Introduction
Application programming interface
C/C++ API
Other APIs
Examples
Feedback control
System identification
Detailed hardware description
Circuit design
Parts
PCB
About
Authors
The BOBShield belongs to the family of control engineering education devices for Arduino that form a part of the AutomationShield project and presents a low-cost ball-on-beam (BOB) control experiment. The device uses a micro servo motor to adjust the position of a steel ball inside a tilting plastic transparent tube, measured by a time-of-flight distance sensor. The goal is to control the inclination of the tube such that the ball tracks a desired reference trajectory, which creates a simple single-input single-output feedback loop. This underactuated, fast, nonlinear system is an excellent tool to test and teach concepts of control theory, modeling, identification, microcontroller technology and signal processing. The hardware is exceptionally low-cost and small, making it ideal for take-home experiments or long-term student projects.
The basic application programming interface (API) serving the device is written in C/C++ and is integrated into the open-source AutomationShield Arduino library. This library contains hardware drivers and sample exercises for control systems engineering education. All functionality associated with the BOBShield is included in the BOBShield.h header, which contains the BOBClass class that is constructed by default as the BOBShield object. The functions specific to this shield mostly perform input/output peripheral communication.
The summary of basic functions and the illustration below should get you started quickly:
- Output (sensor):
BOBShield.sensorRead(); - Input (actuator):
BOBShield.actuatorWrite();
The following subsections describe the methods used to access the input and output of the BOBShield. Note that before you begin an experiment you must initialize the hardware by calling
BOBShield.begin();
which starts the servo motor functionality by assigning its hardware pin for later use.
The sensor readings can be calibrated by using the
BOBShield.calibrate();
method, which tilts the beam towards ±30° and selects the maximal, respectively, the minimal distance readings. These calibrated readings are accessible to other methods.
Using
y = BOBShield.sensorRead();
will output the absolute position of the ball y (mm) as a floating-point number by polling the ToF sensor. Similarly, the percentual position based on calibration may be accessed by the sensorReadPerc() method.
The manual reference from the potentiometer is read by the
r = BOBShield.referenceRead();
method, returning the reference r as a floating-point number.
Finally, the desired angle u (°) is sent to the servo motor by
BOBShield.actuatorWrite(u);
where the input parameter is a floating-point number.
Expansion of the current API to MATLAB, Simulink and Python is currently in progress.
Currently, there are four examples offered for the BOBShield Arduino API.
The file BOBShield_SelfTest.ino implements a basic self-test routine, aiding the verification of the hardware functionality. First, the ToF sensor is initialized and, if the MCU cannot connect to the sensor chip, will report the failure. Then, the servo turns to each of its extreme operation points and the routine compares the distance readings with expected values.
The project file BOBShield_Identification.ino performs an open-loop test by supplying a range of actuator settings, while BOBShield_Identification_aprbs.ino does the same but applies an amplitude-modulated pseudo-random signal.
Finally, BOBShield_PID.ino demonstrates the proportional-integral-derivative (PID) position control of the steel ball.
Besides the engaging visual action of the hardware, results are also printed to the serial line. The figure below demonstrates a step change of the reference, where the process variables r (blue), y (red) and u (green) may be followed in the oscilloscope-like rolling window of the Arduino IDE Serial Plotter. Similar to the open-loop case, any other terminal program is suitable for monitoring and data logging.
The PID demonstration example is sampled with a T=10 ms period, and a pre-set reference vector of [30 50 8 65 15 40]T mm is requested for 1000 samples (10 seconds) providing sufficient time for transients. The PID controller is manually tuned to the KP=0.3 (mm/°), TI=600 and TD=0.22 constants. An integral windup clipping strategy is set to ±10 (°), while the input to the servo motor is saturated at u=30 (°).
The results of this experiment, processed in MATLAB, are illustrated in the figure below, where reference position r is compared to achieved output y on the top and the corresponding servo input u is presented at the bottom.
The ball follows the reference position trajectory faithfully across a wide span of the working range. The tracking is worse when the ball is close to the sensor. We believe this is caused by sensor noise originating from unwanted reflections or other physical effects. Note that the response may be further improved by more advanced control and/or appropriate signal processing.
In progress.
The BOBShield is an open hardware product, you are free to make your own device. If you come up with improvements, please let us know so we can improve our design as well. The discussion below should help you to improvise a similar setup for experimentation on a breadboard or perforation board. You may even order a professionally made PCB by a PCB fabrication service.
The circuit schematics has been designed in the Freeware version of the DIPTrace CAD software. You may download the circuit schematics of the FloatShield from here.
The inclination of the tube and, ultimately, the position of the ball, is manipulated by a standard micro RC Servo motor M (i) with analog feedback. The total rotation range of the motor utilized in this application is only ±30°. The motor is connected to the D9 pin of the standard Arduino R3 layout, since this equivalent MCU pin handles interrupts and timing on the Uno as well as other prototyping devices. The current consumption of the servo motor is low enough to be directly powered from the 5V supply of the Arduino board without the need of an external wall-plug adapter. We added a capacitor C1 (k) parallel to the motor to smooth out possible transients affecting the board supply and a diode D1 (l) to prevent possible back-EMF damaging the circuitry.
