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| 1 | +--- |
| 2 | +title: "Vehicle Motion Management" |
| 3 | +date: 2019-08-04T12:46:30+02:00 |
| 4 | +weight: 7 |
| 5 | +--- |
| 6 | + |
| 7 | +## Introduction |
| 8 | + |
| 9 | +In modern vehicles multiple electronic controlled systems are interacting to realize the overall vehicle motion in all physical directions. |
| 10 | +Typically, the driver gives input e.g. using the steering wheel and the accelerator and brake pedals, but additional vehicle functions may have also requests towards the motion actuators. |
| 11 | +One example is that a traction control system may want to limit the performance due to slippery road conditions or that the emergency braking system requests braking. |
| 12 | + |
| 13 | +A number of signals have been added to VSS related to powertrain (eAxle), steering and braking. |
| 14 | +These signals may be used to define actuator interfaces to support a highly flexible functional deployment on different electronic control units. |
| 15 | +Vehicle motion functions like driver brake request, cooperative regenerative braking and traction control system may request target values for longitudinal control on vehicle, axle and wheel level. |
| 16 | +Therefore, generic interface signals for the braking systems are introduced which support an arbitration of the requested target values by minimum and maximum values. |
| 17 | +The signal definitions for powertrain and steering are based on state-of-the art interfaces which are widely used in the automotive industry. |
| 18 | +VSS does not specify the implementation of the interface signals and the arbitration cascade, but possible examples are given below. |
| 19 | + |
| 20 | + |
| 21 | +## Definitions and Assumptions |
| 22 | + |
| 23 | +If nothing else is specified, the following definitions and assumptions apply |
| 24 | + |
| 25 | + |
| 26 | +* The specified data types are chosen based on state-of-the art interfaces for vehicle internal communication between chassis ECUs. |
| 27 | + All datatypes int/uint may represent decimal numbers, therefore additional computational methods could be specified in upcoming VSS releases. |
| 28 | + |
| 29 | +* Main guiding units for vehicle motion brake control: |
| 30 | + * Vehicle level: Force requests |
| 31 | + * Axle level: Torque requests |
| 32 | + * Wheel level: Torque requests, omega limits (wheel-spin velocity according to ISO 8855) |
| 33 | + |
| 34 | + Brake forces on vehicle level are defined by the sum of all wheel forces that control the vehicle along the driving path. |
| 35 | + Brake torques on axle level are defined as the sum of torques on all wheels on that axle. |
| 36 | + Brake torques and omega limits on wheel level are defined for each individual wheel of the vehicle. |
| 37 | + |
| 38 | +* Signal orientation brake system: |
| 39 | + Requests are positive in desired path direction of the vehicle. |
| 40 | + The desired path direction is defined by the intended driving selection (P,R,D,N). |
| 41 | + This means that a positive value indicates acceleration, a negative value deceleration with respect to the intended driving selection. |
| 42 | +* Omega limits for brake system defined as relative to actual driving direction. This means that omega is never negative. |
| 43 | +* Torque and Force distribution signals (between front/rear axle or left/right wheel) are based on the assumption that all wheels/axles exercise torque in the same direction, i.e. a single axle/wheel cannot fulfil more than 100% of the total request. |
| 44 | + |
| 45 | +* Signal orientation steering system: |
| 46 | + Requests are defined according to ISO 8855. |
| 47 | + For steering related signals, a positive request on the front axle unless otherwise stated yields to steer the vehicle to the left. |
| 48 | + |
| 49 | +* Signal orientation eAxle system: |
| 50 | + Requests are defined according to ISO 8855. |
| 51 | + This means that a positive torque yields to a force in vehicle forward direction and a negative torque yields to a force in backward direction. |
| 52 | + Omega limits for eAxle system defined according. Positive sign for rotation in forward direction, negative sign for rotation in backward direction. |
| 53 | + So the sign of current omega of a eAxle indicated the current driving direction. |
| 54 | + |
| 55 | +* All signals are defined in an automotive safety context, i.e. ISO 26262 has to be considered while using specified signals in vehicle applications. |
| 56 | + |
| 57 | +----------------------- |
| 58 | + |
| 59 | +## Sensors vs. Actuators |
| 60 | + |
| 61 | +The type `actuator` is selected to define that the signal may be used as a "request" by vehicle systems/services towards the actuator. |
| 62 | +The type `sensor` is selected for signals that are processed by the actuator and may be used as actuator feedback towards the requesting vehicle systems/services. |
| 63 | + |
| 64 | +## Braking System |
| 65 | +The defined braking signals are the main signals for interacting between vehicle motion features and the braking system including the arbitration of longitudinal vehicle requests. |
| 66 | +The interface signals are specified on vehicle-, axle- and wheel-level and the arbitration concept supports a flexible integration of vehicle motion features on different ECUs. |
| 67 | +The arbitration of multiple vehicle motion features can result in a sophisticated arbitration logic to ensure the vehicle stability and driver safety, which is not part of VSS signal catalogue. |
