(Revision, 24 January 2002)

The automatic control system aiming at autonomous operation is drafted to be based on the GT 2000 terminal from GeoTec. The terminal will be connected to the tractor functions using CAN bus to an electronic control unit (ECU) or job computer, ESX from Sensor-Technik Wiedemann. The actuators and sensors will be connected to the ESX job computer.

The system will have three (major) operational modes:

1. Off: This mode is characterized by no power to the electronic control system, except for the GT 2000 and the GPS system. The ESX may be constantly powered according to the document ESX_HILFE and the power switching performed using the power control input D+ (0 Volt by Off mode).
   
2. Remote: The system (mainly steering, CVT and engine rpm) is functioning in remote control mode. The actuator control inputs are potentiometers and switches in a box (control panel) and are connected to an electric switch by cable. The ESX outputs are kept in the state corresponding to switching to mode Automatic. (It is assumed that later the ESX will perform simulation reporting to the RS 232 monitoring output which the controls to the outputs would have been if in mode Automatic).
   
3. Automatic: The control system is functioning for autonomous operation. In this mode the actuator control inputs are switched to the ESX outputs by relays.

By switching to mode Automatic the ESX outputs must be in the automatic driving start-up conditions, which are: The steering is set to centre (straight forward), Diesel pump is set to idle, CVT is set to neutral and relays for 3-point linkage, PTO and engine stop are un-powered.

(By extension of the ESX functions it is expected that e.g. sensor signal inconsistencies will be used for failure checking to ensure a gracefull system degradation).

1 Steering
In the driving modes “Remote control” and “Automatic” a Danfoss PVG 32 proportional valve controls the hydraulic steering normally controlled by the steering wheel hydraulic steering unit. The controlled hydraulic flow will approximately depend linearly of the control voltage. The actuation of the PVG 32 is controlled by an analogue signal from the ESX.

The steering angle is measured by potentiometers at both front wheels. The potentiometer signals are conditioned to suit the ESX input voltage range.

Set point for average steering angle is determined by GT 2000.

Remarks:
Hydrostatic steering layout: Estimated hydraulic flow 1.7 l/min by maximum power steering angular velocity, PVG32 spool suggested: Type AA ( 5 l/min, smallest available type. Standard PC spool). (Smallest available type with a linear flow variant is type B, 25 l/min).

Danfoss suggests proportional electrical actuation type PVES. Mech@tronic IT specifies the valve actuation unit suiting the ESX.

AgroTechnology makes the voltage range conversion electronics for the potentiometers.

Hakotrac: Steering angle end to end, estimated 90°. Wheel base: 1597 mm.

2 Diesel pump
A Linak LA 12 electromechanical linear actuator controlled by a Soft Servo 20 controller will perform the control of the diesel pump. The ESX will output the position set point for the Soft Servo 20 as a pulse with modulated signal (PWM). A specially made PMW/analogue converter will convert this signal to the analogue input signal for the Soft Servo 20.

Feed back of engine RPM to the ESX will be from an inductive sensor by the engine shaft.

Engine RPM set point is determined by the GT 2000.

Remarks:
Engine RPM range: 1250-2550 RPM.

AgroTechnology makes PWM/analog converter to control SoftServo 20 from ESX PWM output.

3 Continuously variable transmission (CVT)
The Hako tractor has a hydraulic continuously variable transmission without a conventional clutch. Standstill is near the middle of the control range, which goes from full backwards to full forwards.

A Linak LA 12 electromechanical linear actuator controlled by a Soft Servo 20 controller will perform the control of the CVT. The ESX will output the position set point for the Soft Servo 20 as a pulse with modulated signal (PWM). A specially made PMW/analogue converter will convert this signal to the analogue input signal for the Soft Servo 20.

The corresponding driving velocity feedback to the ESX will be from a non-contacting sensor at the shaft to the front wheel differential.

The set point for the tractor velocity is determined by the GT 2000.

Remarks:
AgroTechnology makes PWM/analog converter to control SoftServo 20 from ESX PWM output.

The velocity sensor cannot show if driving is forwards or backwards, instead the velocity sign has to be derived from the actuation setpoint output by the ESX (open loop control conditions). The CVT neutral position is signalled by a micro switch, which is sensing the mechanical position of the CVT control arm.

4 Three-point linkage rear

The control of the 3-point linkage is by an electric 3-position switch. Actuation from the ESX will be performed by two electromagnetic relays.

The upper lift arm angle is measured by a potentiometer. The potentiometer signal is conditioned to suit the ESX input voltage range.

Remarks:
AgroTechnology makes the range conversion electronics for the potentiometer.

5 Binary controls

5.1 PTO rear:
Relay.

5.2 Engine stop:
Relay with contact in series with solenoid valve for diesel pump fuel supply.

Remarks:
Before stopping the steering is set to centre (straight forward), Diesel pump is set to idle, CVT is set to neutral and relays for 3-point linkage and PTO are un-powered. The motor stop relay stays powered until operation mode is switched to Remote, however it must at least be powered for 20 seconds independent of operation mode.

5.3 ESX ready:
Signal showing if ESX has been initiated and is functional for the defined tasks.

5.4 Emergency stop:
(It was discussed if it would be relevant having a possibility for the ESX to break the hardwired safety loop circuit. The possibility will be left out until a relevant argument appears.)

5.5 Warning signal for driverless operation:
(The function will be part of the hardwired system.)

6 Binary feed backs to ESX

6.1 Operation mode:
See system concepts.

6.2 CVT in neutral position:

7 Action monitoring system via RS 232

• Selected parameters
• Sample rate
• (Preliminary)