The absolute distance of the ball from the reference located at one end of the tube is measured by the ST Microelectronics VL6180X ToF sensor J (m). We have decided to utilize a widely available breakout board implementing the VL6180X for hobbyist use. This breakout may be purchased as an end-product and implements the sensor on a small-outline PCB with standard 0.1'' pitch. The sensor communicates through the I2C protocol and we have utilized the standard A4/SDA and A5/SCL pins of the layout. The breakout connects to the base board via a flexible 4-wire ribbon cable (n). A flat flexible cable (FFC) would have been a better choice, unfortunately the sensor module would need to be custom-made for this.
Finally, there is a potentiometer POT1 (o) with a small plastic shaft (p) allowing to manually input the desired reference signal. The potentiometer is fed by the 3.3V source for its maximal setting, so that the wiper connected to the A0 analog port does not damage other microcontroller boards, such as the Due or Zero, operating with CMOS logic levels. To maximize analog resolution, we have also connected the 3.3V source to the AREF pin and compensate for this change in code. The sensor board is also powered by the 3.3V source.
To make a BoBShield you will need the following parts or their similar equivalents:
| Part | Name | Type/Value/Note | PCS |
|---|---|---|---|
| (a) | PCB | 2-layer, FR4, 1.6mm thick (e.g. JCLPCB) | 1 |
| (e1-5) | 3D print | 18g, Ø1.75mm PETG filament, bright green, at 240°C (90°C bed), 3h 20min | 1 |
| (o),POT1 | Trimmer | 10kΩ, 250mW, single turn THT trimmer (e.g. ACP CA14NV12,5-10KA2020) | 1 |
| (h) | Screw | DIN 7971C 2.2×6.5 | 1 |
| (l),D1 | Diode | Generic diode, DO214AC (e.g. Vishay Semiconductor BYG20J) | 1 |
| (g1) | Screws | M3×8 DIN963A | 4 |
| (g2) | Nuts | M3 DIN439B | 4 |
| (b3) | Header | 10×1, female, stackable, 0.1'' pitch (e.g. SparkFun 474-PRT-10007) | 1 |
| (p) | Pot shaft | For ``POT1'', (e.g. ACP CA9MA9005) | 1 |
| (b1) | Header | 8×1, female, stackable, 0.1'' pitch (e.g. SparkFun 474-PRT-10007) | 2 |
| (b2) | Header | 6×1, female, stackable, 0.1'' pitch (e.g. SparkFun 474-PRT-10007) | 1 |
| (c) | Tube | transparent, plexiglass (PMMA), L=100mm, 12/10mm | 1 |
| (m),J | Sensor | VL6180X breakout, time of flight ranging SNSR | 1 |
| (i),M | Motor | Metal geared 9g, 5V micro servo | 1 |
| (k),C1 | Capacitor | 100uF, 6.3V, 20, 0805, tantalum | 1 |
| (d) | Ball | Steel ball, diameter 8mm | 1 |
| (f) | Bearing | Miniature ball bearing, 3×6×2mm, (e.g. MR-63-ZZ) | 1 |
| (n) | Cable | Ribbon cable, 4-7 wires, cca. 0.2m (e.g. EL-2468) | 1 |
The total cost of the above components and thus of the entire BOBShield is no more than $10 excluding labor and postage.
The assembled BOBShield is shown below. The equivalents of the circuit components described above for the schematics are marked on the assembled device with the same letters.
One end of the tube is closed by a 3D printed cap (e1), while the other end is terminated by another 3D printed part (e2) designed to hold the distance measurement unit. The tube is fixed at its middle by a 3D printed circular clamp (e3) that permits the inclination of the tube. The clamp is held in place at both sides by L-shaped 3D printed brackets, one (e4) containing a miniature ball bearing (f) to facilitate smooth movement and the other (e5) fixed to the clamp through the shaft of the actuator. The figure below shows all of the 3D printed components (e1-5), for which we provide downloadable CAD files. They were printed on an original Prusa i3 MK3/S 3D printer using a total of 18g filament under 3.5h time.
A 3D CAD sketch of the entire assembly may also be downloaded from here.
The printed circuit board has been designed in the Freeware version of the DIPTrace CAD software. The PCB is two-layer and fits within the customary 100×100mm limit of most board manufacturers. The DIPTrace PCB layout and circuit schematics can be downloaded here and here, respectively, while the ready-to-manufacture Gerber files can be downloaded from here.
This shield was designed and created as a term project at the Institute of Automation, Measurement and Applied Informatics. The Institute belongs to the Faculty of Mechanical Engineering (FME), Slovak University of Technology in Bratislava in 2019/2020.
- Hardware and 3D model design: Tibor Konkoly, Patrik Šíma, Marko Michal, Marek Krippel, Erik Mikuláš, Gergely Takács
- Software design: Lukáš Vadovič, Matúš Bíro, Anna Vargová, Gergely Takács
- Wiki documentation: Martin Gulan, Erik Mikuláš and Gergely Takács