tem used in 2 wheel drive tractors. It consist of 1.steering wheel, 2.steering gear box ,3. Drop arm, 4. Draglink ,5. Spindle arm , 6. Steering arm ,7. Tierod , 8. Front axle beam, 9. Tyre assy.
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Steer By Wire In Agricultural Tractors
Jyothis Balakrishnan
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N integral design change to achieving these objectives was switching from a hydraulic steering system to a steer-by-wire (electronic) steering system. Incorporating
an electronic system would eliminate the many mechanical parts inherent to a typical hydraulic steering system, thereby reducing the overall weight of the tractor and automatically increasing fuel efficiency. This design change would also im- prove product reliability simply by eliminating the number of parts potentially subject to failure. Fewer mechanical parts also meant a significant reduction in manufacturing costs. These long-term benefits overcame any short-term costs re- quired to develop the electronic steering.
There has been a general trend away from hydraulics in other applications as well. Many manufacturers are looking to cut back or eliminate the use of hydraulics, so it is becoming much harder to find spare capacity on a hydraulic pump for the steering system. If spare capacity is not available then it be- comes necessary to add a hydraulic system dedicated to steer- ing which substantially raises the cost of this approach. Elec- tronic steer-by-wire systems, on the other hand, are complete- ly self-contained and do not require external pumps or hoses. This means that they are usually considerably less expensive than hydraulic steering when the cost of the pump, valve, hos- es and fittings are taken into account.
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• Jyothis Balakrishnan is currently pursuingmasters degree program in Au- tomobile engineering in Anna University India, PH-
+919543363916,Email:jyothis.balakrishnan@gmail.com
Tractor steering systems are of two types in the major popularity – manual steering system and hydro static steering system. Shown in the above picture is a manual steering sys-
tem used in 2 wheel drive tractors. It consist of 1.steering wheel, 2.steering gear box ,3. Drop arm, 4. Draglink ,5. Spindle arm , 6. Steering arm ,7. Tierod , 8. Front axle beam, 9. Tyre assy.
An integral design change to achieving these objectives was switching from a hydraulic steering system to a steer-by- wire (electronic) steering system. Incorporating an electronic system would eliminate the many mechanical parts inherent to a typical hydraulic steering system, thereby reducing the overall weight of the tractor and automatically increasing fuel efficiency. This design change would also improve product reliability simply by eliminating the number of parts potential- ly subject to failure. Fewer mechanical parts also meant a sig- nificant reduction in manufacturing costs. These long-term benefits overcame any short-term costs required to develop the electronic steering.
There has been a general trend away from hydraulics in other applications as well. Many manufacturers are looking to cut back or eliminate the use of hydraulics, so it is becoming much harder to find spare capacity on a hydraulic pump for the steering system. If spare capacity is not available then it becomes necessary to add a hydraulic system dedicated to
steering which substantially raises the cost of this approach.
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Mechanical steering systems are considered safe,
For motor vehicles for carriage of passengers, goods and their trailers, Framework Directive 2007/46/EC applies for type approval. Among others it lists for the steering effort the Directive UNECE R79. The cur- rent ECE R79 in Revision 2 now covers the possibility of full steer-by-wire systems without mechanical backup.
For Agricultural or Forestry Tractors, especially for faster tractors above 40km/h, directive 70/311/EEC may be applied. This directive, and the more general
75/321/EEC do not cover full steer-by-wire systems, but the latest UN ECE regulations may be applied to
some systems. In particular, regulation UN ECE R79
R2 may be applied, allowing full steer-by-wire sys-
tems.
For mobile machinery there exists no separate Euro-
pean Framework Directive. These vehicles are typical-
ly nationally type approved for on-road use. Many countries use ECE R79 R2, 70/311/ECC or
75/31/ECC concerning steering as possible basis for national approval. Therefore an independent test re- port on the steering system on that basis usually con-
siderably simplifies the national type approvals, espe- cially for those countries that also accept EC Direc- tives and UN ECE Regulations as an alternative to the national type approval regulations.
Besides regulatory requirements from the type ap- proval procedure, the manufacturer of steer-by-wire
systems also needs to consider liability aspects includ- ing the electronic control and communication system. Standards such as IEC 61508 are used to avoid and control any systematic design faults and to deal with random hardware faults.
1. Steering wheel
Fig. Steer by wire working
The system basically consists of two separate systems;
1. a control system that enables the driver to control the steerable wheels of the tractor
2. a feedback system that allows the driver to sense the reaction of the steerable wheels in the steering wheel.
