Air suspension control system

Field of the invention

The present invention relates to a system and a method according to the preambles of the independent claims, and relates generally to the field of active suspension systems used in vehicles. More particularly, the invention relates to an air suspension control system, and method, in a vehicle having an active cab air suspension system for achieving a control of the air suspension system that improves the driver's comfort, e.g. for trucks running on bad road conditions.

Background of the invention

In a vehicle, e.g. a truck or a work vehicle, that does not include a cab suspension system, the ride quality and operator comfort of the vehicle is adversely affected by vibrations or movement transmitted from the frame or chassis of the vehicle to the operator's cab. As the vehicle travels across a surface, movement of the chassis induces the operator's cab to pitch, roll and bounce. Movement of the cab can be particularly severe in agricultural and construction equipment vehicles (e.g., tractors, combines, backhoes, cranes, dozers, trenchers, skid-steer loaders, etc.) because such vehicles typically operate on off-road surfaces or fields having a high level of bumpiness.

Operator comfort may also be adversely affected by the operation of various systems on a work vehicle. In particular, operation of various work vehicle systems can cause forces to be applied to the chassis of the vehicle which, in turn, are transmitted to the cab. Examples of these forces include the following: draft forces exerted on the hitch of an agricultural tractor by an implement (e.g., a plow) which can cause the cab to pitch; normal forces applied to a vehicle as the vehicle turns in response to a steering device which can cause the cab to roll; clutch forces generated when a work vehicle clutch (e.g., a main drive clutch; four- wheel drive clutch) is engaged or disengaged which can cause the cab to pitch; gear shift forces generated when a transmission of a work vehicle is shifted which can cause the cab to pitch; braking forces generated as brakes of a work vehicle are operated which can cause the cab to pitch; acceleration forces generated when a speed actuator changes the speed of a vehicle which can cause the cab to pitch; etc. To improve ride quality and operator comfort, vehicles have been equipped with passive, semi-active or active suspension systems to isolate the operator from vibrations caused by surface bumps. Such systems include vibration isolators mounted between the chassis and cab or seat. Passive systems use passive vibration isolators (e.g., rubber isolators, springs with friction or viscous dampers) to damp vibrations with different isolators used to damp different frequencies.

Active systems use sensors to sense cab movement and a controller to generate control signals for an actuator which applies a force to the cab to cancel vibrations transmitted to the cab by the chassis. The power needed to apply the force is supplied by an external source (e.g., hydraulic pump).

In US-2001/0044685 is disclosed an active suspension system used in work vehicles. The apparatus, which is in a work vehicle that includes a chassis, an operator's cab and an active cab suspension system, includes a sensor that is configured to sense a quantity representative of the vibration experienced by the component of the work vehicle and to develop a first signal indicative of that quantity. The cab suspension system is then controlled, via a communication bus that works under the SAE J-1939 protocol standard, in dependence of the determined quantity. The baud rate of SAE J-1939 is 500 kbit/s (version J- 1939/14).

US-6029764 and US-2007/0045067 relate to similar active suspension control systems. Although the present active cab suspension control systems have improved the cab suspension and the driver comfort, these systems still have drawbacks, e.g. with regard to being slow to react on sudden events, and therefore do not meet the driver's comfort demands.

The object of the present invention is to achieve an improved cab air suspension control, which is fast and precise and thereby meets the requirements set by the drivers.

Summary of the invention

The above-mentioned object is achieved by the present invention according to the independent claims.

Preferred embodiments are set forth in the dependent claims.

The present invention is based upon the inventor's insight that the use of a high- speed communication protocol (e.g. FlexRay™) is a presumption to gain full advantage of the control capability of a PID-controller for controlling the air suspension of a cab. More specifically, due to the high-speed communication the real-time control of the air pressures in the air bellows of the air suspension modules using acceleration measurements of the cab movements is made possible.

Thereby is a cab air suspension system is achieved that greatly improves the truck driver ^' s comfort.

A high-speed communication protocol is necessary to exchange information among the air suspension control units and the control module, and the PID controller is required to have a complete control of all parameters (response characteristics) such as rise time, overshoot, settling time and cancelation of the steady-state error. Short description of the appended drawings

Figure 1 is a schematic drawing illustrating a vehicle provided with an air suspension control system according to the present invention. Figure 2 is a block diagram schematically illustrating an air suspension control system according to the present invention.

Figure 3 is a flow diagram illustrating the method according to the present invention.

Detailed description of preferred embodiments of the invention

The improved air suspension control system will now be described with

references to figures 1 and 2. Throughout the figures the same or equivalent items have the same reference signs.

An air suspension control system 2 is provided to control a predetermined number of air suspension modules 4, e.g. four air suspension modules ( |, 4 ₂ , 4 ₃ , 4 ₄ ), configured to suspend a cab 6 of a vehicle 8, a truck, a working vehicle, or any vehicle discussed in the background section.

