Force Feedback Actuator Hybrid Solution
The INVENTUS Steer-by-Wire Force Feedback Actuator (SbW FFA) hybrid solution combines a small e-motor (e.g. max. 6 Nm) with a powerful yet compact MR-brake (25 to 35 Nm) and thus provides the world’s smallest SbW system combining extraordinary performance, excellent thermal management and cost advantages.
Thanks to our patented MR brake, we can significantly reduce the size of the active component (factor 3-7) and thus the overall system size.
Safety first is our guiding design principle. That’s why our hybrid system offers two options for safe operation in case of an emergency.
Our hybrid steer-by-wire solution generates strong force feedback (active & braking force) for intuitive and safe steering maneuvers.
The use of a significantly smaller active component and the optimization of the entire system result in cost savings in the double-digit euro range.
The INVENTUS SbW system features particularly efficient heat management, ensuring the entire system to remain cool at all times.
There is no ideal solution – not yet
Current SbW technologies are based on Column Electric Power Steering, Direct Drive, and Belt Drive systems at a number of positions at the steering column or steering wheel.
While driving in SbW mode, the force feedback experienced by the driver consists of about 80% braking torque (up to 35 Nm) and active feedback of 6-8 Nm (about 20%).
While all systems mentioned above can generate a high degree of active feedback, it is the high degree of braking performance for which state-of-the-art SbW systems are not optimized. This results in target conflicts regarding:
- the haptic/force feedback (sufficient basic torque with absence of backlash, ripples, and cogging torque…)
- the required installation space
- the warm-up time and
- the overall system weight
To address this challenge, we combine our years of experience in MR technologies and our high-performance braking systems with the advantages of active steering systems.
Our solution: The world’s smallest SbW system with extraordinary performance and efficiency for increased cost advantages.
INVENTUS steer-by-wire solution
INVENTUS MR brake technology
- We make use of our patented MR brake technology with a basic torque of <0.1 Nm.
- Due to the low base torque, the sensitivity of the steering system can be tailored to any customers requirements.
- The MR brake torque changes within milliseconds with an extremely very low power consumption of 4.2 amps / 60 watts (at 12 volts) to generate 25 Nm in <10 ms.
- A torque sensor is not mandatory.
INVENTUS hybrid steer-by-wire system
- Our MR brakes are extremely efficient in generating the required passive torques.
- This way, we can reduce the size of the electric motor for active torque generation by a factor of 3 to 7.
- Combining the advantages of both technologies we provide a smaller, stronger and more efficient SbW solution.
INVENTUS SbW FFA Hybrid Solution
INVENTUS Steer-by-Wire steering column Hybrid-Direct-Drive (HDD) setup
Passive torque generation (MR brake) of 25 Nm
Active torque generation (electric motor) of 5 Nm
Also applicable for Hybrid Belt Drive (HBD) or Hybrid Worm Drive (HWD) solutions
Size comparison between electric motor solutions
and the INVENTUS MR brake technology
INVENTUS hybrid steer-by-wire system
Steering torque of up to 25 Nm
DDA steer-by-wire system using an electric motor
Steering torque of up to 25 Nm
Electric motor VS INVENTUS MR brake
Compared to several electric motors, the INVENTUS system’s symmetrical design on the steering column is another advantage.
INVENTUS MR brake VS Electric motor
INVENTUS MR brake operating principle
The basic design of our INVENTUS MR brake is quite simple.
How it works
- The brake consists of a rotor, a stator, an electric coil, as well as bearings and seals
- The gap between the rotor and the stator is filled with carbonyl iron powder (MRP – magnetorheological powder)
- As soon as a current is applied to the coil, a magnetic field is generated, which closes from the rotor via the active gap to the stator
- The magnetic field in the effective gap/between the surfaces moving towards each other, causes the carbonyl iron particles to build „magnetic particle bridges“, generating shear stresses
- The forces resulting from the shear stresses, multiplied by the distance, produce a torque (braking torque)
- The electric coil can be attached to either the rotor or the stator. The shaft or the housing can rotate
INVENTUS SbW hybrid system
force feedback performance
State of the art solutions
In normal driving operation, >80% of the steering movements are passive torques as most maneuvers are quasi-static or very slow twists. State of the art solutions generate torque with a redundant electric motor, usually with transmission (gear, belt…). However, at low speeds and high torque requirements, electric motors have an extremely high electrical power loss, making them highly inefficient.
Force feedback requirements for intuitive and reliable steering operations
- Steering wheel torque in normal driving mode = 1 to 3 Nm
- Maximum torque = 25 to 35 Nm (e.g., end stop, egress or holding on to steering wheel)
- Maximum reset torque = 5 to 8 Nm
- Maximum angular velocity = approx. 1400 °/sec (evasive maneuvers)
- Up to 35 Nm braking torque / passive torque
(braking steering movement/torque generated by the driver)
- Up to 8 Nm active torque generation
(active rotation of the steering wheel, e.g., after a U-turn)
INVENTUS SbW FFA hybrid solution active & passive steering torque
Examples of adaptive braking curves
The INVENTUS SbW hybrid system can change the braking behavior and thus the perceptible steering behavior in real time.