2. Torque Sensing Device
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Fig. Steer by wire block diagram
The torque applied by the driver on the steering wheel is measured by a torque measuring device and transmitted to a processing unit. The processing unit controls a motor to pro- duce a torque corresponding to the torque applied
by the driver on the steering wheel. If the torque applied by the motor (b) to the steering gear is sufficient to overcome fric- tion and road forces, the wheels will begin to move. The steer- ing gear can be a traditional rack-and-pinion drive
connected to the steered wheels by tie rods. If external forces acting on the steered wheels exceed the force applied by the tie rods from the torque provided by motor (b) through the steering gear, the wheels will move as well.
Fig. Ackermann geometry
COT (IWA) – COT (OWA) = CC/WB IWA – Inner Wheel Angle
OWA – Outer Wheel Angle
WB – Wheel Base
CC – Center to center of pivot. INNER WHEEL ANGLE = 48 DEG OUTER WHEEL ANGLE = 33DEG
Measured steering effort in the manual steer- ing tractor has a maximum value of 10 Kg when vehicle is at stand still condition. As per regulatory requirements, the max- imum steering effort with a malfunctioning steering should not exceed 60 Kg. The objective of this project is to achieve the same specifications as the above manual steering and excel wherever there is possibility.
TURNING RADIUS = 2800MM
Table MF 1035 tractor specification
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Figure shows a schematic diagram of a vehicle steer by-wire system. The steer-by-wire system includes a steering wheel
mechanism (controlled plant) and a road wheel mechanism
(controlled plant). An electronic control module (steer-by-wire controller) controls the steering wheel mechanism and the road wheel mechanism in coordinated fashion. The steer-by- wire controller effectively links the steering wheel and road wheel mechanisms by wire through control signals.
Fig. Steer By Wire Layout
As shown in above Figure, the conventional hydraulic steering assembly has been replaced by an electric motor actuator to drive the road wheels in the road wheel mechanism. Road wheels are connected to a rack and pinion mechanism by tie rods. An angle sensor mounted in the motor or the rack and pinion mechanism is used to sense the road wheel angles. The steer-by-wire controller receives road wheel angle signals and produces a control signal to the permanent magnet brushless Direct Current (DC) motor through its electricdrive. The pri- mary goal for controlling the road wheel mechanism is to keep the road wheel tracking for the reference road wheel angle. The reference road wheel angle signal comes from the steering wheel assembly and changes according to the vehicle driver’s intent and the vehicle dynamics requirements. This system consisting of the road wheel mechanism and its control is re- ferred as the road wheel control subsystem. The steering wheel mechanism consists of a steering wheel mounted to a steering shaft, a steering wheel angle sensor mounted to the steering shaft for sensing a steering wheel angle, a DC electric motor and its drive, and a belt and pulley device (or gear de- vice) to connect the motor and the steering shaft. The primary goal of controlling the steering wheel mechanism is to pro- duce the steering feel and provide the road wheel reference angle signal.
Fig Steer by wire Functional Block Diagram
The driver applies a torque T1 on the steering wheel (1). The steering wheel is firmly connected to the one-way working gear (4) and can therefore not be moved by the driver. The torque T1 applied by the driver is measured by the torque measuring device (2) and is represented by a voltage V1. The voltage V1 is transmitted to the processor (6) and a voltage f(V1) is applied by the processor (6) to the electrical motor (7). The voltage f(V1) is a function of the voltage V1 and can be dependant on defined rules concerning, for instance, vehicle speed and/or operator defined characteristics which is desired to influence the handling. By applying a voltage f(V1) to it the motor (7) will apply a torque T2 on the steering gear (8). The torque T2 is thus a function of the torque T1 applied on the steering wheel by the operator, T2= f(T1).
Obtaining the required output torque T2 from the motor (7) can be done by either relying on known characteristics of the motor so a given input gives a given output, or by refining the system by adding an additional torque measuring device (10) between the motor (7) and the steering gear (8). By adding an additional torque sensor (10) the voltage applied on the motor (7) can be fine-tuned to achieve the target value torque T2. Friction between the mechanical parts in the steering gear (8) and the tie rod steering connections (11) between the steering gear (8) and the controlled wheels (12), as well as the friction between the road surface and the controlled wheels (12), re- quires a torque T3 to be overcome. As long as the torque T2=f(T1)< T3 none of the parts move. If the torque T2 gets larger than the torque T3 the controlled wheels (12) start to move. The acceleration, and thus the speed by which the con- trolled wheels (12) begin to move, is determined by the result- ing torque TResulting= T2- T3, being the difference between the torques T2 and T3. The motor (7) shall keep up and main- tain the torque T2= f(T1) which means that the current to the
motor must beincreased. This part of the system, the control
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part, is represented in the upper row in the information flow chart.
