Assessment and characteristic of road-feeling for wheeled tractor hydraulic steer-by-wire system

Abstract: The hydraulic steer-by-wire system has no mechanical connection between the handwheel and front steering wheel. In other words, the road-feeling has no direct force contact with the tires, and it can be designed freely or by driver's preference. The design of road-feeling is one of the most difficult problems for steer-by-wire technology. Most drivers prefer lighter road-feeling, but at the same time, too light road-feeling can't help driver to supervise the vehicle operation and judge the condition of road face. So it's worthy to study the road-feeling characteristics. The paper chose the wheeled tractor as the study objective. In order to finish the study better and design road-feeling characteristic more reasonably, the paper first analyzed the evaluation index with the help of abundant references. Then the generating mechanism of road-feeling for tractors was relearned, because wheeled tractor not only works in the field but also travels on the road, and the driving environment of tractors has the large difference compared with other farm vehicles. When tractor works in the field with low speed and without pedestrian and other vehicles, the driver doesn't need to pay much attention to safety and prefers lighter road-feeling, but when tractor travels on the road from one place to another, such as dry land, dirt road, cement road and asphalt pavement, the speed is higher, nearly reaching up to 30 km/h, and with many pedestrians and vehicles around, so the driver needs to supervise the vehicle operation carefully and judge the information of road rightly with the help of high road-feeling. The paper measured the steering resistance of JS-754 tractor on the above 4 kinds of roads to guide the design of road-feeling characteristic. The modeled road-feeling was designed based on tractor speed and steering resistance by taking the driving environment difference into account, including cropland working mode and non-cropland traveling mode. The road-feeling value was calculated by tractor speed and steering resistance. The cropland working mode mainly was targeted to reduce the driver's work intensity. The tangential force that drivers exerted on the handwheel was 2-10.127 N in cropland working mode with the speed below 11 km/h. At the same time, the non-cropland traveling mode mainly focused on safety, supplying more distinct road-feeling to help driver to supervise vehicle operation accurately and do right operation. The tangential force that drivers needed to exert on the handwheel was 2.5-52.5 N, and the vehicle speed was lower than 30 km/h according to government standard under non-cropland traveling mode. In addition, the identification system was designed to fit the modeled road-feeling, including manual and automatic identification. The driver could choose identified way by three-position switch, and the controller determined the road-feeling mode and road-feeling value according to current speed, steering resistance and drivers' choice. Furthermore, the non-cropland mode was defined as safe mode and must be applied when speed was higher than 11 km/h. At last, the paper finished the objective evaluation about the designed road-feeling characteristic with the help of the developed experiment platform, and the steering resistance loading system was designed for the test. Ten experienced tractor drivers attended the questionnaire survey after they finished driving the experiment platform with the designed road-feeling. Drivers felt the speed just by value displayed on monitor instead of dynamic feeling. During the test, the linear, step, and sinusoidal steering resistances were loaded to simulate the physical truth. The results showed that the objective evaluation score was reasonable, which meant the designed modeled road-feeling could be used in all wheeled tractors and satisfy most drivers' preference. In summary, the designed road-feeling can make driving feeling better and promote the development of hydraulic steering by wire.