Active Suspension Systems: A Leap in Railway Vehicle Comfort and Safety

Introduction

The railway industry has been developing new technologies to improve the comfort, safety, and performance of railway vehicles. One of these technologies is the active suspension system, which aims to improve the stability and ride quality of the vehicle. The active suspension system employs various actuators, such as hydraulic and electromechanical, to control the vertical, lateral, and longitudinal movements of the vehicle. In particular, the lateral secondary suspension of a railway vehicle is responsible for controlling the lateral forces acting on the vehicle. The lateral forces can cause instability and discomfort for passengers, especially during cornering or high-speed turns. Therefore, an active lateral secondary suspension system is crucial for ensuring passenger safety and comfort.

One of the key components of an active lateral secondary suspension system is the actuator, which generates the required forces to counteract the lateral forces. The tubular permanent magnet actuator (TPMA) is a type of electromagnetic actuator that has been widely used in the railway industry due to its high-power density, low weight, and compact size.

Topologies of a Tubular Permanent Magnet Actuator

The TPMA consists of a cylindrical stator and a mover, which is composed of permanent magnets and ferromagnetic materials. When a current is passed through the stator winding, a magnetic field is generated that interacts with the permanent magnets on the mover, producing a force that moves the mover. By controlling the current in the stator winding, the TPMA can generate the required force to counteract the lateral forces acting on the railway vehicle.

Here are some common configurations and topologies of TPMAs used in Active Lateral Secondary Suspension (ALSS):

                 

                   

                 

Fig. 2. Various Tubular Configurations: (a) Moving Magnet; (b) Moving Coil; (c) Short Mover; (d) Long Mover; (e) Slotless;  (f) Slotted [1]

(a) The Moving Magnet Topology is a type of Tubular Permanent Magnet Actuator (TPMA) configuration where the magnet moves along the length of the stator. In this topology, the stator is a cylindrical tube with multiple poles on its inner surface, and the magnet is a cylindrical rod that slides inside the stator. The magnet is usually mounted on the moving part of the system, such as the suspension arm of a railway vehicle, while the stator is fixed to the vehicle body.

(b) The Moving Coil Topology is another type of Tubular Permanent Magnet Actuator (TPMA) configuration that is commonly used in industrial and aerospace applications. In this topology, the stator is a cylindrical tube with multiple poles on its outer surface, and the coil is a cylindrical winding that moves along the length of the stator. The coil is usually mounted on the moving part of the system, such as the suspension arm of a vehicle or the control surface of an aircraft, while the stator is fixed to the vehicle body or airframe.

(c) The Short Mover Topology is a type of Tubular Permanent Magnet Actuator (TPMA) configuration that is used in linear motion applications where a short stroke and high force density are required. In this topology, the stator is a cylindrical tube with multiple poles on its inner or outer surface, and the mover is a short cylindrical rod or sleeve that moves along the length of the stator.

The short mover is typically made of permanent magnet material and has multiple poles on its outer or inner surface that align with the poles on the stator.

(d) The Long Mover Topology is a type of Tubular Permanent Magnet Actuator (TPMA) configuration that is used in linear motion applications where a long stroke length is required. In this topology, the stator is a cylindrical tube with multiple poles on its inner or outer surface, and the mover is a long cylindrical rod or sleeve that moves along the length of the stator.

The long mover is typically made of a soft magnetic material, such as iron or steel, and has multiple poles on its outer or inner surface that align with the poles on the stator. When a current is applied to the stator windings, it generates a magnetic field that interacts with the magnetic field of the long mover, producing a force that can be used for linear motion.

(e) The Slotless Topology is a type of Tubular Permanent Magnet Actuator (TPMA) configuration that is used in high-performance applications that require low cogging, high speed, and precise control. In this topology, the stator is a cylindrical tube without any slots or teeth on its inner or outer surface, and the mover is a cylindrical rod or sleeve that moves along the length of the stator.

The absence of slots or teeth in the stator eliminates cogging, which is the tendency of the actuator to stick or jump when the mover passes over a tooth or slot. This results in smoother operation and more precise control of the mover position and speed. In addition, the absence of slots or teeth reduces eddy current losses and increases the efficiency of the actuator.

(f) The Slotted Topology is a type of Tubular Permanent Magnet Actuator (TPMA) configuration that is commonly used in industrial and automotive applications. In this topology, the stator is a cylindrical tube with multiple slots or teeth on its inner or outer surface, and the mover is a cylindrical rod or sleeve that moves along the length of the stator.

The slots or teeth in the stator are used to concentrate the magnetic flux and increase the force density of the actuator. When a current is applied to the stator windings, it generates a magnetic field that interacts with the magnetic field of the mover, producing a force that can be used for linear motion.

The slotted topology uses a winding arrangement that matches the number and arrangement of the slots or teeth in the stator. This is typically achieved by using a concentrated winding that is placed in the slots or teeth, or by using a distributed winding that covers the entire length of the stator.

Design Challenges

The design of the TPMA is critical for achieving optimal performance and efficiency of the active lateral secondary suspension system. The following are some key design considerations:

Stator and Mover Dimensions: The dimensions of the stator and mover should be optimized to achieve the required force output and minimize the weight and size of the actuator.

Stator Winding: The stator winding should be designed to generate a magnetic field that interacts with the permanent magnets on the mover efficiently. The winding should also be designed to minimize power losses and thermal heating.