The position measuring device (9) measures the posi- tion P9 of the steering gear (8) and the position is represented electrically and constantly (or very frequently) transmitted to the processor (6). As long as the controlled wheels (12) do not move the readings from the position measuring device will not cause anything to happen. As soon as the controlled wheels (12) move the readings P9 of the position measuring device (9) change. The processor (6) will control the motor (5) in such a way that the position P3 of the steering wheel (1) as quickly as possible is made to correspond with the position P9 measured by the position measuring device (9) and thus P3=f(P9). The position P3 of the steering wheel (1) is measured by the position measuring device (3) It is evident that all sig- nals between the units, both in the control part and in the feedback part of the system, can be represented as electrical voltages or currents or as digital values.. In an analogue sys- tem, voltages and currents are regulated continuously and controlled by operation amplifiers.In modern digitalised com- puter based systems the all-important motor controllers can operate with by fixed time intervals. By measuring the posi- tions P3 of the steering wheel (1) and the position P9 of the steering gear (8) by fixed time intervals of Ät the processor (6) can determine a target value position P3Target of the steering wheel (1) for each timeframe of Ät. The processor (6) controls the motor (5) through a motor controller by adjusting the speed of the motor (5) so that the desired position P3Target of the steering wheel (1) can be obtained within a timeframe of Ät. If at the time Ät a position P91 of the steering gear (8) is measured by the position measuring device (9) and the posi- tion measuring device (3) measures a position P31=f(P91) of the steering wheel (1) no adjustment is required so the motor
(5) does not move.At the time 2Ät the position measuring de-
by now desired position of the steering wheel determined by the processor
(6) as P33=f(P93). If Ät is made adequately small it will in practical use appear as if the positions measured by the posi- tion measuring devices (3) and (9) change simultaneously. Using modern digital electronics frequencies of 100Hz or more should be achievable.
Fig .Steer by wire system dynamics
Fig. Closed loop system identification block diagram.
The closed loop transfer function of the system is as follows:
vice (9) measures the position P92 of the steering gear (8)
where the position P92 is different from the previously meas-
𝜃(𝑠)
=
𝜔𝑛 𝐾
2 =
ured position P91 measured at the time Ät. This means that
the steering gear (8), and thus the controlled wheels (12), has
started to move. The position measuring device (3) is still
𝜃𝑑 (𝑠)
Ө - pinion angle
𝑆2 + 2𝜉 𝜔𝑛8 + 𝜔𝑛
𝐽𝑠 𝑠2 + 𝑏𝑠 𝑆 + 𝐾
measuring the position P31 of the steering wheel (1), since the operator can not move the steering wheel (1) due to the one- way working gear (4). The motor (5) shall then drive the steer- ing wheel with a speed S1=(P32-P31)/Ät, where P32 is the desired position of the steering wheel determined by the pro- cessor (6) as P32=f(P92). At the time 3Ät the position measur- ing device (9) measures a position P93 of the steering gear (8) and a position P’32 (which is somewhere between P31 and P32) is measured by the position measuring device (3). Motor
(5) now changes speed to S2=( P33- P’32)/Ät where P33 is the
