Control for electric power steering

A system for an electric power assisted steering (EPS) apparatus comprising at least two inverter bridges each connected to a multi-phase motor configured to provide power assist to steering of a vehicle, wherein each inverter bridge comprises a plurality of switching elements each associated with a phase of the motor, the system further comprising a controller which is arranged such that:

• during a first mode of operation of the EPS system the controller drives the motor using both inverter bridges simultaneously, the nominal current that is applied to the motor at any time by each bridge being limited to a first value, and
• during a fault mode of operation the controller drives the motor using only one of the two inverter bridges whereby the nominal current that is applied to the motor at any time in the fault mode is limited to a second value, the second value of a non-faulty bridge of the inverter bridges initially being set at a 100 percent value of the applied nominal current of the EPS system,
• the controller dynamically varying the second value throughout the duration of the fault mode of operation as a function of one or more operating parameters of the EPS system by decreasing the second value in response to the one or more operating parameters indicating potential overheating of the EPS system and by increasing the second value in response to the one or more operating parameters of the EPS system indicating a decrease in temperature of the EPS system.

A method of controlling an electric power assisted steering (EPS) system comprising a first lane and a second lane, each of the first and second lanes comprising an inverter bridge that is electrically coupled to at least one motor, the method comprising:

• generating a driving current having a nominal amplitude;
• providing a first portion of the driving current through the first lane at a first amplitude and a second portion of the driving current through the second lane at the first amplitude to drive the at least one motor in a normal mode of operation of the EPS system;
• in response to a fault of the first lane, disabling the first lane and providing the second portion of the driving current through the second lane to drive the at least one motor in a fault mode of operation;
• initially setting a current limit of the second portion of the driving current to the nominal amplitude in the fault mode of operation;
• monitoring operating parameters of the EPS system in the fault mode of operation;
• and
• dynamically varying the current limit by decreasing the current limit to a second amplitude that is greater than the first amplitude in response to the operating parameters indicating potential overheating of the EPS system, and by increasing the current limit from the second amplitude in response to the operating parameters indicating a decrease in temperature of the EPS system in the fault mode of operation.