The suspension position of each wheel was measured using linear potentiometers installed on individual suspension linkages. Potentiometers are analogue sensors, and operate on the basis that their electrical resistance is proportional to length of the resistor, and that the output voltage is proportional to resistance. They generally consist of a moveable component that makes contact with an internal resistor and forms a circuit, with the resistance amount defined by its position. The movement of this contact is normally either in translation (linear) or rotation (angular). When a resistance element has a voltage applied to it the motion of the moveable contact results in a change in output voltage across the sensor that is linearly proportional to the contact position, as is depicted in Figure 4.12.
Figure 4.12: Potentiometer operation [115]
Four 100mm stroke linear potentiometers were selected for this application, and were supplied by Gefran (model PZ12A). These sensors are considered a standard size for automotive suspension measurement. They consist of anodised aluminium cylinder cases with internal moveable control rods made from stainless steel, and have a useful electrical stroke of 100 mm and a mechanical stroke of 105mm. At the end of the
moveable rod and at the bottom of the sensor there are two M5 self-aligning rod ends used for mounting, with a minimum “eye to eye” length of 228mm due to the size of the sensor.
The sensors can survive speeds of up to 10m/s and forces of less than 0.5N. The 40 resistor can also withstand up to 10mA (although 0.1A is recommended) and 60V, and has an independent linearity error 0.1% with infinite resolution. As with the Hall sensors, these sensors also entail the use of a supply voltage wire (brown), a ground wire (blue) and the signal wire (yellow).
4.2.2.1 Sensor Considerations
The placement of the linear potentiometers to measure suspension movement is critical in obtaining useful, reliable and accurate data. Nonetheless, mounting these sensors to the suspension system is a difficult proposition, and many design compromises need to be made.
Since wheel position relative to the vehicle in the vertical plane is being measured, it is obvious that mounting the sensor directly from the body of the vehicle to the wheel in a vertical direction would provide direct results. This is, however, fraught with difficulties. Firstly, the wheel moves on its suspension much more that 100mm which would require a much larger sensor. Secondly, having the sensor mounted at the wheel would expose it to adverse operating conditions, with a strong likelihood of failure. Thirdly, the steering action of the front wheels would alter the geometry of the sensor, affecting results. Finally, physically mounting the sensor in the centreline of the wheel is impossible.
As a result of these problems potentiometers are often mounted to the suspension springs or dampers. This has the benefit of providing spring and damper position and velocity (which plays a very important role in evaluating suspension performance [116]), but with knowledge of the suspension geometry or experimental testing it can be used to estimate the wheel position – with the assumption of no free play in joints. This often reduces the displacement for measurement, places the sensors in a safer position, negates steering effects and is much easier to mount.
4.2.2.2 Sensor Installation
While the above is the case on most racecars, the suspension on the test vehicle makes mounting suspension sensors directly to the springs or dampers difficult. As a result, it was decided to mount the sensors from the vehicle body to an arbitrary point on the suspension system that moved with the wheel. Using this method it was possible to easily mount the sensors in a way that maximizes their stroke length, and places them in a reasonably safe position. Using an experimental technique it would then be possible to calibrate the sensed displacement to movement at the wheel, or movement at the spring/damper with a reasonable degree of accuracy. This is a common mounting method.
4.2.2.2.1 Front Suspension Position Sensors
The front wheel suspension is independent and of a “Macpherson Strut” type [20], and as such, comprises of a lower pivot and a spring/damper from which the wheel is suspended, as depicted in Figure 4.13. The installation of the front linear potentiometers for suspension movement measurement is shown in Figure 4.14. The lower end of the linear potentiometer is attached to the lower pivot about half way along its length, and the upper end is connected to the chassis through the wheel well.
