If you are developing a steer-by-wire system, you may need help selecting a LORD Tactile Feedback Device (TFD). While the vehicle central controller is responsible for managing the actual turning of the vehicle, a TFD works at the front end of the steering system to perform two functions. One function is to mimic the traditional steering feel of a mechanical or hydraulic system, with the goal of ensuring the operator’s comfort, control and safety. The second function is to act as a steering sensor, accepting input from the steering wheel and communicating that information to the vehicle's central controller.
See Part 1 of this series for more details on how to select the appropriate torque capacity of a LORD TFD for your application.
Selecting the Correct Sensor Output
Integrated into each LORD TFD are multiple non-contact rotational sensors which translate operator movement of the steering wheel into electrical signals for controlling vehicle direction. These steering signals can be sent to the vehicle’s central controller as either an analog or pulse width modulated (PWM) output or can be communicated across the vehicle’s controller area network (CAN). Selection of an analog versus PWM versus CAN signal is typically determined by the capabilities of the vehicle’s central controller in the steer-by-wire system.
Analog signals are the most basic sensor output, and are suitable for vehicle controllers with limited digital I/O. In an analog system, steering motion is converted into corresponding voltage changes which can be measured over time. Analog signals are smooth and continuous and have a large array of potential values within the measured range.
LORD offers two different Analog signal options – the first called a “ramp” signal, and the second called a “triangle” signal.
The ramp output increases in voltage as you rotate the steering wheel. Each sensor is 180-degrees out of phase with each other. If one sensor fails, you can still use the second sensor to detect the correct steering position. Care must be taken to manage the transition between maximum and minimum output states, since samples taken while the signal is changing between these two states may be mistaken for an erroneous steering position. However, there are ways to detect the correct steering position in the signal processing to ensure a reliable and safe steer-by-wire system.
The triangle output increases in voltage from 0-degrees to 180-degrees, then reduces in voltage from 180-degrees to 360-degrees. In this type of output, both signals are required to obtain a unique steering position, since each single voltage can represent one of two steering positions. However, by comparing the voltage measurement from both sensors, the correct steering position can be assured in the signal processing. This type of output avoids the rapidly changing transition between maximum and minimum output states that occurs with a ramp output configuration.
Pulse Width Modulated (PWM) Signals
PWM signals are digital signals, which have a finite number of possible values, typically two – either “high” (5V) or “low” (0V or ground). While the signal can only be high or low at any time, PWM signals consider the proportion of time the signal is high compared to when it is low over a consistent time interval. The amount of time that the signal is high compared to low over a specified time interval is called the duty cycle. In a TFD, the steering position can be correlated to a specific duty cycle. In general, PWM signals are less susceptible to noise, and are less affected by variations in sensor supply voltage.
Controller Area Network (CAN) Signals
Many vehicle architectures are migrating toward use of CAN – a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer. LORD CAN 2.0 enabled TFDs communicate steering position to the central vehicle controller but also have an internal amplifier and PCB to control the steering torque, so that the vehicle controller can adjust the steering feel by sending simple CAN messages. With CAN-enabled devices, steering engineers have access to a lot more vehicle information which can be used to create highly sophisticated steering controls, allowing the user to program and store customized steering profiles to give vehicles a unique steering identity, all while using the same hardware.
LORD offers both dual sensor (tolerate up to 1 sensor failure) or quad sensor variants (tolerate up to 2 sensor failures) to achieve the required functional safety requirements for the vehicle.
As you can see, there are many variants and details to weigh when considering which TFD Steering Unit is right for your project. Torque capacity, steering wheel size, and sensor output should all be included in the decision process.
Additional information is available and if you still have questions regarding which LORD TFD is the best one for your project, please contact us and we would be happy to help recommend a product.