| 68 | +However, the specified vehicle signals shall support different arbitration logics. |
| 69 | +The following section shows some hypothetical examples for arbitration. |
| 70 | + |
| 71 | +### Example 1 - vehicle force requests (`Vehicle.MotionManagement.Brake.VehicleForceMaximum`) |
| 72 | +Driver request via brake pedal -2000N. |
| 73 | +Advanced driving assistance system for automated emergency brake has a request of -3000N. |
| 74 | +As a result, a vehicle force of -3000N would be applied, to ensure maximum deceleration. |
| 75 | + |
| 76 | +## Example 2 distribution request (`Vehicle.MotionManagement.Brake.VehicleForceDistributionFrontMinimum`/ `Maximum`) |
| 77 | + |
| 78 | +Vehicle force request of -3000N of example 1. |
| 79 | + |
| 80 | +A vehicle energy function requests a AxlePercentRangeDistributionFrontMaximum of 100% to be most energy efficient. |
| 81 | +The vehicle stability function could limit this distribution of 20% < x < 80% due to the current driving situation. |
| 82 | +As a result, only 80% axle distribution would be arbitrated, e.g. -600N force (braking) on rear axle and -2400N force (braking) on front axle. |
| 83 | + |
| 84 | + |
| 85 | +## Powertrain System (Electrical Axle) |
| 86 | + |
| 87 | +A vehicle may have an arbitrary number of electrical axles. Rotational speed and torque for an electrical axle refer to the axle coming out of the electrical axle unit. |
| 88 | +There may be a transmission/gearbox between the electrical axle and the actual wheels served by the electrical axle. |
| 89 | + |
| 90 | +There are two operation modes of an eAxle: torque control or speed control. |
| 91 | + |
| 92 | +### Torque Control |
| 93 | +A torque request (e.g. 50Nm) is realized by the eAxle. In addition, limit for the rotational speed can be defined, e.g. -5000 rpm ... +5000 rpm. These limits are useful to avoid too high wheelspin when the vehicle is on a slippery road. This mode is used most of the time, when the eAxle is active. |
| 94 | + |
| 95 | +### Speed Control |
| 96 | +A rotational speed request (e.g., 1000rpm) is realized by the eAxle. This mode is useful for low speed till standstill, because so the vehicle can be controlled easier to a standstill, especially on a rough road. |
| 97 | + |
| 98 | +## Steering System |
| 99 | + |
| 100 | +The defined steering signals are the main signals for interacting between vehicle motion features and the steering system on the front axle and are widely used in automotive industry. |
| 101 | +These signals are specified in a generic way to support various steering systems like electric power steering as well as Steer-by-Wire. Therefore, not all signals must necessarily be available/applicable. |
| 102 | +The signal specification supports requests on steering wheel (torque and angle) and the steering rack position which is linked to the steered wheels of the vehicle. |
| 103 | +The signal requests are further divided in "offset" and "target" signals, where an offset value is used additive to the functionality of the steering system and a "target" value is used as absolute external set-point for steering actuation. |
| 104 | +The different requests are controlled by dedicated "mode" signals for enabling and disabling the steering requests. |
| 105 | + |
| 106 | +Examples: Driving in a left curve and applying external requests to steering system. |
| 107 | +Based on the stationary driving without external request (config 1) the principle effects of the different steering interface signals are explained for each interface signal exlusively (config 2...6). |
| 108 | +Depending on the concrete use case a combination of parallel requested interface signals is applicable. |
| 109 | + |
| 110 | +Config | Current.SteeringWheelTorque (Torque Applied by Driver) | Current.SteeringWheelAngle | Current.RackPositionFrontAxle | Result/Comment |
| 111 | +-------------------------------------------------------------------------|--------------------------------------------------------|-----------------------------|--------------------------------|---- |
| 112 | +`1`: Regular turn - No external request | 3 Nm | 20 degree | 4 mm | "Steering support" (e.g. power steering) require driver to apply 3 Nm to continue turning with same radius. |
| 113 | +`2`. Lane Assist Intervention - `SteeringWheelTorqueOffsetTarget = 2 Nm` | 5 Nm* | 20 degree | 4 mm | Driver torque needs to be increased to continue turning with same radius |
| 114 | +`3`. Torque Target Changed - `SteeringWheelTorqueTarget = 2 Nm` | 2 Nm* | 20 degree | 4 mm | Required driver torque set to 2 Nm (e.g. external steering feel for steer by wire), offset ignored. |
| 115 | +`4`. Autonomuos Driving - `SteeringWheelAngleTarget = 0 deg` | ---** | 0 degree | 4 mm | Steering wheel set to 0 deg (e.g. automated driving with steer by wire and fixed steering wheel) |
| 116 | +`5`. Stability intervention - `RackPositionOffsetFrontAxleTarget = 3 mm` | 3 Nm | 20 degree | 7 mm | Rack position increased by 3mm (e.g. vehicle stability intervention with steer by wire) |
| 117 | +`6`. Autonomuous Driving - `RackPositionFrontAxleTarget = 3 mm` | ---** | 0 degree | 3 mm | Rack position set to 3 mm (e.g. automated driving), offset ignored |
| 118 | + |
| 119 | +`*` Assumption is that driver is holding the steering wheel at the same position as without external request |
| 120 | + |
| 121 | +`**` Steering wheel torque depending on driver input. Target request may be ignored if driver is applying torque (driver override). |
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