Figure 4.13: Front suspension
This choice of placement was made based on observations of the suspension movement, and on possible mounting points. The wheel well contains a small number of nuts welded into the body of the car that provide mounting possibilities, and one of these was observed to be in a useful location. The upper rod end of the potentiometer was thus installed in this location using a custom built “double threaded” bolt, as shown in Figure
4.15. This bolt was designed to convert the ¼” thread of the vehicle to a M5 thread to suit the sensor, as well as to provide sufficient clearance for the sensor to operate correctly.
Figure 4.14: Front suspension position linear potentiometer mounting
Figure 4.15: Front suspension “double threaded” bolt for mounting
Figure 4.16: Front suspension lower strut mounting method
The installation of the lower potentiometer mounting required some small modification to the strut, and is shown in Figure 4.16. The mounting was produced by drilling and tapping a M5 thread into the hollow strut. It is noted that this will reduce the strength of the strut, but a location was chosen whereby the effect would be negligible. A M5 bolt was installed into the hole so the thread was pointing outwards and “Loctite” used to provide a stud to mount the sensor on. This was difficult in practice because there was no way to access the bolt head, so the threaded end of the bolt was modified to allow
alignment and tightening from the other end. The potentiometer mounting was held in place using two split washers, a nut and additional “Loctite”.
Finally, it was observed that this mounting arrangement took full advantage of the length of the sensor for the full range of suspension movement. The sensor was installed with approximately 5mm extra travel with the suspension in full droop and has some additional travel to compensate for unknowns when in full rebound. It is noted, however, that even though the sensor has been installed to accommodate extremes of movement to avoid failure, its normal operating range will be limited to approximately 10% of its travel because full droop and full rebound are rarely encountered in practice.
4.2.2.2.2 Rear Suspension Position Sensors
The rear suspension is not independent, and is of a “Trailing Twist Axle” type [20]. This type of suspension is increasingly used in small front wheel drive vehicles, and consists of two trailing arms connected by a single transverse member, as shown in Figure 4.17.
Figure 4.17: Rear suspension
Unfortunately, this means that a movement from one wheel can directly affect movement of the other, so it is difficult to consider each as separate entities. They are not rigidly connected, however, with the bar between them designed to be rigid in bending (locating the wheels in plan view) and torsionally compliant during roll (providing anti roll and camber gain characteristics). It could be argued that it is fair to approximate the workings of the rear suspension to an independent trailing arm arrangement, with an anti-roll bar linking each wheel. This assumption reduces the degrees of freedom of the
system, and as a result it can be assumed that the wheels move in the arc of the trailing arm. This makes it possible to relate the movement at the trailing arm as proportional to movement at the wheel. By making this assumption a very small amount of error would be introduced into the wheel position calibration, as the arc of movement of the trailing are would vary slightly in reality.
Nonetheless, the placement of the suspension sensors must be chosen to measure only the movement of the wheel, and not incorporate any of the suspension flex into the readings. This means that the sensors must be mounted to a part of the trailing arm not likely to flex with movement of the transverse member. This is compounded by the desire to use existing features around the suspension to provide rigid mounts, and to avoid mounting options that would reduce suspension strength. Nonetheless, the final mounting points are shown in Figure 4.18, and fulfil these requirements.
The lower mount bolts directly into the trailing arm. This location was chosen to provide a vertical mount for the sensor as well as to avoid reducing the strength of the arm (being mounted at the edge of the formed metal sheet, well away from the welds). The upper mount is installed in a similar way, with an M5 thread tapped into an overhanging edge of metal sheet. Both mounts also utilise “Loctite”, and have spacing washers installed to maximise the rod end movement potential.
Figure 4.18: Rear suspension position linear potentiometer mounting
4.2.2.3 Sensor Calibration
Suspension position sensors can be calibrated to determine three important, yet proportional, variables. When evaluating damper performance it is important to be able
to measure the position of the damper, which can be differentiated into speed. In this case it is useful to reference the calibration to the damper. When evaluating kinematic suspension performance, however, it is important to determine the load on the tyre, which for the most part is determined by the spring position. In this case it is useful to reference the calibration to the spring, which is often in the same position as the damper. When evaluating the dynamics of the vehicle, as is the aim of this investigation, it is useful to determine the position of the wheel relative to the body. These three variables are related to each other by the “Motion Ratio” of the suspension (MR = Wheel Movement/Spring Movement), which is approximately constant, and, for example, allows the force at the spring to be related to the force at the wheel.