The air suspension module 4 is of a conventional type and will therefore not be described in detail herein. It comprises an air bellow, an air valve for injecting or withdrawing air from the air bellow, a pressure sensor to measure the pressure within the air bellow. In one embodiment two air suspension modules are arranged in the front of the cab and two modules in the rear.

The control system 2 comprises a predetermined number of air suspension control units 10, each configured to control one air suspension module 4.

Preferably, each air suspension control unit 10 is mounted at an air suspension module. The air suspension control unit 10 is an electrical control unit configured to control the air valve of the air bellow via air valve control signal 1 1 (1 11 , 1 1 ₂ , 1 13, 1 14 in the embodiment illustrated in figure 2). In addition it is configured to receive a pressure signal 13 (13i, 13 ₂ , 13 ₃ , 13 ₄ in the embodiment illustrated in figure 2) being the pressure measured inside the air bellow (see figure 2).

Furthermore, a control module 12 is provided, configured to apply an air suspension control algorithm, defining desired performance of the air suspension. The control algorithm uses a mathematical representation determined for the behavior of each air suspension module, and a compound mathematical representation is determined for the whole system, i.e. all air suspension modules used to suspend the cab. The mathematical representation includes parameters such as damping coefficients, stiffness, and time constants.

More specifically, and in addition, the control algorithm includes a set of rules that translates a measure to an activity, e.g. one rule could be: the truck turns left, the cab then probably will tilt to the right - one possible control of the air suspension would then be to increase the air pressure in the right air suspensions, and possibly decrease the air pressure in the left air suspensions.

The air suspension control system 2 further comprises an acceleration sensor module 14 arranged at the cab 6 and configured to measure the acceleration of the cab 6 in a number of predefined directions.

Advantageously, the acceleration sensor module 14 comprises one or many separate accelerometers, e.g. piezoelectric accelerometers. Preferably, the predefined directions being the X and Y directions of the vehicle, i.e. along the longitudinal direction of the vehicle, and perpendicularly to that direction, in an essentially horizontal plane. The accelerometers are preferably placed in the cab in different places in order to sense roll and pitch angles.

The acceleration sensor is configured to generate an acceleration signal 16 that includes real-time acceleration values measured by the acceleration sensor module 14, and to apply the acceleration signal 16 to the control module 12.

The control module 12 is configured to receive the acceleration signal 16 and to calculate, based upon at least said real-time acceleration values, a set of movement measures being a representation of the movements of the cab 6. The calculated movement measures represent the measured real-time state of the cab and that these measures include e.g. the acceleration, direction and magnitude of a movement. The control system 2 further comprises a communication bus 18 connected to the air suspension control units 10 and to the control module 12. The communication bus 18 is configured to perform the communication between the air suspension control units 10 and the control module 12 using a real-time high-speed communication protocol.

The high-speed communication protocol works with the speed of approximately 10 Mb/s or higher

According to one embodiment the high-speed communication protocol is

FlexRay™, which will be discussed in detail below.

The air suspension control algorithm is applied, by the control module 12 using the set of movement measures, in a PID controller 20 to control the air

suspension modules 4, in order to achieve the desired performance of the air suspension.

More specifically, the PID controller 20 is configured to control the air pressures in air bellows of the air suspension modules 4, by determining specific control parameters to be applied by the respective air suspension control units to the air suspension modules, i.e. by applying air valve control signals 1 1 to the air suspension modules. When determining the control parameters, other already available signals in the truck may also be used, such as truck speed, tire pressure, truck weigh, etc.

The tuning of a PID controller must be made taking the mathematical

representation and all different parameters into account. A general discussion of tuning of a PID controller is presented at the end of the description, which is applicable for this specific application.

With reference to the flow diagram illustrated in figure 3, a method in an air suspension system will be described. When describing the method it is generally referred to the above description of the system where the system is described more in detail.

A method is provided to be applied in an air suspension control system 2 to control a predetermined number of air suspension modules 4 configured to suspend a cab 6 of a vehicle 8. The control system 2 comprises a predetermined number of air suspension control units 10, each configured to control one air suspension module 4, and a control module 12, configured to apply an air suspension control algorithm, defining desired performance of the air suspension.

The method comprises the steps of:

A - measuring, by an acceleration sensor module 14 arranged at the cab 6, the acceleration of the cab 6 in a number of predefined directions. B - generating an acceleration signal 16 that includes real-time acceleration values measured by the acceleration sensor module 14.

C - applying the acceleration signal 16 to the control module (12). D - calculating, in the control module 12, based upon at least said real-time acceleration values, a set of movement measures being a representation of the movements of the cab 6.

Advantageously, the movement measures represent the measured real-time state of the cab and that these measures include the acceleration, direction and magnitude of a movement.

The method further comprises the steps of:

E - applying the air suspension control algorithm, using the set of movement measures, in a PID controller 20.

