Cameron Motor Works Electric Vehicle Conversion

Installing the Electro-Hydraulic Power Steering System

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December 2004

Toyota MR2 Electro Hydraulic Power Steering

Design Notes

Rather than the elaborate VW plumbing used to cool the power steering fluid, a compact lightweight cooler will be used.
The stock reservoir will be used
An optical encoder will be placed on the steering column, triggering a 100 Amp relay to activate the power steering pump.
The accessory battery will be used to handle the surge power of DC-DC current
The MR2 EHPS has two small contactors which open when two of the four brushes become too worn. These will be wired into the EV ECU for a warning

Power Steering System Design

Hydraulic System

The following schematic depicts the power steering system hydraulic plumbing design:

Electrical System

There are a variety of ways to control the power steering to be used only when needed:

The majority of need is when running at lower speeds, making larger turns. Sheer's design uses an optical encoder and a microcontroller to detect the amount of change in steering. If over a certain amount, the controller will turn on a FET which will in turn power a 100Amp relay which will in turn send power to the motor of the power steering pump. [1]
(suggested by Neon John off the EV List) is to rig the EHPS motor to be two speed. Low speed for normal driving, and then high speed for higher demand. A control unit would have to be devised to determine demand. This can also involve and encoder to anticipate the need. The idea is not to have the unit "turn on" suddenly.
One thought was to use a hydraulic pressure accumulator. The pump feeds the accumulator only when the pressure gets low. It turns out that this idea will not work at all, because power steering systems are "open spool" sllowing oil to always freely circulate. Any built-up pressure would soon dissipate and the motor would constantly be running.
Attach an optical encoder to the steering shaft. Feed this to a PWM controller which varies the speed of the power steering unit with the amount the steering wheel turns.
Swap out the whole steering unit and put in a European electric power steering unit (most likely **very** difficult to retrofit). See BBA-reman for a list of European units.

Misc Notes

A special fuse and relay will be used to control the PS Pump:

P&B High amperage automotive relay
Large fuse from Wytek

NOTES about using a MOSFET to control the relay

From Jon Pullen:

use an induction protection dioe on the load side of the relay To drive a solenoid with a pic and a Mosfet (n channel) Connect the Source of the Mosfet to the Pic Vss or ground. Connect the Drain of the Mosfet to the Solenoid Connect the other terminal of the Solenoid to the proper V+ voltage (It might be 5, 12, or 24 volts)(doesn't matter if it is higher than pic Vcc) Connect the gate of the Mosfet to any output pin of a pic. When you drive the pin high, the solenoid turns on.

Use a "logic level" FET such as a 2N7000 or an MTP3055 and directly drive the gate from a PIC output. A 100k resistor from Gate to ground is helpful as it keeps the FET turned off if you remove the PIC for reprogramming, etc.

Very little chance at all. It would probably be a Good Idea [TM] to include a series resistor between the pic output and the Fet gate. You're more likely to get spikes transmitted through the power rails - good decoupling is essential.

Calculating the Size of Ultracaps for EHPS Pump

An option to using the accessory battery to handle the surge of the EHPS Pump, is to use ultra capacitors. To determine the approx size of the capacitors, the following calculations can be used:

Assumptions Surge 100 Amps
Voltage 12 Volts
Time period = 4 seconds

Power P = I * R
    = 100 A * 12 V
    = 1200 Watts

Energy Energy = P * t
    = 1200 W * 4 s
    = 4800 Joules

Capacitance Capacitance = 2 * Energy / V²
    = 2 * 4800 J / (12 V)²
    = 33 Farads

Installation

Installation of the power steering pump required:

Mounting the pump, close to the steering rack
Having hydaulic hoses fabriacted for the high pressure side of the pump
Mounting the power steering reservoir in a convienient location
Installation of a small power steering fluid cooler

Reference

[1] Electronic Control for DC Motors Using Discrete Bridge Circuits January 1997, Nuts and Volts Column #23, Scott Edwards

[2] Motor Control Made Easy February 2000, Nuts and Volts Column #58, Lon Glazner

[3] Using Encoders

[4] Mouse and Trackball

[5] Motion System Design

[6] Honeywell Sensors