Modifying Electric Power Steering

This article was first published in 2005.

More and more cars are now being fitted with fully electric power steering. In these systems, hoses, pumps and reservoirs are dispensed with – instead, an electric motor attached to the steering column does all the work. The advantages to the manufacturer include lower servicing and assembly costs, and to the consumer, less likelihood of failure through power steering fluid leaks. Fuel consumption is also improved.

But there’s another advantage to the modifier, one which so far has been completely overlooked. Because it’s an electronically-controlled system, it’s easy to alter the characteristics of electric power steering to suit individual preferences. Specifically, you can alter the steering weight to radically improve steering feel and high speed stability.

And you want the good news? You can give your electric power steer car user–adjustable control over the steering assistance for under ten bucks.

The modification covered in this story was carried out on a Toyota. However, we’d expect that very similar changes would be possible on any car with electric power steering that uses a torque-based measuring system to determine the amount of electric assist.

Electric Power Steering Systems

It’s now over four years since we covered the basics of electric power steering (see Electric Power Steering) but what do most electric power steering systems look like today? Well, some are simpler than covered in that story.

Typically, an electric power steering system consists of:

• - a powerful electric motor geared to the steering shaft
• - torque sensor(s) that detect how much effort is being put into the steering
• - an electric power steering Electronic Control Unit (ECU)
• - a road speed input to the ECU

The ECU looks at the steering torque and steering direction being applied by the driver, and at the road speed, and directs the electric motor to provide the required amount of assistance in the correct direction.

Torque refers to the strength of twist being applied to a shaft. The higher the twisting force, the higher the torque.

Since the key ingredient in modification is the torque sensor, let’s take a closer look at it.

As with some conventional hydraulic power-assisted steering systems, a torsion bar is used measure the relationship between the torque being applied to the steering wheel by the driver and the resistance being posed by the tyres.

It’s important to realize that this measured torque is a two-way process – if the front wheels are on wet grass they’ll turn very easily, so despite the driver turning the steering wheel hard, not much torque will need to be applied to alter the steering angle of the tyres. However, if the front tyres are on coarse bitumen, they will resist turning and so the amount of torque needing to be applied by the driver will be much higher to get the tyres to turn.

In other words, the torque sensor indicates both the driver’s input of torque and the torque reaction of the tyres.

The use of the torsion bar therefore takes into account the real steering effort needing to be applied – irrespective of road surfaces, tyre inflation pressures, and road speed.

So how does this torsion bar system work? The torsion bar forms part of the steering column – it twists when subjected both to high input torque and high tyre reaction torque. Two sensors are used. Each measures the amount of twist and outputs a voltage that is proportional to this. When no twist is occurring, the voltage output of each sensor is in the middle of its range. So, with sensors with an output range of 0-5V, each sensor reads close to 2.5V when there’s no steering torque being input.

However, when subjected to torsion, the sensors’ output voltages change. When there’s increasing left-turn steering torque being applied, one sensor increases in its output voltage while the other sensor decreases in its output. The opposite occurs on right-hand corners.

Therefore, the larger the difference between the output voltages of the two sensors, the more steering effort that is occurring.

This graph shows the output of the two torque sensors and how this relates to the amount of steering effort. It can be seen that at the point where there is no steering torque being applied, the sensor voltage output lines cross.

One of the hard points to grasp about these torque-measuring systems is that the output difference between the two sensors is not proportional to the amount of steering lock applied. This is because the tyres mostly resist being turned when they are being turned – once a certain amount of tyre angle has been adopted, the effort required to maintain that steering lock is much less than the effort required to first gain it. Instead, the greatest difference between the two sensor outputs occurs when steering input is being rapidly applied on a grippy surface at low speed... which is fine, because that’s when you most need the assistance!

Because of the way the output signals of the torque sensors are configured, the ECU knows both the direction that the torque is being applied in and how great it is. The ECU then instructs the electric motor to assist appropriately, and as a result, the required steering torque effort by the driver decreases, resulting in a lower difference in the output voltages of the torque sensors. The assistance provided by the motor is therefore reduced.

Modifying the System

So to summarise the above paras for those just skipping along: the greater the difference in the output voltages of the two torque sensors, the greater the amount of steering torque that the ECU knows is being applied to the steering.

In nearly all cases, the desired outcome of modified electric power steering will be more steering feel – or in other words, you want less power assist. At higher speeds this results in better turn-in cornering feel, better straightline stability and a far more secure on-road feel. Sure, there will be slightly heavier steering when parking, but unless you’re very frail, that’s unlikely to be a problem.

So to achieve the outcome of less power assistance, the ECU needs to be fooled into thinking that there is less steering input effort than is really occurring. To achieve this, all that we need to do is reduce the difference between the voltages of the torque sensors.

This can be achieved very simply by the use of just two multi-turn potentiometers (pots). Even including the cost of a box to mount the pots in, the total bill will be under 10 bucks.

So, how do you know if your late model car has electric power steer? The easiest way is to look for the underbonnet presence of a hydraulic power steering fluid reservoir. If the car has power steering and there’s no reservoir, it must be electric – or the car uses a combined hydraulic system that powers the steering and brakes.

How to Do It

The first step with any electric power steering system is to disable the system and go for a drive. Usually, switching off the system is just a case of pulling the electric power steering fuse or relay. Of course the steering will be much heavier when moving slowly, but the car will still be driveable. What you are looking for is the change in steering weight at speed – say 80 km/h.

