Studies on the Automatic Control of Hydrostatic Transmission Tractor (I)

Design of the Control Device with Hydro-mechanical Servo-valves

Abstract

The weak points of present load driving systems consisting of mechanical transmission are as follows.
(1) The increase in specific fuel consumption and the lowering of mechanical efficiency, at light load, is remarkable.
(2) The engine is apt to stall by over-load.
(3) The velocity of the tractor can not be changed continuously and smoothly.
Thus, it is the purpose of this study to suggest new control systems by which the weak points above mentioned can be eliminated, and the engine is always operated with the optimal efficiency in spite of disturbance of torque. To start with, we designed and tried our hands at control device with hydro-mechanical servo-valves. (Fig. 1) Secondly, using this control device and closed circuit PV-MF hydrostatic transmission system, we should try to control pump stroke angle of variable displacement type axial plunger pump, namely, pump output-flow, according to disturbance of torque.
These control systems are divided into the following two new ones;
(I) ENGINE OUTPUT POWER CONSTANT CONTROL SYSTEM
In spite of great and small sizes of the disturbance of torque, the pump output-flow, namely hydraulic motor speed, is controlled on the basis of the following action principle of control device used
p₃·q=constant.
where, p₃ is pump output pressure, q is pump displacement. As a result, if an engine speed is given by governor setting, this control system usually keeps constant engine output power at its engine speed. Therefore, if this control system is always controlled along p₃-q (α) diagram in hydraulic system corresponding to engine speed-torque diagram where the engine gives the best efficiency, the control purpose above mentioned should be certainly accomplished.
(II) ENGINE OUTPUT POWER VARIABLE CONTROL SYSTEMS
Keeping alive many advantageous points which power constant control system has in it, especially in this control system we should try to control the engine output speed in addition to the hydraulic motor speed for the purpose of operating with the output power and the fuel consumption in proportion to the disturbance of torque.
In this paper we described mainly design method of this control device.
A summary of design process is shown below.
(1) Seizing accurately the characteristics of the Diesel engine and the hydraulic pump used by experiments, we require pump torque efficiency, η_pT and engine maximum horse-power, P_epmax, at each pump speed, n_p→v
(2) The desired value is calculated by the following equation.
q=P_epmax·η_pT·0.85/p₃·n_p
where, the constant 0.85 is load factor which gives the best engine efficiency. Besides, the purpose and condition for usage are some of the considerations that we must take into account. So, in high speed range, the higher the speed, the more load we gave to the control system as the desired value. Theoretical p₃-q (α) diagrams corresponding to each engine speed which has been obtained in the way described above are shown in Fig. 5.
(3) The theoretical p₃-q (α) diagram is nonlinear in most cases. Therefore, design target p₃-q (α) diagram is drawn by linearizing approximation of it. (Fig. 5) If the p₃-q (α) diagram has been simplified, the desired value of this control system can be given easily by using of combination springs. An noticeable error between actual desired value set in proportion to pump input speed and the desired value previously calculated was recognized. This error is based on the nonlinearity of p₃-q (α) diagram and unavoidable In order to improve the accuracy, we must