EV.4.5 Accelerator Pedal Position Sensor

T.4.2 Accelerator Pedal Position Sensor - APPS

T.4.2.1 The Accelerator Pedal must actuate the APPS T.1.4.1

a. Two springs must be used to return the foot pedal to 0% Pedal Travel

b. Each spring must be capable of returning the pedal to 0% Pedal Travel with the other disconnected. The springs in the APPS are not acceptable pedal return springs.

Video - APPS Troubleshooting

A large portion of the T.4.2 rules are elaborated upon in the video shown above. Many of the graphics shown in the explanations are pulled from this source. For more in-depth information on troubleshooting the APPS, this is worth a watch!

T.4.2 focuses on the specifications for the Accelerator Pedal Position Sensor (APPS). The main purpose of this component is to analyze signals coming from two independent potentiometers mounted along the acceleration pedal, and to decide whether the measurements are plausible or not. A breakdown of each subsection within this Technical Aspect rule is as follows:

Photo - Manufactured Pedal Box

T.4.2.1 stipulates that pressing the Accelerator Pedal must cause the Accelerator Pedal Positioning Sensor (APPS) to operate, as determined in T.1.4.1. In order to return the foot pedal to its defined neutral (0%) state, there is an aspect of redundancy specified in T.4.2.1.a. Two springs, which as specified in T.4.2.1.b must independently be able to accomplish this task. This implies that these springs must be connected to the pedal in parallel, not series.

T.4.2.2 Two or more electrically separate sensors must be used as APPSs. A single OEM type APPS with two completely separate sensors in a single housing is acceptable.

Rules Reference - T.1.4.1 (Referenced in T.4.2.1)

T.1.4.1 Accelerator Pedal

a. An Accelerator Pedal must control the Powertrain output

b. Pedal Travel is the percent of travel from a fully released position to a fully applied position. 0% is fully released and 100% is fully applied.

c. The Accelerator Pedal must:

Return to 0% Pedal Travel when not pushed
Have a positive stop to prevent any cable, actuation system or sensor from damage or overstress

Graphic - Example Design

T.4.2.2 requires that at least two sensors that are electrically independent be used to track the positioning of the accelerator pedal, for the purpose of redundancy. To be electrically independent, these sensors must be provided two different supply voltages. If these sensors are purchased as a commercially available module, this rule stipulates that they must be sold by an original equipment manufacturer (OEM) to ensure that it works as intended without discrepancy. Additionally, as long as the purchased sensors are electrically independent, they can share a single housing.

T.4.2.3 The APPS sensors must have different transfer functions which meet one of the two:

Each sensor has different gradients and/or offsets to the other(s). The circuit must have a pull-up or pull-down resistor to bring an open circuit input to 0% Pedal Travel
An OEM pedal sensor with opposite slopes. Non OEM opposite slope sensor configurations require prior approval.

The intent is that in a short circuit the APPSs will only agree at 0% Pedal Travel

T.4.2.3 provides guidelines for the transfer functions produced by the APPS. As defined, each sensor must produce transfer functions which are non-intersecting as a result of either or both cases of:

An offset applied to one transfer function which is greater than 10% of the difference between both functions

Differing slopes which do not intersect within the full range of pedal travel (0% to 100%).

Scenarios for these cases are provided below:

Graph - Parallel Transfer Functions

Schematic - Parallel Transfer Functions

Graph - Non-Parallel Transfer Functions

Schematic - Non-Parallel Transfer Functions

Alternatively, teams are allowed to use sensors which are made by an original equipment manufacturer (OEM) that provides transfer functions whose slopes are inversely proportional.

T.4.2.4 Implausibility is defined as a deviation of more than 10% Pedal Travel between the sensors or other failure as defined in this Section T.4.2. Use of values larger than 10% Pedal Travel require justification in the ETC Systems Form and may not be approved

Spreadsheet - ESF APPS Transfer Function

T.4.2.4 defines an implausibility as more than a 10% difference between the travel of each sensor with respect to its own voltage.

Note that this rule does not stipulate a 10% difference between the two sensors (the offset), but actually a 10% difference between the individual sensors and their respective pedal positions. If their percentages are off by more than 10%, the system should trigger an Implausibility.

For Example, say sensor A says the pedal is at 50% and sensor B says the pedal is at 53%. This is allowable, because they are within 10% of each other. However, if sensor A says the pedal is at 50% and sensor B says the pedal is at 61%. This represents an implausibility - the two sensors are reading pedal positions that are too far apart.

T.4.2.5 If an Implausibility occurs between the values of the APPSs and persists for more than 100 msec, the power to the (IC) Electronic Throttle / (EV) Motor(s) must be immediately stopped completely.

(EV only) It is not necessary to Open the Shutdown Circuit, the motor controller(s) stopping the power to the Motor(s) is sufficient.

T.4.2.5 requires that more than 100 milliseconds of implausibility be met with an immediate loss of power to the EV Motor(s), so as to ensure the vehicle is not accelerating outside of the driver's control. Furthermore, this loss of power does not need to come as a result of opening the Shutdown Circuit, which essentially allows for it to be carried out as a software-based function within a motor controller or Electronic Control Unit (ECU).

T.4.2.6 If three sensors are used, then in the case of an APPS failure, any two sensors that agree within 10% Pedal Travel may be used to define the (IC) throttle position / (EV) torque target and the 3rd APPS may be ignored.

