Beginnings of Implement Control by Hydrostatic Hitches
The beginnings are connected with the name of Harry Ferguson, who developed in the early 1920s the idea of the closed-loop draft control for rear-attached implements, by a three-point hitch [121]. At that time Ferguson was mainly engaged in plow design, where he recognized the poor technical level of tractor-plow combination [122]. His new ideas initiated one of the most important innovations in tractor design, first applied to a Fordson F in the 1920s (prototypes), later to british tractors (Black Tractor 1933, David Brown 1936) in small quantities, and since late 1939 commercially to Fordson tractors (the “Handshake Agreement” [122]). The design was characterized by a coil spring upper link force sensor and a one-piston pump working directly on the lift cylinder and
controlled by a pump intake throttle valve (scheme see Fig. 1.151 and [123–125]). A general breakthrough of the Ferguson System as a common principle of tractor design took place directly after World War II. By the end of the 1960s there were very few tractors worldwide that did not make use of a draft control.
Concept and Dimensions of the Three-point Hitch
The basic concept is plotted in Fig. 1.144. As the three-point hitch has been for many decades the most important interface for tractor-implement combinations, a strong standardization has been developed [123, 126, 127]. ISO 730-1 (third edition 1994) class- ifies the dimensions by four hitch categories according to nominal tractor PTO power, Figs. 1.145 and 1.146 and Table 1.45.
Figure 1.144. Classic concept of rear three-point hitch (modern version: cylinder(s) also outside).
Figure 1.145. Dimensions of tractor rear hitch points (ISO 730-1: 1994). Front hitch almost the same
(ISO 8759-1).
Table1.45.Importanttractorrearhitchdimensions(ISO730-1:1994) 1234L4H HitechCategorieswithPTOPower inkWatRatedEng.Speed:Upto48Upto9280to185—150to350— UpperHitchPoint D1Diameterofhitchpin19025.5031.750450450 −0.08−0.13−0.2−0.8−0.8 b1Widthofball44max.51max.51max.64max.64max. b2Linchpinholedistance76min.93min.102min.140min.140min. dDiameterforlinchpinhole12min.12min.12min.17.5min.17.5min. LowerHitchPoint d2Diameterofhitchpinhole22.4+0.2528.7+0.337.4+0.3551+0.551+0.5 00000 b3Widthofball35045045057.5057.50 −0.2−0.2−0.2−0.5−0.5 11Lateraldistancefromlowerhitchpointto359435505610or612610or612 centerlineoftractor 12Lateralmovementoflowerhitchpoint100min.125min.125min.130min.130min. LDistancefromPTOtolowerhitchpoints500to575550to625575to675575to675610to670 hMastheight460±1.5610±1.5685±1.5685±1.51000±1.5
Figure 1.146. Distance from PTO to rear hitch lower link points
(ISO 730-1: 1994).
Kinematics and Forces of the Three-point Hitch Kinematics
The three-point linkage is a three-dimensional system. Plane views, Fig. 1.147, lead to four-bar linkages with virtual hitch points and convergence distances (ISO 730-1:
1994). ISO 730-1 recommends following values for horizontal convergence distances:
Cat. 1: 1700 to 2400 mm Cat. 2: 1800 to 2400 mm Cat. 3: 1900 to 2700 mm Cat. 4: 1900 to 2800 mm
If a certain implement (weight) shall be carried, the necessary torque at the lift shaft can be determined graphically as demonstrated by Fig. 1.148. Theoretical lifting force maps have been developed by [129], practical values see OECD Test Reports.
Forces in Case of Floating Implement.
Floating position is characterized by “no pressure in the cylinders” (connected with oil sump). Implement is controlled only by the equilibrium of implement forces and the virtual hitch point, as demonstrated for a mounted plow, Fig. 1.149. The total plow force results from soil resistance, plow weight and supporting heel force (with friction). The line of draft must cross the virtual hitch point, vertical load transfer from implement to tractor and from front to rear axle is not very important. Kinematics of front three-point hitch have been analyzed by [132].
Forces in Case of Hydraulic Implement Control
Implement is mainly carried by the tractor hitch in this case, demonstrated by Fig 1.150.
Soil resistance and plow weight are the same as for the above explained floating position, but the heel force is reduced considerably, influencing the line of draft as shown. This principle (as invented by Harry Ferguson) has three important advantages:
1. Improved implement control by hydraulics (draft control, position control and others)
Figure 1.147. Horizontal and vertical convergence (ISO 730-1: 1994).
