Electro-hydraulic power steering is an extension of the well-known hydraulic power steering. The main difference is how the hydraulic pump that supplies the pressure necessary for the power steering is driven. In electro-hydraulic power steering, an electric motor drives the pump, which is controlled according to the required assisting steering force.
At very low speeds, or when stationary, the delivery of the hydraulic pump is increased to provide a large amount of steering power. At high speeds, the pump rotational speed is lowered because assistance is not necessary.
The advantage of electro-hydraulic power steering is the improved comfort: When manoeuvring, steering is light to handle. But at high speeds, steering is firm. This enhances overall fuel economy, as the power uptake adjusts as required.