The power steering rack fitted to the SY and SZ series is driven by an engine driven hydraulic pump, as was typical in the cars of its day. In the absence of an engine, it is possible to substitute this with an electrohydraulic pump to provide hydraulic pressure to the existing rack, which keeps modifications to a minimum.
Citroen C5 pump
However, hydraulic pumps can be a source of noise, and the RR steering rack design has a history of being prone to leaks due to poor seal design. Both of these problems are solved by using electric power steering , or EPS.
There are 2 ways to do provide EPS , either by using the existing rack with a servo motor in place of the lower steering column link, or by replacing the rack with a suitable EPS rack. TeslaRR uses the latter method using a rack designed for large SUVs.
The EPS is supplied by the LV battery, which in turn is maintained from the main HV traction battery via the DC-DC converter.
If the vehicle power supply fails, the driver can continue to steer the vehicle due to the mechanical connection between the steering wheel and the wheels, although far greater effort than normal is required to turn the steering wheel.
The sensitivity is adjustable, allowing the system to maintain the renowned Rolls Royce feel.