Figure 1.148. Graphical determination of moments for tractor rear three-point hitch [128].
2. Improved traction by transfer of vertical implement forces to the tractor chassis 3. Reduced friction losses due to reduced heel forces or rolling resistance of support-
ing wheels.
Control Strategies for Three-point Hitches
The various systems are addressed by Table 1.46. They relate mainly to rear hitches (control strategies for front hitches have not yet been developed as well). Floating and
Figure 1.149. Three-point linkage operating in floating position; forces simplified (more in detail see [130, 131]).
Figure 1.150. Three-point linkage operating with hydraulic implement control; forces simplified (more in detail see
[130, 131]).
force control by upper link were the first strategies. An early stage of the Ferguson System is represented by Fig. 1.151. Upper link force is transformed to a clockwise movement of the control linkage operating the control valve. This opens the pump intake port, creating a certain flow to the lift cylinder, lifting in the same time the implement and finally reducing the upper link force (approaching the task set by the control lever).
Figure 1.152 shows a common system that combines strategies 3 and 4. They can be run both separately or in a mixed mode. Compared with 2 and 3 the mix reduces depth variations in case of soil density variations and improves dynamic control stability (problem of force control [133]).
Development of control strategies and systems is influenced considerably by sensor and processor development. Figure 1.153 demonstrates some important principles of forces sensing. Principles M1 and M2 are typical for low-cost upper link sensing systems (Ferguson). M3 is very economic, as it uses the lower link fastenings at the same time
Table 1.46. Basic tractor hitch control strategies [29]
No Control Strategy Typical Applications, Comments
1 “Free floating implement” Set-in procedure of plows, all implements having (valve in “sinking position”) supporting wheels (such as power tiller, rotary mower) 2 “Force control by upper link” Mounted implements (smaller tractors) for tillage
(original Ferguson-System)
3 “Force control by lower links” Mounted and semi-mounted implements for tillage (bigger tractors)
4 “Position control” Mounted implements (i.e., for distribution of chemicals and seeds)
5 “Mix of force and position control” Tillage implements (in relation to 1, 2, 3: reduced depth variations)
6 “Transmission torque control” Tillage implements
7 “Hydraulic pressure control” Load transfer from trailer or implement wheels or other supporting elements to tractor wheels
8 “Wheel slip control” Tillage implements
9 “Control strategies” working Mounted implements, i.e., tillage implements with with implement sensors depth sensor
10 “Control strategies including Semi-mounted tillage implement with actuated rear external cylinders” gage wheel
Figure 1.151. Early configuration of the
“Ferguson System” with upper link force sensing [123, 124 modified].
as a force sensor. (J. Deere 1965). H2 has been used by Case for many years and E3 represents the electronic force sensor bolt first introduced by Bosch in 1982. Electronic data processing (Bosch/Deutz 1978) has had a dramatic increase since the mid 1980s. The first analog working generations have been replaced more and more by digital systems offering higher flexibility.
Figure 1.152. Hitch control system with lower link force control and position control – also combined (mixed);
very popular system (mechanically or electronically) [29].
Figure 1.153. Principles of force sensors [125].
Fluid Power Systems: Symbols and Vocabulary
Communication in hydraulics can be simplified considerably by graphic symbols, as in other disciplines such as electrics, electronics, software engineering, transmissions etc.
The first international standard ISO 1219 (1976) was replaced by ISO 1219-1 in 1991 introducing some modifications (i.e., concerning pressure control valves and hydraulic cylinders). An extract has been formed by Table 1.47. ISO 1219-1 (1991), focusing more on general design rules than ISO 1219 (1976) and containing, for example, standardized relations of typical dimensions (circles, squares, rectangles, triangles etc.).
Table 1.47. Extract of ISO 1219-1 (1991) “Fluid Power Systems and Components - Graphic Symbols and Circuit diagrams. Part 1: Graphic Symbols”
Pumps and Motors Directional Control Valves
Hydraulic pump with one Directional valve 3/2:
direction of flow, fixed Three ports and two
displacement and one distinct positions,
direction of rotation muscular control
Hydraulic pump with Directional valve 4/3: Four
two directions of flow, ports, three distinct positions.
variable displacement and Actuated by electrical
one direction of rotation solenoid and return spring
Hydraulic motor with one Directional valve 4/3: Four
direction of flow, fixed ports, three distinct positions
displacement and one and infinite intermidiate
direction of rotation positions (throttle effects).
