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POWER STEERING
Each year, the Formula Student Team named High-Octane Motorsports e.V. of the Friedrich-Alexander-University Erlangen-Nürnberg, designs, constructs, and builds innovative and unique racing cars. These are then entered in the Formula Student Competition in three different categories, competing teams from all over the world.
All three categories, Electric, Driverless, and Combustion, come with their challenges. This article explains how the students tackled power steering for the driverless vehicle. The concept describes all the necessary forces that are required in order to perform a turn on the steering column. To ensure the safety of all persons involved, and especially the drivers, strict rules generally apply to the autonomous steering system in Formula Student.
Since the autonomous racing car must also be steerable in manual mode, the vehicle has a manually operated steering wheel with a classic steering column. For transmitting the torque of the electric motor to the steering rod, it was decided to integrate a second steering rod, which is coupled to the manual steering rod by means of a bent metal plate. The BLDC motor is geared by a planetary gearbox, after which the output shaft of the planetary gearbox is transferred to our self-developed steering gearbox, in which a pinion goes to a rack.
Through various measuring techniques, the team could determine a relatively precise torque at the steering column and thus at the pinion of the manual steering gear, which is required for steering when the vehicle is stationary. Since the gear ratio in High-Octane Motorsports e.V.’s manual steering gear is the same as in the “autonomous steering gear”, the same torques are required at the output shaft of the compact drive as the required torques and rotational speeds determined by means of sensors. The required torque is 15 Nm at a standstill. However, this is a value that’s seldom found in the history of a race car, as it’s hardly ever necessary to maneuver at a standstill. Since the torque is reduced many times over when driving, this fact may well be taken into account in the design of the drive.
From the average steering speeds, the rotational speed for the servo motor can be determined. The average value (in absolute value) amounts to 403.2°/s, which is equivalent to 67.2rpm. For an autonomous race car, however, the average value can be set lower.
As mentioned above, a maximum torque of 15Nm is required at the output shaft at a speed of approx. 100rpm. According to the datasheet of the brushless DC motor, a nominal torque of Mnenn = 0.47Nm may be assumed, which results in an output torque of: Mab = 0,47Nm ∗ 32,72 = 15,4 Nm. Furthermore, it can be found in the datasheet that a nominal speed of nnenn=3500rpm is present at 24V. However, since only a maximum voltage of 16.5V is available with the battery used, a reduced speed of approximately nred~3500 * 16/24=2333rpm can be calculated. With nred the rotational speed at the output shaft of the planetary gear can then be calculated: nab=2333rpm / 32,72=71,3rpm. The calculation shows that both the maximum torque and a sufficient nominal rotational speed of the motor are achieved, resulting in a sufficient rotational speed at the output shaft. Also, the planetary gear with a specified maximum output torque of 42.7Nm, as well as a maximum output speed of 10913rpm, is not overloaded at any time.
On the part of the electronics, a functioning control and regulation for a stepper motor has already been developed. This season, however, a switch was to be made to a brushless DC motor (BLDC), which generally makes it possible to control the steering more quickly and accurately with a corresponding reduction ratio. In order to be able to evaluate the switch to a BLDC, a new, more powerful stepper motor and a BLDC were requested from Nanotec. In order to be able to test both engines, the team used the TMC4671-10A70V-EV-KIT.
Since this concerns the first BLDC control board, all conductor track thicknesses were designed for their maximum peak current. In addition, more measurement options of the motor currents (phase measurement and total current measurement) were included, so that as many measured values as possible are available for the validation of the overall system on the first board. The TMC4671-LA from the already mentioned evaluation kit was chosen as the basis.
The design of the board is separated into two parts. It consists of the section where the high motor currents (up to 30A) can flow in paths as short as possible. This reduces disturbance characteristics like a “ground bounce” or unwanted voltage collapse of the small-signal electronics. In order for the currents to gain the widest possible flow range, the board was developed on a 4-layer design with a copper thickness of 70μm.
The measurements of the board have become quite large due to the many current measurements and the prototype production and are to be reduced in future seasons. For this purpose, a new layout has already been considered. Also, in future seasons, the team’s plug problem of this year will be solved and all plugs will be integrated in a smaller format on the board again.
To make sure that the circuit board can be stored water and dustproof in the monocoque, High-Octane Motorsports e.V. designed a housing which is 3D-printed and then attached to the monocoque via a vibration-proof dual lock connection.
May 28, 2021 / Trinamic / 0
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