F - communicating control parameters to the air suspension control units via a communication bus using a real-time high-speed communication protocol. G - controlling the air suspension modules 4, in order to achieve said desired performance of the air suspension. Preferably, step G comprises controlling, by said PID controller 20, the air pressures in air bellows of the air suspension modules 4.

The high-speed communication protocol works with the speed of approximately 10 Mb/s or higher. Preferably, the protocol is FlexRay™.

According to another aspect of the invention a computer program is provided, including a program code P to cause a control module 14, or a computer connected to the control module, to perform the method steps described above. Tuning of a PID controller

A proportional-integral-derivative controller (PID controller) is a generic control loop feedback mechanism (controller) widely used in industrial control systems. A PID controller calculates an "error" value as the difference between a measured process variable and a desired setpoint. The controller attempts to minimize the error by adjusting the process control inputs.

The PID controller algorithm involves three separate constant parameters, and is accordingly sometimes called three-term control: the proportional, the integral and derivative values, denoted P, I, and D. Simply put, these values can be interpreted in terms of time: P depends on the present error, / on the accumulation of past errors, and D is a prediction of future errors, based on current rate of change. The weighted sum of these three actions is used to adjust the process via a control element such as the position of a control valve, a damper, or the power supplied to a heating element.

In the absence of knowledge of the underlying process, a PID controller has historically been considered to be the best controller. By tuning the three parameters in the PID controller algorithm, the controller can provide control action designed for specific process requirements. The response of the controller can be described in terms of the responsiveness of the controller to an error, the degree to which the controller overshoots the setpoint, and the degree of system oscillation. Note that the use of the PID algorithm for control does not guarantee optimal control of the system or system stability.

Some applications may require using only one or two actions to provide the appropriate system control. This is achieved by setting the other parameters to zero. A PID controller will be called a PI, PD, P or I controller in the absence of the respective control actions. PI controllers are fairly common, since derivative action is sensitive to measurement noise, whereas the absence of an integral term may prevent the system from reaching its target value due to the control action.

In the journal article "Experimental Investigation on Road Vehicle Active

Suspension" (Strojniski vestnik - Journal of Mechanical Engineering 59(2013)10, pp. 620-625, accepted for publication on 2013-06-20) is presented an

investigation report for an electronically controlled pneumatic suspension system. The performance improvement in the passenger's comfort and attitude behaviour are evaluated for a proportional integral derivative (PID) controlled pneumatic suspension design. In the article is discussed the design and implementation of a PID controller using the Zeigler-Nichols and refined Zeigler-Nichols (RZN) tuning methods while designing the PID controller. In particular the proportional gain Kp, integral gain Ki and derivative gain Kd are the parameters that influence the controller design.

According to one embodiment of the present invention the PID controller 20 is tuned by applying the Zeigler-Nichols or the refined Zeigler-Nichols (RZN) tuning methods. The FlexRay communication protocol

According to one implementation of the air suspension control system the highspeed communication protocol is FlexRay™, which will be described in the following.

FlexRay™ is an automotive network communications protocol developed by the FlexRay™ Consortium to govern on-board automotive computing. It is designed to be faster and more reliable than CAN (controller area network). The FlexRay™ standard is now a set of ISO standards ISO-1 - 5.

A FlexRay™ system consists of a bus and processors (Electronic control units, or ECUs). Each ECU has an independent clock. The clock drift must be not more than 0.15% from the reference clock, so the difference between the slowest and the fastest clock in the system is no greater than 0.3%.

As cars get smarter and electronics find their way into more and more automotive applications, existing automotive serial standards such as CAN and LIN do not have the speed, reliability, or redundancy required for X-by-wire applications such as brake-by-wire or steer by-wire. FlexRay™ fills the voids with a faster, fault tolerant, and time-triggered architecture that ensures dependable delivery of messages for safety applications. FlexRay™ is a differential bus running over either a STP or an UTP at speeds up to 10 Mb/s, which is significantly faster than LIN's 20 kb/s or CAN'S 1 Mb/s rates. FlexRay™ uses a dual-channel architecture that has two major benefits. First, the two channels can be configured to provide redundant communication in safety-critical applications to ensure the message gets through. Second, the two channels can be configured to send unique information on each at 10 Mb/s, giving an overall bus transfer rate of 20 Mb/s in less safety-critical applications. FlexRay™ uses a time-triggered protocol that incorporates the advantages of prior synchronous and asynchronous protocols via communication cycles that include both static and dynamic frames. Static frames are time slots of predetermined length allocated for each device on the bus to communicate during each cycle. Each device on the bus is also given a chance to communicate during each cycle via a dynamic frame which can vary in length

(and time). The FlexRay™ frame is made up of three major segments: the header segment, the payload segment, and the trailer segment. The present invention is not limited to the above-described preferred

embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.