Electric input control signals, internal operation by internal Hydraulic variable speed
pilote pressure, centering spring drive unit, pump and motor
with variable displacement,
two directions of output Pressure Control Valves
rotation
Pressure relief valve. Character of control line indicates internal control function: Input pressure Hydraulic Cylinders
ballanced by spring load Single acting hydraulic
cylinder
Flow Control Valves Double acting hydraulic
Adjustable restriction cylinder
valve Lines
Working line
Flow control valve with two ports: Flow is about constant independant of pressure levels Control line
Flow control valve with three ports: Flow is about constant independant of pressure levels.
Flexible line
Bypass for surplus flow Enclosure for integrated
Flow divider valve: two output functions
flows of a fixed ratio being widely independant of pressure levels Diverse Components
Reservoir
Filter
Non-return Valves and Quick Coupling Non-return valves (check-valve), right: valve can be opened by a Cooler (left)
pilot control pressure Heater (right)
Accumulator
Quick-release coupling with mechnically opened non-return valves. Above: coupled, below:
disconnected
Figure 1.154. Hydraulic circuit with fixed displacement pumps and use of flow control valve bypass output
[134, Fendt].
A second part ISO 1219-2 (1996) contains typical circuit diagrams by examples.
The ISO 1219 standardization has been well accepted worldwide (in Germany, for example, by taking it over as national standard DIN-ISO 1219).
A separate vocabulary (English – French – German) has been developed by ISO 5598.
Hydraulic Circuits: Basic Systems
Most existing circuits of tractor hydraulics can be classified into five basic systems, which are explained below. The terms open center and closed center address the neutral position of the service valves.
Systems with Fixed Displacement Pump(s) and “Open Center” Control (“PF-open Center”)
This is the most used system worldwide, commonly working with one or two gear pumps. Output flow is approximately constant, if speed at the input shaft is constant (therefore also called constant flow system). Figure 1.54 demonstrates a typical European concept. A first gearpump I provides a flow for one remote line and the hitch, a second gearpump II for power steering (priority by flow control valve) – the bypass flow is used for front-end loader and remote II. As the front end loader requires high flow rates, bypass and pump I flow can be summarized by the 3/2 valve. The “PF-open-center”
creates the lowest initial cost; disadvantages are: full flow circulating all the time, high losses in case of low required flow rates at high pressures, low flows when engine is idling.
Constant Pressure Systems with Fixed Displacement Pump and Closed Center (“PF-constant Pressure”)
A fixed displacement pump is used to charge accumulators by a pressure-controlled charge valve (unsteady). This principle has been used previously by Ford and John Deere.
Constant Pressure Systems with Variable Displacement Pump and Closed Center (“PV-constant pressure”)
This type of circuit was introduced for mass-produced tractors by John Deere in 1960 (3010, 4010) [135]. The pump is only delivering the required flow, Fig. 1.155.
Adjustment of system pressure to load induced pressure needs flow controller (losses), but all actuators can be operated without reciprocal influence.
Figure 1.155. Hydraulic circuit with variable displacement pump and constant pressure control
[134, J. Deere 1985].
Figure 1.156. Load sensing hydraulic circuit [141].
Flow on Demand Systems with Variable Displacement Pump and Closed Center (“PV-load-sensing”)
Circuits of this type are usually called load-sensing (LS) and were introduced first to agricultural tractors by Allis Chalmers (USA) in 1973 [136]. Their potential of energy saving by on demand flow and pressure is well confirmed [134, 137–141]. The second objective of LS-systems is the excellent ability for proportional flow control valves, Fig. 1.156. A variable displacement pump is controlled by the pressure drop of the ser- vice valve; the pump thus delivers for any valve setting just that flow rate necessary to keep the value1p constant. Therefore the flow through the valve is directly proportional to the valve opening area (assuming constant orifice flow coefficient). When the valve is closed, the pump automatically moves to a “near zero flow” position, reducing at the same time its pressure to1p (i.e., 20 bar). If the working pressure exceeds a given limit, the pump also reduces flow output, saving a pressure relief valve and its energy losses (heat).
In the case of more than one actuator (with the task of no reciprocal influence), the highest actuator pressure is used for the pump control (shuttle). In order to have the same pressure drop1p also at the other directional valve, pressure compensator valves are added.
Flow on Demand-systems with Fixed Displacement Pumps (“PF-load Sensing”) They use pressure drop controller to ensure the constant1p at the service valve [141].
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