So you’re designing a steer-by-wire system, you’ve watched our introduction video on the LORD Tactile Feedback Device (TFD), and you’re interested in evaluating a sample part for your system. BUT, how do you decide which of our TFD Steering Units is best for your system?
Recall that a TFD performs two main functions in a steer-by-wire system:
First, it provides continuously variable steering resistance to mimic the feel of traditional steering systems. This provides the operator a greater sense of connectedness with the vehicle during operation – improving control, productivity and safety.
Second, it is a steering sensor used to communicate steering position to the vehicle as an operator turns the steering wheel. All LORD TFDs include at least two sensors to provide redundancy and diagnostics of the steering signal, improving the overall safety and reliability of the steer-by-wire system. The sensor signals are continuously compared against each other to ensure an accurate steering position. If the signals ever vary, then fault codes are generated requiring the vehicle to safely come to a stop.
Selecting the Correct Torque Capacity
A portfolio of TFD Steering Units with various steering torque capacity including 5Nm, 12Nm and 20Nm torque configurations is available. During normal steering operation of the vehicle, algorithms in the vehicle’s central controller (or optionally integrated within the LORD TFD) modulate torque feedback to the operator as a function of steering wheel rotational position, velocity, acceleration, steered wheel position, vehicle speed, or other parameters. For most steering events, only a small amount of electrical current (or possibly no current) is needed to generate a smooth steering feel for the operator. Maximum steering torque is typically demanded only at the end-stops of steering travel (i.e. full turn left or full turn right). End-stops are generated by applying maximum current to the TFD (typically ~1A) which changes the magnetically responsive material inside the device from a free state to a semi-solid. The amount of torque required to create a firm end-stop and a high-quality steering system is subjective, but it's generally a function of how large the steering wheel is on the vehicle.
For applications with smaller-sized steering wheels (i.e. 4- to 8-inch diameter) like many forklift vehicles, our 5Nm products are generally well accepted.
For medium-sized steering wheels (i.e. 10- to 14-inch diameter) used in many industrial vehicles, our 12Nm products are typically a better fit.
For larger-sized steering wheels (i.e. >14” diameter) like those found in marine applications, our 20Nm products are typically the preferred solution.
Since an operator’s perception of the steering “feel” can be quite subjective, it may be necessary to evaluate such differences during the development phase to determine the most appropriate torque capacity required for your specific application.
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.
For an in-depth look at the characteristics of each signal type, and to understand which vehicle controllers each is compatible with, look for Part 2 in this series coming soon.