AUTOMATION OF A HYDRAULIC TEST BENCH FOR AGRICULTURAL TRACTORS CERTIFICATION

The IMAMOTER (Istituto per le Macchine Agricole e Movimento Terra) of CNR (Consiglio Nazionale delle Ricerche) is one of the official OECD (Organisation for Economic Co-operation and Development) testing institute, the OECD defines a set of rules that specify how the certification tests must be done. The aims of the work were: ? To find an automated and efficient way to perform the tests requested by the certification code ? To provide an efficient system to use the test bench in various ways: o To set the command voltage o Direct control of the inlet pressure ? To study the dynamic response of the system ? To build a user-friendly interface The hydraulic test bench is a system composed by: ? 2 pressure sensor (Wika electronics S-11 sensors) ? 1 flow rate sensor (VSE EF series model 4) ? 1 RTD (Pt100) ? 1 filter ? 1 proportional valve (Hydraforce SP16-20) The resultant hydraulic system is shown in the following figure. The most important device of the system is the proportional valve, it is used to control the flow rate through the circuit. To command the valve, providing the requested value of pressure or flow rate, a Siemens PLC was used, because it is equipped (adding its expansion boards) with requested input-output ports, it permits data-logging (storing the data for later analysis) and it is reliable and robust. The PLC system is composed by: ? Step 7 1212c AC/DC/RLY CPU module, is the core of the PLC, it executes the program, analyse data coming from the input-output boards and communicate them to the HMI panel, showing to user the parameters of the system. ? SM 1231 AI 4x13 bit analog input module, it is responsible of the pressure data acquisition, analysing the 4-20mA signals coming from the pressure sensors. ? SM 1232 AQ 2x14 bit analog output module, it is responsible of the valve command (0-10V signal) ? SB 1231 1x16 bit RTD 16 bit resolution RTD signal board, used with the Pt100 make possible to know the oil temperature in the oil tank of the tractor. ? KTP700 Basic Panel, it is the HMI panel that permit the user to interact with the system, checking the pressure, flow rate and temperature, but also setting the desired values. The PLC program was designed to estimate the physical quantities through the analysis of the sensor's signals, to organise all the logical connection for the correct operation's sequences, to provide the algorithm (PID) to control the valve. In order to execute the OECD tests, a control system was necessary to drive the valve and to reach desired conditions in the hydraulic circuit, to obtain a control algorithm to implement in the PLC some tests must be done, characterizing the hydraulic system. The previous figures describe the behaviour of the valve related to the command voltage (0-10 V). As it can be seen the system response is non-linear for what concern the inlet pressure, in fact with a low command range the pressure rises really fast and non-linearly, instead different conditions exist for the flow rate where the behaviour is more linear due to the features of the valve. In order to obtain a model that describes the dynamic of the system, an open loop test was done, by the analysis of these results the model of the system was approximated to a second order system type, by using the model a PID algorithm was developed due to control the system in an efficient way. The PID algorithm was tested in 3 cases to evaluate the behaviour of the controlled system in different conditions, a low range test (the pressure of the inlet pressure required starts from the minimum of 75 bar and reaches 100 bar), a medium range test (from 75 bar to 150 bar), a high range test (from 75 to 200 bar); all these tests generated different responses of the system in settling time, oscillations and rising time. The figure shows the response of the controlled system compared with the modelled one, in a medium range test, from a minimum pressure of 75 bar to a medium pressure of 150 bar, as it can be seen, at any rate in these conditions, the model and the reality are similar as expected. These tests were done only for model evaluation, because the one designed for the agricultural tractors certification was similar to a ramp of command pressure that passes from the minimum pressure (valve totally open) to the maximum pressure (valve totally closed) with a 1 bar step every 2 seconds, the controller responds to this command variation opening or closing the valve. As it can be seen in the figure, the pressure follow the setpoint imposed in an acceptable way, until the pressure reach the maximum possible value (depending on the tractor). This procedure permits to the hydraulic quantities to cover the whole range possible. By the use of a dedicated and integrated acquisition system all these variables could be recorded in a common USB key device, precisely in a text file, that could be easily analysed later. About this topic, more words must be spend, the acquisition system used arranges the values recorded in a disadvantageous order, so a special software was designed in Visual Basic in order to rearrange the variables disposition and make possible an easier analysis, it was also designed a user interface that provide chart and some setting option based on the user necessities. The HMI Panel, coupled with the PLC, provides a comfortable Human Machine Interface organized in pages, the user can browse through them thanks to the touch screen feature. The operator could easily choose the OECD tests, set the opportune parameters and also directly command the valve in voltage or setting the pressure, these last features were particularly important because they give to the system a special flexibility for general testing operations. About this work, it can be said that: ? Combining the usage of the PLC software and the VB program, it was projected an easy way to compile the table requested by the Code. ? The goal to provide various ways to command the system was reached, in fact the valve could be commanded directly in command voltage or selecting the desired inlet pressure. ? Studying the whole system, many issues become known, like nonlinearities in the fluid, in the tractor's pump and in the valve caused by frictions, inertias and elasticities neglected in the model. These omissions made the model easy to project moreover the usage of a PID controller, combined with the simplicity of the model, was proved as the winning choice. In fact, although the model presents some gaps, the principal specifications imposed were respected. ? By the aim of the dedicated HMI panel a user-friendly interface was projected and tested, making the interactions between system and user easy and clear. For what concern the future developments, some things need to be improved, for example: ? The model could be refined in order to consider the nonlinearities of the system. ? The data acquisition system may be modified in order to acquire more samples per second. ? A calibration of the hydraulic test bench permits the emanation of calibration certificates. ? The margin of error could be evaluated.