DPA-EPS(Dual Pinion Assist EPS)

 Showa has been the leading developer and supplier of motor-equipped electric power steering systems including pinion-assist and rack-assist types. The “DPA-EPS® (Dual Pinion Assist Electric Power Steering)” released in 2015 incorporates the benefits of each system. While delivering good fuel economy, the DPA-EPS® also provides comfortable steering feel better or higher than the conventional hydraulic power steering (HPS) systems.

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 Since its first production of gear pumps in 1977, Showa has accumulated nearly 40 years of experience in power steering business. We launched rack-assist EPS for passenger cars in 1997; we always aim to be a technical leader of the EPS systems.
 While the conventional hydraulic power steering requires a continuously running pump, the electric power steering generates assist power as needed, keeping the energy consumption low for improved fuel economy. The electric motor not only provides power assist but also controls reaction forces, allowing fine controls. The issues with the column-assist or pinion-assist types are that because the motor is placed on the steering axis, the vibrations from the motor are directly transmitted to the steering wheel and driver’s steering inputs are not linearly communicated to tires. The rack-assist type provides improved steering performance, however the use of a special hollow motor made the cost down difficult. DPA-EPS® was developed by incorporating the benefits of different EPS’s with a concept of “achieving the rack-assist steering feel with the cost of pinion-assist.”
 DPA-EPS® uses two separate pinions on two different locations on the steering rack, one on the steering axis and the other connected to the power assist motor. Placing the motor away from the steering axis allows the independent setting of the vehicle’s overall ratio and gear reduction ratio, allowing for the easier optimization of motor size and efficient output. Incorporation of the control unit in the motor provides improved layout flexibility.

Optimized friction enables smooth steering feel when starting and reversing the steering wheel.

 Other advantages of DPA-EPS® are that, with the power assist mechanism located away from the steering axis, vibrations from the motor are controlled, and the use of a motor with damping means reduces unpleasant inputs from road surfaces, both of which contribute to a “crisp” steering feel. With a focus to realize linear steering feel “at the start of turning the steering wheel”, new control methods were developed and on-vehicle evaluations are being carried out on Showa’s own proving ground.
 The control of mechanical friction is critical for improving the smooth steering feel at the start and reverse of the steering wheel. In order to ensure responsiveness to very small inputs, a non-contact Hall IC sensor was adopted, which enables fine control by means of linear signals. By combining the conventional torque feedback control with a control to optimize the assist amount with respect to the steering angle speed, DPA-EPS ® provides both the excellent steering steel and vibration damping effect.

DPA-EPS(Dual Pinion Assist EPS)