Development of Waterproof Hall IC Torque Sensor

Transcription

1 TECHNICAL REPORT Development of Waterproof Hall IC Torque Sensor K. HOTTA T. ISHIHARA JTEKT introduced a Hall IC torque sensor into electric power steering systems in 2006 as a non-contact type torque sensor and has steadily expanded its application since then, while in recent years there have been demands for its further cost reduction and higher performance. These demands have been compensated by the development of a new Hall IC torque sensor designed in waterproof structure, characterized as a lower cost, a higher reliability and a better sensing property. Key Words: electric power steering, torque sensor, Hall IC, waterproof structure, temperature compensation 1. Introduction Since 1989, JTEKT has mass-produced non-contact 2-bobin type torque sensors for electric power steering (hereinafter referred to as EPS) systems, and it developed and started to mass-produce, in 2006, a Hall IC torque sensor with new torque detection method, which is designed for easy mounting on EPS and an increased redundancy for a fail-safe function. In recent years, however, a lower cost and higher performance torque sensor has been needed. In response to this need, JTEKT comes to develop a new Hall IC type torque sensor by improving the conventional type, as presented hereunder. 2. Overview of Hall IC Type Torque Sensor for EPS 2. 1 Structure of EPS System Structure of an EPS system is shown in Fig. 1. Its electric parts consist mainly of a torque sensor, a control unit and an electric motor. Any steering force via the steering wheel is detected by the torque sensor, which sends a torque signal to the control unit. The control unit uses the torque signal and a vehicle speed signal to provide an appropriate current to the electric motor for an appropriate assistance to given steering force. Input shaft Torque sensor Reduction gear Tire Steering wheel Input torque 2. 2 Operating Principle of Hall IC Torque Sensor Figure 2 shows the structure of the Hall IC torque sensor. While the steering wheel is not operated, the teeth portion of the yoke short-circuits surface magnetic flux between N and S poles of the magnet, and hence no magnetic flux is transmitted to the Hall IC (Fig. 3, above). Torsion bar Magnet Yoke Tie-rod Torque signal Pinion Control unit Electric motor Rack Fig. 1 Structure of EPS system Current control Vehicle speed signal Hall IC Ring core Fig. 2 Structure of Hall IC torque sensor 60

2 N pole Yoke fringe portion S pole Hall IC Ring core When any steering force is applied, there arises an angular difference between the magnet and yoke's teeth mutually connected by a torsion bar and the magnetic flux in the magnet is transmitted from the yoke to the ring core. The magnetic flux proportional to a torsional angle of the torsion bar is transmitted to the Hall IC placed between protrusions on the ring core, which allows a given steering torque to be detected (Fig. 3, below). 3. Objectives of Development Yoke tooth portion Magnetic flux through Hall IC Input torque Angular difference generates between yoke and magnet Fig. 3 Hall IC torque sensor's detection principle Hall IC torque sensors currently in mass production have offered a great improvement in performance compared to 2-bobin type torque sensors and are used in many vehicles. As a torque sensor is one of the essential parts of EPS system, both its reduced cost and higher performance must always be pursued. Therefore, JTEKT have approached the development of a new Hall IC torque sensor, aiming at the following: Improve reliability by waterproofing and reduce cost Improve temperature property of torque sensor's output performance 4. Waterproof Structure Technology 4. 1 Principle of Waterproof between Sensor Body and Cable Figure 4 shows the new Hall IC torque sensor. If EPS system is installed in the engine compartment, it is necessary to get all the parts waterproof. In order to waterproof them, O-rings, thermal shrinkage tubes and waterproof connectors are commonly used and highly reliable. However, as far as the waterproofing between the torque sensor and its cable (Fig. 4, A portion) is concerned, there have been few methods that could achieve both objectives of productivity and reliability, which has been a factor of increased cost. New Hall IC torque sensor has a circuit board assy (hereinafter referred to as ASSY) made with polyamide and a cable sheath made with polyurethane. A structure of waterproofing between the ASSY and its cable comes to be achieved by a melted connection of the ASSY (polyamide) and the sheath (polyurethane) under thermal effect for resin molding of the ASSY (Fig. 5). This method not only facilitates the production without introduction of any new special process or equipment, but also ensures a highly reliable waterproofing capability by a melted connection of the materials, thus making it possible to ensure compatibility between the productivity and reliability. Furthermore, molding conditions of the resin come to be optimized, with the melting state checked, so prudently as to ensure a sufficient bonding strength between the polyamide and polyurethane, which in fact depends on a temperature at the melting (Fig. 6). Circuit board ASSY Ring ASSY Waterproofing additional to connector Adoption of waterproof connector Waterproof between harness and cable sheath Adoption of thermal shrinkage tube A: Waterproofing between circuit board ASSY and cable sheathmelted connection between resins Waterproofing between ring ASSY and circuit board ASSY Adoption of O-ring Waterproofing between sensor housing and torque sensor ASSYAdoption of O-ring Fig. 4 Torque sensor waterproof parts Polyamide 2 µm Polyurethane Fig. 5 Melting state of polyamide and polyurethane (Result of phase measurement in scanning microscope) 61