Also, watch the videos below to experience the adaptive force feedback of our system.
Active steering torque
Increasing steering torque from <0,1 Nm to 8 Nm
The test setup shows an INVENTUS SbW hybrid system in freewheeling mode (< 0.1 Nm) followed by a continuous increase of the steering torque up to 8 Nm, the maximum active torque for comfortable and controlled steering maneuvers.
Max. braking torque
Barrier torque of 25 Nm (up to 35 Nm)
By combining a small electric motor and our patented MR braking technology, we achieve a maximum braking torque of up to 25-35 Nm, as demonstrated by the test setup with a distinct end stop.
Fail safe – functional safety to ISO 26262
Functional safety is of high importance for all steer-by-wire systems.In order to ensure a proper and safe operation of the INVENTUS SbW FFA hybrid system even in case of an incident, two fail safe options are available:
Option 1 – Battery solution
- We use an additional small battery to generate a failsafe driving torque of 1 to 3 Nm (<1 Watt).
- Unlike conventional SbW solutions based on an electric motor, the INVENTUS system does not require any sensors to detect the direction of rotation.
- This way the INVENTUS SbW system can be operated with just a small battery, accumulator or capacitor over a longer time.
Option 2 – Permanent magnet solution
- We supplement the SbW MR brake with a permanent magnet in the magnetic circuit, generating a magnetic field without any current applied – e.g. 2 Nm failsafe driving torque.
- While normal driving, this permanent torque can be canceled by counter current (-) or amplified by the same current direction (+).
- Our customers confirm that our MR brakes featuring a permanent magnet provide a major safety advantage over other solutions on the market, greatly simplify the safety architecture (in accordance with ISO 26262 and ASIL A to D), and at the same time save costs.
INVENTUS SbW permanent magnet fail safe demonstration setup
Battery pack with a 9 V battery to demonstrate emergency situations with a permanent magnet fail-safe setup
In normal driving mode (+ button pressed, increased steering torque), the system generates natural steering feedback.
In case of an emergency, the fail-safe drive torque of 1-3 Nm ensures controlled steering maneuvers even without any power supply (no 2nd power circuit required).
Cost savings potential
Due to the simple design of our braking technology, various cost-saving potentials can be utilized
- The INVENTUS MR brake requires significantly less raw materials such as copper, iron and other materials compared to an electric motor with the same torque, which is directly reflected in the manufacturing costs
- The INVENTUS MR brake requires no rare earths
- Also a torque sensor is not essential
- For this reason, we predict the manufacturing costs of the INVENTUS MR brake with 25 Nm braking torque to be in the low double-digit euro range for medium quantities.
- For large quantities, the costs can be in the single-digit euro range.
By combining a MR brake and an electric motor in our INVENTUS steer-by-wire force feedback actuator the size of the motor can be significantly reduced
The electric motor must generate a 3 to 7 times lower active torque (25 Nm braking – 8 Nm active torque; 35 Nm braking – 5 Nm active torque)
The significantly smaller motor is not only cheaper, but also all related components, including cable cross-sections, connectors, ECU components (MOSFETS, shunts, H-bridges, power filtering, etc.) can be optimized for this efficient SbW solution.
Similarly, the redundant/double winding of the electric motor including double contacts, connectors, cables, etc. can be omitted.
Additionally, the saved installation space is another major advantage, as the INVENTUS Steer-by-wire is substantially smaller than state of the art solutions.
INVENTUS technology is reliable and proven in serial production
In cooperation with automotive OEMs and suppliers, we have already developed SbW FFA setups and successfully evaluated them on test tracks.
Moreover, we developed a magnetorheological steer-by-wire feedback brake for the Institute for Driving Assistance and Connected Mobility (IFM) Kempten.
Our patented MR technology is used by XeelTech, our joint venture with the automation and mass production specialist STIWA Group, as a serial product in industries such as gaming, computer peripherals, audio equipment, white goods, or industrial machinery.
Key advantages of the INVENTUS MR brake
compared to an electric motor:
- Significantly higher efficiency and power density
- Same performance
- Better steering feedback (force feedback) thanks to lower base torque and absence of motor ripple
- Smaller dimensions / installation space of overall system
- Weight reduction
- Symmetrical system design
- Low power consumption / increased energy efficiency (resulting in potentially higher range for electric vehicles)
- Simple & reliable failsafe setup
- Inhomogeneous redundancy (or hybrid redundancy)
- Cost savings in the double-digit euro rang
- Industrialization with STIWA, our strong industrialization partner, known for high-quality mass production in e.g. the automotive or gaming sector