Is it much heavier, or the same as usual?

If it’s the same as usual, the amount of power assistance being applied at this speed must normally be zero. (In that case, you’re not going to be able to improve steering weight by modifying the system!) However, if you notice a firmer, meatier steering weight, you can be sure that there’s too much assistance normally being given at this speed – and so there’s room to make improvements.

The next step is to find some of the functions of the power steering ECU pins. At a pinch you can get away without a workshop manual but it’s always best to have one. Earth one lead of a multimeter and then use the other to backprobe the plugged-in power steering ECU. Have the car running and use an assistant to waggle the steering while you’re taking the measurements.

On the Toyota Prius on which this modification was performed, the following important voltages were found:

• Torque sensor #1 – 2.5V output with no torque input, varying downwards with left-hand torque and upwards with right-hand torque
• Torque sensor #2 - 2.5V output with no torque input, varying upwards with left-hand torque and downwards with right-hand torque
• 5V regulated output

Either of the two sensors can be intercepted – the ECU is just looking for the difference between the output voltages. So how is the modification done? The following diagram shows just how easy it is.

Let’s take it step by step. Pot 1 is placed across the 5V-to-earth connections. If this pot is set to its middle position, 2.5V will be available on its wiper.

Pot 2 is wired with one end connecting to this 2.5V supply and the other to the sensor output. This pot’s wiper goes to the ECU.

If the wiper of Pot 2 is placed closer towards Pot 1, the signal the ECU sees will be held more and more at 2.5V – that is, no torque change. On the other hand, if the wiper of Pot 2 is placed closer to its other end, the ECU will see more and more of the unaltered signal.

So with Pot 1 set to provide 2.5V on its output, by adjusting Pot 1 you can alter the signal from being always held at 2.5V at one extreme, to being dead standard at the other extreme. Set Pot 2 to ‘in-between’ positions and you can get ‘in-between’ values.

The two pots used are 10 kilo-ohm multi-turn designs. If you use small trimpots these are very cheap, or if you use full-size multi-turn units, more expensive. We set the system up with the latter, simply because we had them already on the shelf. (Always use multi-turn – eg 10-turn - pots as this makes the setting-up much easier.)

Install Pot 1 - it goes between the 5V regulated supply and earth. Use a multimeter to measure the voltage on the central wiper terminal (the meter connected with one probe to the wiper and the other to earth) and then adjust the pot so that its output voltage is the same as the ‘at rest’ sensor output voltage. In this case, that was 2.5V.

Then cut the signal wire between the sensor and the ECU. Connect the sensor end of this wire to one end of Pot 2, and connect the other end of Pot 2 to the wiper of Pot 1. The wire to the ECU then connects to the wiper of Pot 2.

When doing the wiring it’s easiest to ignore the description and simply look at the diagram.

Adjust Pot 2 so that its wiper fully at the end closest to the signal input. Start the car and drive it – it should drive normally. If it doesn’t, check your wiring.

Then adjust Pot 2 so that the wiper starts to move towards the other end. The steering should now get heavier. If you go too far, it’s likely that you’ll trigger a fault condition – when setting this pot, drive the car lots to make sure that (a) the weight is good across a variety of driving situation, and (b) no fault condition is triggered.

Results

Using a Fluke 123 Scopemeter to data-log both the input signal from the sensor and the modified output shows the changes that have been made.

As can be seen, the input trace (bottom) and the output trace (top) appear to have the same shape. However, close inspection shows that the upper trace always moves less distance from the midpoint of about 2.5V. In fact the recorded minima (circled) show that the output dropped only as low as 1.853V, compared with 1.102V for the input. Other data (not shown here) indicates that the maximum voltage recorded on this drive from the sensor was 3.673V, versus 3.029V on the modified output.

In other words, the output voltage holds closer to the ‘no-torque’ value of about 2.5V, telling the ECU that there was less steering torque being input than there really was.

The result is less power assist and so greater road feel.

Because the ECU has a road speed input, the steering still alters in weight with speed. In this car, the parking weight is slightly increased – which is neither here nor there – but from about 60+ km/h there is noticeably better road feel than standard. Turning into a high speed corner gives far more reassuring feedback as to what the front tyres are doing, in addition to giving weight against which the steering is worked – allowing more precise inputs of lock.

As we said with the last car where we modified the steering weight (Modifying Speed-Sensitive Power Steering), when you have the ability to alter this characteristic, you suddenly realise with startling clarity that the amount of steering weight makes a huge and instant difference to how the car feels on the road.

Thanks to Silicon Chip magazine’s John Clarke for technical help during the development of this modification.

Wiring–Up Pots A potentiometer (pot) is a simple electronic component. Most pots are rotary designs - like the volume control on an older radio, as you turn the shaft, the internal wiper moves along a resistance track. There are only three terminals – shown here as A, B and W. Most pots have clearly laid-out terminals but if the pot you are using is confusing, a simple check with a multimeter (set to resistance) will show you what’s what. Between terminals A and B should be the full value of the pot, eg with a 10 kilo-ohm pot, around 10K resistance. Measuring between either A and B and W (the wiper) will give a resistance that alters as you adjust the pot. In the application shown in this story, the A and B terminals of the pot can be connected either way around – this will just alter the direction that you turn the pot to go up or down in signal.