Өd – commanded steering angle
Js – moment of inertia of the system
bs – effective viscous damping coefficient
K – feedback gain
ξ – damping ratio of the system
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Fig Signals of Input and Output
𝜏 = 𝐽𝑠 𝜃 + 𝑏𝑠 𝜃 + 𝐹𝑠 𝑠𝑔𝑛(𝜃)
Fs – Coloumb friction coefficient
τ - actuator torque
τ = τ feedback + τ feedforward + τ friction + τ aligning
𝜏𝑓𝑒𝑒𝑑𝑏𝑎𝑐𝑘 = 𝐾𝑝 (𝜃𝑑 − 𝜃) + 𝐾𝑑 (𝜃𝑑 − 𝜃)
Өd – desired steer angle
Kp – proportional feedback constant
Kd – derivative constant
𝜏𝑓𝑒𝑒𝑑𝑓𝑜𝑟𝑤𝑎𝑟𝑑 = 𝐽𝑠 𝜃𝑑 + 𝑏𝑠 𝜃𝑑
Taking the inverse Laplace transform leads to a system of equations
𝜃𝑑 = 𝜔2 (𝜃 , 𝑠𝑒𝑛𝑠𝑜𝑟 − 𝜃 ) − 2𝜔 𝜃
𝜏𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛 = 𝐹𝑠 𝑠𝑔𝑛(𝜃𝑑 )
Fig. Feedback with feed forward and friction compensation
First order filter :
Ɵ𝑑 (𝑆)
Ɵ𝑑 , 𝑠𝑒𝑛𝑠𝑜𝑟(𝑆)
𝜔𝑐
=
𝑆 + 𝜔𝑐
Fig slip angle
Taking the inverse laplace:
𝜃𝑑 = 𝜔𝑐 (𝜃𝑑 , 𝑠𝑒𝑛𝑠𝑜𝑟 − 𝜃𝑑 )
Second order filter :
𝜏𝑎 = (𝑡𝑝 + 𝑡𝑚 )𝐹𝑦,𝑓 (𝛼𝑓 )
Ɵ𝑑 (𝑠)
Ɵ𝑑 , 𝑠𝑒𝑛𝑠𝑜𝑟(𝑠)
2
= 𝑐
𝑆2 + 2𝜔𝑐 8 + 𝜔𝑐 2
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𝑒 =
1
𝐽𝑠
�𝐾𝑝 𝑒 + 𝐾𝑑 𝑒�
+ �𝐹𝑠 𝑠𝑔𝑛�𝜃𝑑 � − 𝐹𝑠 𝑠𝑔𝑛�𝜃� + 𝑘𝑎 𝜏𝑎
− 𝑘𝑎 𝜏𝑎 �
= 1 �𝐾 𝑒 + 𝐾 𝑒� + 𝛿𝑒
𝐽𝑠 𝑝 𝑑
Fig Front axle geometry
𝛕 𝐚𝐥𝐢𝐠𝐧𝐢𝐧𝐠 = 𝐤𝐚 𝛕�𝐚 (𝛂𝐟 )
e = Өd – Ө
Ka – scale factor to account for torque reduction by steering gear.
Fig Steering controller with alignment compensation
Fig Steering controller block diagram.
Fig steering system model
𝐽𝜔 𝛿 + 𝑏𝜔 𝛿 + 𝜏𝑓 + 𝜏𝑎 = 𝛾𝑠 𝛾𝑝 𝜏𝑀
Jω - Moment of inertia at road wheel
bω – damping at road wheel
τf – Coulomb friction
τa – tire self alignment torque rs – steering ratio
rp – torque magnification factor
τM – steering actuator torque
Fig Bicycle Model
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𝐾(𝑑𝑒𝑔⁄𝑔) =
K= 0 ; Neutral steer (αf = αr)
K>0 : Under steer (αf > αr) K<0 ; Oversteer (αf < αr)
1
𝑎𝑦
(𝛿𝑓 − 57.3
𝐿
𝑅)
tor is the road to ground friction which varies drastically due to unpredictable operating conditions. The road wheel angles for the corresponding steering wheel angles are mathematical- ly simulated and torque requirements are arrived.
In major critical operations like deep pud- dling the steering wheels are non functional and the direction control of the tractor is executed by differential braking of in-
dependent rear wheels. Steer by wire will be used in light du-
ay : vehicle’s lateral acceleration
L : wheel base of the vehicle
R : Radius of vehicle cornering
δf : front wheel angle
LOCK TO LOCK ANGLE = 97 DEG
NO.OF TURNS FOR LOCK TO LOCK = 3.5 NOS TIME TAKEN PER TURN = 2.5 SEC MIN
Fig Steering wheel Vs Road Wheel angle
For single side turn
Steer by wire has turned up as a great technology to be used in agricultural tractors for easy manufacturing and drive ability. In this phase of the project the torque requirements of the steering system and the control algorithm for the steering sys-
tem is simulated. The major threat in case of agricultural trac-
ty operations like dry land preparation and haulage.
[1] 1. Newlaunch.com. Steer-by-wire concept cars exhibited at tokyo motor show.
[2] 2. Kassakian Dominguez-Garcia. Haptic interface for automotive steer-by- wire systems.
[3] 3. Jon Demerly Sanket Amberkar, Farhad Bolourchi and Scott Millsap. A
control system methodology for steer by wire systems.
[4] 4. Joachim Langenwalter and Tom Erkkinen. Model-based design with pro- duction code generation for steer-by-wire system development.
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