T.4.2.6 allows for use of more than two sensors for the sake of redundancy. Furthermore, it states that as long as at least two sensors out of a potential three agree within 10% pedal travel, the torque target is considered satisfactory. By design of utilizing more sensors, it decreases the likelihood of a false alarm (redundancy).

T.4.2.7 Each APPS must be able to be checked during Technical Inspection by having one of the two:

A separate detachable connector that enables a check of functions by unplugging it
An inline switchable breakout box available that allows disconnection of each APPS signal.

The purpose of T.4.2.7 is to ensure that the each APPS has separate connections to the system, so that they may be easily disconnected and troubleshooted for testing at Technical Inspection. This can be achieved by using either a detachable connector that physically unplugs one of the sensors to isolate the other, or a breakout box which often uses a dual-inline-package (DIP) switch to isolate signals.

T.4.2.8 The APPS signals must be sent directly to a controller using an analogue signal or via a digital data transmission bus such as CAN or FlexRay.

Schematic - APPS Passthrough

T.4.2.8 requires that the signals from all APP Sensors be sent to an Electronic Control Unit (ECU) using either analog or digital signal. In our case, the Controller is a Teensy2.0++ which is directly connected to our potentiometers. By requiring this direct connection, teams are prevented from routing to potentially unsafe user-made communication lines.

T.4.2.9 Any failure of the APPS or APPS wiring must be detectable by the controller and must be treated like an Implausibility, see T.4.2.4 above

As per T.4.2.9, the controller that we use must be able to detect any potential failure modes of the wire. Whether an open circuit, loss of power to the APPS, break in GND line, break in Potentiometer/ECU, or other failure. This should all be completed using a voltage threshold check, hopefully which allows for minor calibration failures.

T.4.2.10 When an analogue signal is used, the APPS will be considered to have failed when they achieve an open circuit or short circuit condition which generates a signal outside of the normal operating range, for example <0.5 V or >4.5 V.

The circuitry used to evaluate the sensor must use pull down or pull up resistors to ensure that open circuit signals result in a failure being detected.

Graphic - Signal Calibration

T.4.2.10 addresses analog signals, requiring that the APPS reach a fail condition if an open or short is detected which causes a signal to be read outside of the voltage threshold. It is reasonable to allow for calibration errors in this scenario, and also a good idea to set the 0% pedal position equal to a value greater than 0V.

If both of these are instituted, it will be easier to detect an open circuit fault before low voltage is powered, because the ground reference will be out of bounds. This is called a Live Zero State. Furthermore, allowing for minor errors (10% error total, 5% upper and 5% lower bounds) reduces the risk of false fault detection.

In order to detect the open circuit signals, we are required to use Pull-Type Resistors.

But what are Pull-Type Resistors?

Schematic - Pull-Type Resistors

Pull-up resistors are resistors which are used to ensure that a wire is pulled to a high logical level in the absence of an input signal, whereas pull-down resistors are resistors which are used to ensure that a wire is pulled to a low logical level in the absence of an input signal.

Pull-up and pull-down resistors are often used when interfacing a switch or some other input with a micro-controller or other digital gates. They provide a controlled current flow into a resistive sensor prior to analog to digital conversion of the sensor output voltage signal.

T.4.2.11 When any kind of digital data transmission is used to transmit the APPS signal,

a. The ETC Systems Form must contain a detailed description of all the potential failure modes that can occur, the strategy that is used to detect these failures and the tests that have been conducted to prove that the detection strategy works.

b. The failures to be considered must include but are not limited to the failure of the APPS, APPS signals being out of range, corruption of the message and loss of messages and the associated time outs.

Table - Failure Modes

T.4.2.11 refers to digital data transmission, and requires that the Electronic Throttle Control (ETC) Systems Form contain a Failure Mode and Effects Analysis (FMEA). As per T4.2.11.a, A form is provided by Formula SAE Online to guide these analyses, but ultimately the potential failures, methods of detecting failures, and tests that justify that these methods work should be documented in-depth for troubleshooting in crisis.

Spreadsheet - FMEA and ETC Template

2022_ETC_FMEA_Template_(2).xls

T.4.2.11.b defines a few common modes of failure which must be considered, being an internal failure of the APPS, APPS signals being out of the voltage threshold, corruption of the serial communication message, or loss of messages and the associated time out messages. Further error considerations need to be made for each team's specific system.

T.4.2.12 The current rules are written to only apply to the APPS (pedal), but the integrity of the torque command signal is important in all stages

Beyond that, T.4.2.12 clarifies that although these rules are written with regard to the APPS, teams should also be wary of the integrity of a torque command signal, which is specific to torque vectoring.

But what is torque vectoring?

It is a multifaceted system which controls how much power the vehicle's engine and motor send to each individual wheel. This can be done using a differential (and the process varies depending on which type is used- an 'open' differential cannot control which wheel receives the most torque, while a 'limited slip differential' does so mechanically) by using small clutches within the mechanism. Additionally, they can use braking systems to stop a wheel from slipping and improve the car's ability to accelerate in slippery conditions.

Ultimately, torque vectoring is helpful for traction, cornering, and stability, but should not be used to amplify the torque requested by the driver. Instead, power output should be redistributed and/or decreased.

Video - Torque Vectoring in Action

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