software design mainly introduces the design of main controller and high precise source.In order to improve the stability and reliability of the device, a lot of anti-disturbance measures was used when design the hardware and software of the device. The device was proved to have high stability,reliability and repeatability,high precision (0.2%),big simulated speed range (10～240km/h) by field test, it can be put into market to use. |