3 Bonding strength between polyamide and polyurethane High Temperature for sufficient adhesion Body resin (polyamide) Check for no clearance Polyamide resin injection molding temperature Cable sheath (polyurethane) Enlarged Polyurethane melting start temperature Polyurethane complete melting Fig. 7 Cross section of circuit board assembly and cable sheathe bonding part Low Temperature High Fig. 6 Bonding strength dependent on a molding temperature of the ASSY 4. 2 Evaluation Results Table 1 shows the results of endurance tests. The test conditions are based on JTEKT in-house specifications defined in reference to each car maker's specifications. The test results ascertain that the new Hall IC torque sensor satisfied all the evaluation items, showing its high reliability. Also, a visual inspection of test sample's cross section after testing reveals an adequate waterproofing capability as well as no clearance between the cable and the resin ASSY (Fig. 7). Table 1 Reliability test results 5. Improved Temperature Property As the waterproof type torque sensors are supposed to be installed in the automobile engine compartment, they are used in a more stringent environment than the nonwaterproof types which are mounted in the passenger cabin. In addition to being waterproof, the torque sensor must provide a stable performance under widely varying temperatures (particularly high temperatures). Influential to steering feeling, a temperature property of the torque sensor needs to be excellent. Particularly in Europe, automotive manufacturers have recently upgraded their requirements, and thus it is necessary to further improve the property. To meet such requirements, the Hall IC torque sensor has incorporated a temperature compensation function to realize a torque detection mechanism immune to the effect of temperature. Especially, the newly-adopted Hall IC incorporates a temperature sensor capable of measuring a temperature of its junction, which is then compensated by digital signal so much as to provide a temperature property better than that of the conventional type (Table 2). Table 2 Comparison in temperature properties of Hall IC μ + μ The effect of temperature variation on performance of the product is considered to be attributed mainly to a temperature property specific to the material of each component comprising the product. For instance, in the case of Hall IC torque sensor, the sum of variation in 62

4 properties, under temperature change, of its components such as the magnet, the yoke, the ring core and the Hall IC element is represented by a variation in performance of the torque sensor at a given temperature. The temperature compensation function integrated into the Hall IC torque sensor can compensate in such a way that it kills a tendency of variation in the performance at each temperature, so that at any temperature within the guaranteed operating temperature range, the same performance as that at normal temperature (25) can be realized. Figure 8 shows the results of temperature property measurement for the Hall IC torque sensors installed in the engine compartment. Both the conventional and new torque sensors, being provided with temperature compensation, show a satisfactory performance level that would not affect steering feeling within the guaranteed temperature range ( 40 to 125). In addition, the new type has some improvement confirmed in temperature property in relation to the conventional type, consequently enabling its safety factor to be more marginal than the conventional type against the requirements of European automobile manufacturers. Fluctuation of torque sensor performance Rightward steering Large variation Small variation Leftward steering Large variation 0 Conventional type Temperature, Performance requirements by European manufacturers New type Fig. 8 Improvement achieved by temperature compensation 6. Other Structures PPS resin Products installed in the engine compartment need to maintain their basic performance under stringent environmental conditions, as well as to have resistance to corrosion and aging effects. As mentioned in the previous section, particularly in Europe, automotive manufacturers have recently made the requirements more and more severe, and thus it is necessary to improve the environment-resistant performance. In this respect, the new type of ring assy has adopted a resin of PPS superior in environmental resistance to PBT on the conventional type with favorable results (Table 3). Conventional guaranteed performance Table 3 Comparison of environment-resistant performance Cost reduction The conventional type Hall IC torque sensor was originally designed to be non-waterproof, so that the addition of a waterproof function to this type (base) has resulted in increased cost. On the other hand, the new type incorporates a waterproof structure into its newly-adopted basic design originally in favor of waterproof, together with its parts internally manufactured. As a result, cost reduction was successfully achieved compared with the conventional type (Fig. 9). Also, a circuit board assy of the new type, which is of a split structure, is designed to be available for both waterproof and non-waterproof. This limits a difference by vehicle models to only the connector shape and the harness length, which also contributes to the cost reduction. Cost 1 Waterproof structure Base design (non-waterproof) Conventional type 30% cost reduction compared with conventional type Fig. 9 Cost comparison Base design (waterproof) New type 63

5 7. Conclusions While it has not been long since JTEKT started its mass production of Hall IC torque sensors, JTEKT has proceeded with such a continuous improvement in aspects of performance and cost in order to respond to a wider range of needs as a supplier. Also, in response to the current demands for energysaving and compact car, JTEKT will continue its development efforts to realize a lighter-weight and morespace-saving design of the sensor that would satisfy the requirements of the EPS systems on which the sensor is installed. K. HOTTA * T. ISHIHARA ** * Electronics Engineering Dept., Steering System Operations Headquarters ** Experiment & Analysis Dept., Steering System Operations Headquarters 64