Steer-by-wire-system

1. A steer-by-wire system comprising:

a steering handle disconnected mechanically from a pair of steerable wheels;

a steering motor for steering the steerable wheels in dependence on the manipulation of the steering handle;

a reaction force motor for generating a steering reaction force against the manipulation of the steering handle;

road reaction force detection means for detecting a road reaction force acting on the steerable wheels;

first made-up value determination means for determining a first steering reaction force made-up value (Hc 1 ) in dependence on the road reaction force detected by the road reaction force detection means;

steering position detection means for detecting the steering position of the steering handle or the steerable wheels away from a neutral position;

second made-up value determination means for determining a second steering reaction force made-up value (Hc 2 ) in dependence on the steering position detected by the steering position detection means; and

command value generation means for determining first and second gains (G 1 , G 2 ) so that relations G 1 ≦C, G 2 ≦C and G 1 +G 2 =C can be satisfied where symbols G 1 , G 2 and C are taken respectively as first gain, second gain and predetermined constant, and for generating a steering reaction command value (H) which makes the reaction force motor generate the steering reaction force, by the following expression:

H=G 1· Hc 1+ G 2· Hc 2

 using the first and second gains (G 1 , G 2 ).

2. The steer-by-wire system as set forth in claim 1 , wherein:

the second steering reaction force made-up value (Hc 2 ) is kept to be a constant value where the steering position exceeds a predetermined value; and

the command value generation means sets the first and second grains (G 1 , G 2 ) to a relation of G 1 =G 2 =1 and generates the steering reaction command value (H) by the following expression:

H=Hc 1 +Hc 2

 using the first and second steering reaction force made-up values (Hc 1 , Hc 2 ).

3. The steer-by-wire system as set forth in claim 1 , wherein:

the command value generation means is constructed to alter the first and second gains (G 1 , G 2 ) in dependence on the behavior of the vehicle or the state of the road surface.

4. The steer-by-wire system as set forth in claim 1 , further comprising:

vehicle behavior detection means for detecting a vertical acceleration of the vehicle or the movement of a suspension provided on the vehicle; and wherein:

the command value generation means is constructed to make the first gain (G 1 ) smaller as the vertical acceleration of the vehicle or the variation in movement of the suspension detected by the vehicle behavior detection means becomes larger.

5. The steer-by-wire system as set forth in claim 1 , further comprising:

vehicle behavior detection means for detecting a vertical acceleration of the vehicle or the movement of a suspension provided on the vehicle; and wherein:

the command value generation means is constructed to make the first gain (G 1 ) larger as the vertical acceleration of the vehicle or the variation in movement of the suspension detected by the vehicle behavior detection means becomes larger.

6. The steer-by-wire system as set forth in claim 1 , further comprising:

an inter-steerable wheel shaft interposed between the pair of the steerable wheels; and

a rack-and-pinion mechanism or a ball screw mechanism provided between the inter-steerable wheel shaft and the steering motor for converting the output rotation of the steering motor into the linear motion of the inter-steerable wheel shaft; and wherein:

the steering position detection means is constructed to detect the linear motion of the inter-steerable wheel shaft as the steering position.

7. The steer-by-wire system as set forth in claim 1 , wherein:

the second made-up value determination means is constructed to determine the second steering reaction force made-up value (Hc 2 ) in dependence on the steering position and a time-differentiated value of the steering position.

8. The steer-by-wire system as set forth in claim 1 , wherein:

the second made-up value determination means is constructed to determine the second steering reaction force made-up value (Hc 2 ) in dependence on the steering position and a vehicle speed.

9. The steer-by-wire system as set forth in claim 1 , wherein:

the second made-up value determination means is constructed to determine the second steering reaction force made-up value (Hc 2 ) in dependence on the steering position, a time-differentiated value of the steering position and a vehicle speed.

10. The steer-by-wire system as set forth in claim 9 , further comprising:

a first data map having a first coefficient correlated with the vehicle speed in advance; and

a second data map having a second coefficient correlated with the vehicle speed in advance; and wherein:

where symbol (θ) denotes the steering position, symbol (ω) denotes a time-differentiated value of the steering position, symbol (V) denotes the vehicle speed, symbol (Kv) denotes the first coefficient determined from the vehicle speed (V), symbol (Dv) denotes the second coefficient determined from the vehicle speed (V), and symbol (Hc 2 ) denotes the second steering reaction force made-up value, the second made-up value determination means is constructed to determine the second steering reaction force made-up value (Hc 2 ) by the following expression:

Hc 2= Kv·θ+Dv·ω.