Asian Journal of Agricultural Sciences 4(2): 145-148, 2012 ISSN: 2041-3890 © Maxwell Scientific Organization, 2012 Submitted: January 19, 2012 Accepted: February 17, 2012 Published: March 26, 2012 Corresponding Author: Móslem Sami, Agricultural Mechanization Engineering Department, College of Agriculture, Shahid Chamran University, Ahwaz, Iran 145 Design, Development and Evaluation of a Digital Tractor Dynamometer 1 Mohammad Reza Havasi, 1 Mohammad Javad Shiekhdavoodi, 2 Navid Alaei Shini and 1 Móslem Sami 1 Agricultural Mechanization Engineering Department, College of Agriculture, Shahid Chamran University, Ahwaz, Iran 2 Electrical Engineering Department, College of Engineering, Shahid Chamran University, Ahwaz, Iran Abstract: In this study, to measure drawbar pull of agricultural tractors, a digital dynamometer was designed, developed and evaluated based on AT89C52 microcontroller. The microcontroller was programmed using assembly language. The inputs of microcontroller supplied from a kind of S shape electronic loadcell that has 4 resistant strain gauges with Wheatstone Bridge configuration. The amplifier AD524 and an analogue to digital converter ADC0804 were used in electronic parts of the set. During dynamometer calibration the correlation coefficient (R 2 ) between force and output voltage was 0.99. Ultimately, the laboratory tests and evaluation were performed and the correct performance of set was proved. Key words: Drawbar pull, loadcell, microcontroller, tractor INTRODUCTION Farm Machinery Testing and Evaluating Centers must be equipped to precise tools to measure and evaluate performance parameters so that by accurate data, they can provide a realistic analysis from a special machine function. Due to more accuracy, also convenient utilization of digital measuring systems in comparison to mechanical systems, use of this type of systems to outfit tractors and measure the technical parameters is growing. Study and research works related to tractors traction assessment need accurate strain force measurement. Precise measurement of strain force is impressive in the calculation of functional parameters of drawbar power, such as specific fuel consumption and drawbar tractive efficiency. For accurate measurement of drawbar power, researchers have used different methods. Safari et al. (1999) designed and developed a kind of hydraulic drawbar dynamometer with the capacity 20 kN that its components included: the main cylinder, the secondary cylinder, pressure gauge, needle valve, connector hose, electronic pressure sensor and booster system. The dynamometer was calibrated using a standard strain-testing machine with the capacity of 600 kN. Their results showed that in all replication the coefficient of correlation, R 2 , for the relationship between electronic pulses read from display screen and strain force was equal to 0.99. During the test, the measurement accuracy was 1%. A drawbar dynamometer was also designed and built by Zarif Neshat and Ranjbar (2004). The dynamometer was able to measure drawbar power of most agricultural implements in category I and II. The dynamometer was formed from both mechanical and electronic parts and able to measure pull and pressure load entered to tractors and agricultural implements. Mechanical part included the main body of loadcell, sub-components of the main body, reactionary segment, ball bearings and other appurtenances, and electronic part included loadcell, the Wheatstone Bridge circuit, data collection mechanism and the mechanism of the data analysis. Designed device was evaluated after laboratory calibration. Test and evaluation results showed that in all replication between the applied force to the dynamometer and output voltage existed a linear relationship with coefficient of correlation R 2 = 0.99. In another study, Turner (1993) made a typical hydraulic forcmeter that only able to measure draft of single hitch point implements. This force transducer was not affected by lateral force or moment and support overload up to 40%. Kheiralla and Yahya (2001) used a draft transducer in which Proof Rings were used as a strain sensor to measure drawbar power. Sensitivity of tractor draft transducer was about 13.6 mV per kN and maximum strain load in the set measuring range was 50 kN. Khan et al. (2006) developed a digital dynamometer to measure tractor draft. They exerted sensitivity, linearity hysteresis and repeatability tests on the designed device. The dynamometer was able to measure vertical and horizontal tension forces up to 10 and 20 kN, respectively. In another related study, Khan et al. (2007) designed a loadcell with an octagonal structure to test the