Principle and Operation of Radio-Controlled Full Size Off-Road Vehicle
1 Introduction
In order to investigate loss of tractor stability, it is necessary not only to analyze the dynamics of the tractor by modeling it but also to perform upset tests of the tractor in field. Testing a tractor for its stability in real life poses problems, such as difficulty in data acquisition, risk to driving personal, and property loss. During testing, when the tractor is overturned, its situation is so dangerous that it cannot be driven by humans. Therefore, a remote controlled tractor is essential in order to execute the tractor-upset test in the field conditions. A radio-controlled tractor and relevant measuring system are optimal solutions.
A radio-controlled tractor is one whose operations are controlled or operated by a radio remote system. Therefore, the radio control system should remotely control the major operations of the tractor. Four radio controlled tractors (Ford 8N, John Deere A, Farmall M, Ford 800) have been developed using a VANGUARD FM 6 CHANNEL radio control unit, and relevant executable parts of those tractors were modified. This chapter will discuss the principle of a radio control tractor, the modification of remotely operating parts of the tractor, and its operating procedures.
2 Principle of a Radio Control Tractor
The diagram of a radio-controlled tractor is shown in Figure 1. It mainly consists of a modified tractor and a radio control set. A commercial AVANGUARD FM 4/6 CHANNEL, AIRTRONICS inc.@ radio control set is used in the radio control tractor. The radio control set (Figures 2 and 3) mainly consists of a radio transmitter, a receiver and six servos (which depends on the number of controlled operations).
An operator of the radio-controlled tractor can virtually drive the tractor by switching the trim levers or switches on a radio transmitter. The receiver has a number of channels, each of which connects to a servo. A servo drives (pushes) one or two switches by rotating servo output arms. Then, the switches can turn on or off the electric circuit of actuators, which execute the operations of the tractor. When an operator shifts the trim lever or a switch on the transmitter, the receiver can receive a signal from the transmitter and then transfer it to an appropriate servo. Further, the servo rotation causes an electrical circuit to execute an operation of an actuator or DC gearmotor, which drives a functioning part such as the steering axle, brake pedal, clutch pedal, starting motor and fuel switch.
In general, when a tractor is operated, the following operations need to be executed:
For a radio controlled tractor, some major parts of the tractor need to be modified and rebuilt in order to be controlled by the radio control system, and another parts such as gear and throttle are still operated manually. Major operations of the tractor include electrical power, ignition, fuel flow, steering, engagement or disengagement of the clutch and brake. The electric system of power and the starting system need to be modified to be operated by either a manual or remote control.
3 Radio Control Set
A radio control set is used to transfer a radio signal from the transmitter into an electrical switch. It includes a transmitter and conventional control box (Figure1). An operator usually holds the transmitter while the conventional control box is installed on the tractor.
The VANGUARD FM 4/6 CHANNEL (Specification, 1995) transmitter (Figure 2) has a number of control sticks, trim levers and switches. Each control stick or switch is remotely connected to individual servo. In order for an operator easily to drive a radio-controlled tractor, all sticks or switches are selected with as much consideration as possible of operator ergonomics. Switch 13, control stick 3 and control stick 10 on the transmitter were selected as functional controllers.
The switch 13 has two positions, is set for a tractor electrical power on or off, and functions as a total controller of the tractor electrical power flow. The control stick 10 has two functions, one of which is to control fuel on (stick up) or fuel off (neutral position). Another function is to execute both clutch engagement and brake disengagement by shifting the stick left, or both clutch disengagement and brake engagement by releasing the spring-centered stick to neutral position. Control stick 3 is set for steering. Shifting it left turns the tractor left, or shifting it right turns the tractor right. Flap switch 16 is used to start the tractor.
The conventional control box consists of a radio receiver, Ni-Cd battery, six servos, seven switches, a switch harness, mounting frame and three electrical outlets. The diagram of the electrical circuit of the receiver, battery, servos, and switch connection is shown in Figure 3.
The principle of the servo arm driving switch and the detail of K6 are shown in Figure 4. Normally, each servo only controls one switch such as electrical power, fuel flow, or ignition. Operations of both brake and clutch are controlled by one servo and switch. The steering K6 needs the cooperation of both two switches and one servo in order to execute a right or left turn by the DC Gearmotor turning the steering wheel. A switch OMRON Z-15GW2-B7-K has two positions: NC (normally close) and NO (normally open), and two are mounted in a certain location relative to the servo arm (Figure 4). The rotation of the servo arm can throw the switch, opening and closing the ground and 12V power switch respectively, and transmit the power to the operating units. Based on the requirement of the torque turning steering wheel, the Dayton permanent magnet DC gearmotor was selected.
4 Modification of Tractor Electrical System
To operate a tractor by the radio control, it is necessary to modify the tractor's electrical system. The electrical power, ignition and fuel flow switches of the tractor are directly related to its electrical system, and are major functions controlled by the radio control set. The electrical system of the tractor should also be designed to provide power for actuators, which drive operating work.
The diagram of the modified electrical system is shown in Figure 3. An electrical transfer box was designed and mounted in the tractor, and has three plugs, which can be connected to the outlets of the conventional control box. Like the original electrical system of the tractor, the modified electrical system has an essential switch K1 for electrical power, which controls the power of the whole electrical system. When the K1 is on, other parts may get power through the battery.
Some extra electrical circuits need to be added to the radio-controlled tractor. A solenoid valve for fuel control was connected between the gas pipe and the gas tank. The battery on the tractor supplies power for steering the DC gearmotor.
5 Modification of Tractor Operational Parts
Normally, turning or steering is a major operation when driving the tractor, and executed by applying torque on the steering wheel. The DC gearmotor is used to drive the steering wheel. The modified steering system of Ford 800 is shown in Figure 5. The DC gearmotor shaft is directly connected to the steering axle by a universal connector. The DC gearmotor=s electrical power is connected to the conventional control box and provided by the tractor battery.
The clutch and brake are pneumatically remote controlled using actuators and solenoid air valves, which can be activated by turning switches in the remote control box (Figure 6). A clamp is designed and attached to the cylinder end of the actuator, and holds the pedal of the brake or clutch. Two actuators can be extended to disengage the clutch and engage the brake, respectively, or retracted to engage the clutch and disengage the brake, respectively.
Shifting gears is still operated by an operator. If the gear needs to be changed, the tractor has to be stopped to do so. The throttle is manually adjusted after starting.
6 Fail-Safe Design
The radio controlled tractor is operated by a set of radio control units. Although it has the advantage that the tractor is not necessarily operated by humans during field upset tests, it also has the disadvantage that it may be out of control if some units do not function properly. In order to secure the operation of the radio controlled tractor and to ensure that the failure of the radio control will not result in injury or damage to property, a fail-safe design was used.
The fail-passive design principle was applied to the radio controlled tractor. Cutting the power or shutting down the engine are two options. As is discussed in Section 3, the power switch 1 is to control the power electrical supply of the whole system. So, if an emergency occurs, the electrical power will shut down, and the spring-extended actuators will disengage the clutch and engage the brake to stop the tractor. In addition, a cable extending behind the tractor is linked to the throttle of the engine to shut down the engine manually in an emergency.
Figure 1 Diagram of a radio-controlled tractor
Figure 2 Airtronics Vanguard FM 4/6 Channel transmitter features and function
Figure 3 Diagram of radio receiver, servos, tractor, and electrical system connection of radio control tractor
Figure 4 Illustration of cooperation of servo and switches for steering system
Figure 5 The modified steering system of Ford 800
Figure 6 The solenoid valve and actuator for brake and clutch of Ford 8N
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