The preferred method of racecar suspension position calibration is to remove the suspension spring and anti-roll bar and measure the position and sensor voltage relationship as the suspension is moved through its entire range. Removing the spring makes it much easier to move the suspension and collect data. In the case of the test vehicle, however, the type of suspension makes this difficult, as the spring/damper unit is integral to the suspension at the front (in the Macpherson Strut arrangement) and at the rear, the transverse member makes independent movement impossible. As such, combined with the amount of effort required, a different method was chosen for suspension calibration.
It was decided that the simplest method of calibration was to model the suspension in CAD. This would not only provide the geometry necessary to complete the calibration, but would compile all required data to undertake a very thorough suspension analysis if required. The resulting CAD models are shown above in Figure 4.13 and Figure 4.17.
By including the suspension linear potentiometer into the models it was possible to relate a movement at the wheel to movement at the sensor and at the spring/damper. By assuming that the linear potentiometers are in fact linear, data was acquired to calibrate the suspension parameters at each wheel, with the results shown in Table 4.1. It can be seen from the data that the suspension position has been zeroed so that the normal ride height when unloaded reads as zero. This was done to provide results that were easy to understand and interpret, although it is noted that the zero is arbitrary. Further, the data for one side of the car is different from the other. This arose from the fact that it was difficult to position the sensors in an identical arrangement on both sides of the car.
Finally, the motion ratios where calculated from Table 4.1 as approximately 0.99 for the front wheels and 1.25 for the rear, the ratios decrease with increasing deflection.
FL Voltage (V) 0.00 0.01 0.50 0.88 1.00 1.15 1.50 1.73 2.00 2.13 2.50 2.89 3.00 3.53 4.04 5.00 Suspension Pos. (mm) -83.0 -82.4 -48.1 -20.5 -11.5 0.0 26.2 43.4 64.5 74.7 104.0 135.8 145.5 193.8 245.4 371.2 Spring/Damper Pos. (mm) 0.0 60.8 124.3 156.6 221.4 284.3 342.4 FR Voltage (V) 0.00 0.26 0.50 1.00 1.13 1.37 1.50 1.97 2.00 2.38 2.50 3.00 3.13 3.78 4.28 5.00 Suspension Pos. (mm) -96.6 -79.8 -63.1 -27.2 -17.9 0.0 10.0 46.0 48.0 77.3 86.8 127.2 138.4 196.4 247.9 337.1 Spring/Damper Pos. (mm) 0.0 60.8 124.3 156.6 221.4 284.3 342.4 RL Voltage (V) 0.00 0.06 0.50 1.00 1.37 1.50 1.54 2.00 2.00 2.50 2.61 3.00 3.19 4.18 4.88 5.00 Suspension Pos. (mm) -83.4 -80.6 -57.6 -30.2 -9.5 -2.0 0.0 26.1 26.3 54.3 60.5 82.7 93.5 157.3 213.4 225.1 Spring/Damper Pos. (mm) 0.0 54.7 82.9 111.3 139.9 196.7 250.8 RR Voltage (V) 0.00 0.01 0.50 1.00 1.32 1.50 1.57 1.95 2.00 2.50 2.56 3.00 3.14 4.13 4.83 5.00 Suspension Pos. (mm) -85.6 -85.2 -59.6 -32.0 -14.0 -3.8 0.0 21.7 24.3 52.5 56.0 81.0 89.0 152.8 208.9 225.3 Spring/Damper Pos. (mm) 0.0 54.7 82.9 111.3 139.9 196.7 250.8
Table 4.1: Suspension and spring/damper position calibration data