Permanent Magnet Selection: The selection of permanent magnets is critical for achieving high force output and efficiency. The permanent magnets should have a high remanence and coercivity, as well as low eddy current and hysteresis losses.

Ferromagnetic Material Selection: The ferromagnetic materials used in the mover should have a high permeability and low magnetic saturation to maximize the force output and minimize the weight and size of the actuator.

Cooling System: The TPMA generates heat during operation, which can affect the performance and lifespan of the actuator. Therefore, a cooling system should be designed to dissipate the heat efficiently.

Design Solutions for Tubular Permanent Magnet Actuator Design by EMWorks

EMWorks provides a suite of powerful simulation tools that can be used for the design and modeling of linear motors. These tools allow users to accurately simulate the performance of linear motors and optimize their design, leading to improved efficiency and performance. One of the key capabilities of EMWorks software is its ability to accurately model electromagnetic fields. This is essential for the design of linear motors, as these devices rely on magnetic fields to generate motion. With EMWorks software, users can accurately simulate the magnetic field distribution in the linear motor, allowing them to optimize the design and ensure optimal performance.

EMWorks software also includes advanced thermal modeling capabilities, which are critical for ensuring the reliable operation of linear motors. By accurately modeling the temperature distribution in the motor, users can identify potential hot spots and make design modifications to prevent overheating and premature failure. In addition to these core capabilities, EMWorks software also offers a range of advanced features for optimizing the design of linear motors. For example, users can simulate the effects of different materials and geometries, evaluate the performance of different winding configurations, and optimize the control strategy for the motor.

Fig. 3 shows a section view of the initially modeled TPMA.

Fig. 3. Section View of the Initial Modeled Linear Tubular Actuator

Design Specifications 

The cogging torque of a tubular permanent magnet actuator for active lateral secondary suspension of a railway vehicle is a result of the interaction between the stator and rotor of a permanent magnet actuator and can cause undesirable vibrations and noise in the system. The cogging torque depends on various design factors such as the number of poles, the number of slots, the shape and size of the permanent magnets, and the dimensions of the stator and rotor.

In this blog, we will investigate an initial Tubular Permanent Magnet Actuator characterized by 15 slots and 8 magnet poles. The main idea is to improve the back-EMF waveform and to decrease the cogging torque amplitude to enhance efficiency and get a more stable torque with fewer ripples. To minimize the cogging torque in a tubular permanent magnet actuator, the design should aim to reduce the variation in the magnetic reluctance between the permanent magnets and the stator teeth. This can be achieved by optimizing the pole and slot configuration, using high-quality permanent magnets, and carefully designing the stator and rotor to ensure good alignment.

The optimized topology, giving reduced cogging torque, is compared to the initial topology in Table 1:

Table 1. Initial Topology vs. Optimized Topology

Transient magnetic study of EMS for Solidworks coupled to motion analysis is used to simulate in no load condition, the initial geometry, and the optimized geometry of the tubular permanent magnet actuator. A comparison between the two geometries analysis results shows that the cogging torque was reduced by 60% compared to the initial configuration, considering the increasing of the parameters: Height of Coil Yoke, Height of Coil, Height of magnet, Height of Magnet Yoke, Slot opening (Figure 6).

This reduction in cogging torque can lead to improved system efficiency, reduced vibration and noise, and improved overall performance of the actuator for Active Lateral Secondary Suspension of a Railway Vehicle.

Fig. 4. Comparison Between Initial Topology and Optimized Topology

In a TPMA for Active Lateral Secondary Suspension of a Railway Vehicle in no-load conditions, the steel and air-gap flux play important roles in the actuator's performance. The steel flux is the magnetic flux that is present in the steel parts of the actuator. It is created by the magnetization of the permanent magnets in the actuator, and it flows through the steel components of the actuator. In no-load conditions, the steel flux will be present, but its strength will depend on the strength of the permanent magnets and the geometry of the actuator.

Below is the section view of magnetic flux density distribution in the optimized topology.

Fig. 5. Magnetic flux density distribution in the optimized topology

The air-gap flux, on the other hand, is the magnetic flux that is present in the air gap between the permanent magnets and the stator. This flux is essential for the actuator's operation because it generates the force that is required to move the stator and the vehicle. In no-load conditions, the air-gap flux will also be present, but its strength will depend on the geometry of the actuator and the strength of the permanent magnets. Therefore, in no-load conditions, the strength of the steel and air-gap flux in a Tubular Permanent Magnet Actuator for Active Lateral Secondary Suspension of a Railway Vehicle will depend on the specific design of the actuator, including the strength of the permanent magnets and the geometry of the actuator.

Fig. 6. Steel & Air Flux Density

Conclusion

In conclusion, the design of a TPMA is critical for achieving optimal performance and efficiency of the active lateral secondary suspension system of a railway vehicle. By considering the key design considerations, the TPMA can provide an effective solution for controlling the lateral forces and improving the ride quality and safety of railway vehicles.

In this blog, we saw that the Finite Element Method (FEM) software, EMS, can be very helpful in optimizing the design of tubular permanent magnet actuators, including the geometry, to reduce cogging torque and improve overall performance. EMS can simulate the magnetic and mechanical behavior of the actuator under different conditions and design parameters, allowing for a rapid evaluation of different design options and can lead to a more efficient and optimized design.

References

[1] Yoon, J.-H.; Kim, D.; Park, N.-C.; Park, Y.-P. Design of a Tubular Permanent Magnet Actuator for Active Lateral Secondary Suspension of a Railway Vehicle. Appl. Sci. 2017, 7, 152.