43
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Figure 25 Lookup table example
In Figure 25 there is a graphical example of the operation that the logic on the ASB control does in order to convert the target pressure coming from the path planning request in a target angle for the servomotor. To understand how to manage the servomotor control is useful to understand before its working principle [28].
In servomotors there are usually three electric inputs:
• A ground.
• A positive input.
• A control input in which the PWM signal is sent.
The PWM of a servomotor is a signal with a frequency and a duty cycle that indicates to the internal servomotor board which is the position that it must assume. For the servomotor HS-1005SGT the PWM range is 800-2200 µs for a motion range of 139º. It means that if on the control input a signal that has voltage equal to 5V and the duration of 800 µs is sent, the servomotor will go on the initial position (angle equal to 0º). If instead the duration of the signal is 2200 µs, the motor will rotate up to a motor angle
Target Pressure
Target Angle
45 equal to 139º. To reach intermediate positions, a linear interpolation must be performed following the equation below.
𝜏 = 800 +2200 − 800
139 ∗ 𝛽
Where:
• 𝜏 is the pulse width corresponding to the target angle.
• 𝛽 is the target servomotor angle.
Thanks to this relation is now possible to directly link the pressure target sent by the path planning not just to a motor angle but to a PWM signal. This signal can be sent to the real motor in order to control it and so starting to link the results obtained by the model to the real system.
Open loop control
The control designed starting from the data obtained by the model and its lookup tables allows to link the PWM signal to the pressure inside each braking lines. Anyway, if in the system there isn’t any feedback about the control’s effectiveness, this is an open loop control. The schematization of the open loop controller is shown below.
Figure 26 Schematization of the open loop control
Target Pressure
ookup Table conversion
Target Angle
Angle to P M conversion
P M signal
ervomotor actuation Actual brake pressure
46 With this type of controller during the actuations is not possible to understand if the target is effectively satisfied by the system, so a closed loop controller has been designed.
Closed loop controller
The idea of this controller is to use the pressure sensor on the front brake line to detect which pressure is actually present inside the system and use it as feedback to calculate an error compared to the target pressure. The controller designed in this section is a PID controller.
A PID controller is a simple controller born at the begin of the last century [29] that takes into account the past, the present and the future in order to control the system. The classic formulation in the continuous time domain is the following.
𝑢(𝑡) = 𝐾𝑃𝑒(𝑡) + 𝐾𝐼∫ 𝑒(𝜏)𝑑𝜏
𝑡 0
+ 𝐾𝐷𝑒̇(𝑡)
Where:
• 𝑢(𝑡) is the controlled input of the system
• 𝑒(𝑡) is the error between an actual signal and its reference
• 𝐾𝑃𝑒(𝑡) is the proportional factor that depends on the constant 𝐾𝐼. It provides a gain that depends on the current error and so it must be properly chosen since if it is too high it will provide big oscillations and if it is too low it will make the controller less effecting.
• 𝐾𝐼∫ 𝑒(𝜏)𝑑𝜏0𝑡 is the integral factor. It provides a gain depending on the constant 𝐾𝑃 that increase the effort of the sum signal in previous instants. Higher are the past errors and higher will be weight of the integral in this formulation.
• 𝐾𝐷𝑒̇(𝑡) is the derivative factor. It depends on 𝐾𝐷 and it is related to the slope of the error.
The controller developed for the ASB system is schematized in the following figure.
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Figure 27 Closed loop controlled system
PID controller development
In order to satisfy the pressure requested by the path planning, a low-level closed loop controller has been developed for the ASB system. The controller designed is a PID and to define it the constants 𝐾𝑃, 𝐾𝐼 and 𝐾𝐷 must be found. The design of this controller has been performed on the simulated system developed on 𝑆𝑖𝑚𝑠𝑐𝑎𝑝𝑒𝑇𝑀 𝑀𝑢𝑙𝑡𝑖𝑏𝑜𝑑𝑦𝑇𝑀 with a trial-and-error approach. The idea is to use the digital twin of the actual system realized in the sections before coupling it with a simulated Servomotor with the same characteristics of the real one and a PID controller that reduces the simulated error. Once the PID parameters will be defined, they will be used and tested on the real system. The optimal PID parameters founded for this application are:
• 𝐾𝑃 = 0.01
• 𝐾𝐼 = 3
• 𝐾𝐷 = 0
Since the value of 𝐾𝐷 is zero, the obtained controller is a PI and not a PID. During tests both on the vehicle and on the model, choose a null 𝐾𝐷 used to lead to a more stable transient dynamic with less oscillations. The results of the controlled system are shown in the following plots.
Target Pressure
ookup Table conversion
Angle to P M conversion
P M signal ervomotor actuation
Actualbrakepressure PID
Error
ref
48
Figure 28 brake oil pressure in the simulated model with a PI controller
Figure 29 pressure error in the simulated model with a PI controller
49 The plots above represent a simple test in which a target pressure is demanded, and the servomotor actuates the pedal to satisfy the request. The PI controller shows a good behaviour reaching the value in less than 2 seconds and without any oscillations around the steady state value.
50