By: Ashlee Chang (E&C)

A Clean Electronic Setup

Breadboard testing

The electronics journey goes as so: Fritzing diagram, breadboard testing, circuit schematic, and then PCB (printed circuit board) fabrication. All electronic components are Arduino microcontroller, four batteries, eight servos, Bluetooth, ultrasonic sensor, and accelerometer. Notably, there are quite a bit of components, meaning a lot of wires needed to interface between them all. That is why the team decided that most of the wiring would be implemented on the PCB to avoid a messy project. Also, the microcontroller will be mounted directly on the PCB for this very purpose.

Servo Load Test

Servos in general draw quite a sum of current, and that current draw is proportional to the load (weight) the arm has to push against. A voltage regulator will be needed for the velociraptor, so it’s essential to see approximately how much current is drawn by these powerful servos so an appropriate regulator can be chosen. A test was conducted to see the servo load versus current draw.

Servo load and current test

A bucket containing miscellaneous materials to reach a particular weight was used and weighed on a scale. Afterwards, a string was used on the bucket and attached to the servo. A simple sweep code was installed on the servo to work against this load. An ammeter was inserted into the servo and power source loop.

Servo load and current results

The results were documented in the above table. The velociraptor currently weighs 544g (not taking into account the PCB, microcontroller, sensors, and Bluetooth). Thus, the project will weigh around a total 570g-600g. Taking the current draw of 0.3A at 506g from the table, this brings the current draw of eight servos at 2.4A. If the project is estimated to be 600g, the total current draw should be close to 3A, not taking into account current going to other parts of the circuit. Thus, the voltage regulator must be capable of handling over 3A.

Voltage Regulator

The voltage regulator is the only major component on the circuit schematic. Choosing a proper one was difficult. A decision between an LDO (low dropout voltage) voltage regulator versus switching regulator was contemplated. The switching regulator is actually highly efficient compared to the LDO voltage regulator. The lesser the inefficiencies, the lower amount of energy will be wasted as heat. Because the velociraptor shall be designed as a child’s toy, hot electrical pieces is a definite no-no that must be avoided for the health and safety of the user. With all switching regulators, there needs an inductor in the suggested schematic, which requires specific placement of the schematic in terms of distance on the PCB. Finding a switching regulator with an internal inductor proved to be impossible. In addition, there are not many switching regulators capable of stepping down small voltages. In the velociraptor’s case, the voltage had to be stepped down from 7.4V to 6.0V, which is only a change of 1.4V. Due to the complexity of switching regulators, the team decided on an LDO voltage regulator.

Some spec considerations that must be taken into account were: 5A maximum current output, input of 7.4V, output of 6V, capable of stepping down 1.4V, efficient, and through-hole (the smaller the package size, the hotter the component can get). The voltage regulator agreed on was MIC29512. Some additional features include current limiting protection, thermal shutdown, and an enable input pin that can be used to turn off the voltage regulator when not in use.

MIC29512 Voltage Regulator

Heat Sink

If the voltage regulator were to be a surface mounted piece, some copper embedding must be implemented. However, seeing the voltage regulator is a through-hole TO-220 package, a screw on heat sink can be used. For calculations based on the datasheet, the math is as follows: Rtsa = [125 – 50] C / 3W – 2 C/W – 1 C/W = 22 C/W. Any heat sink under 20 C/W would be ideal, but one with a specification of 9.6 C/W was chosen because, after all, this is a child’s toy.

Heat sink choice

Eagle Circuit Schematic

Velociraptor circuit schematic

The final circuit schematic is shown above. Both two batteries will be in series, and those two are in parallel. The resistors in between are used to help with any possible offset in between the two battery pairs. Transistors were placed to use the enable pin. A large capacitor needs to be next to the batteries. A TO-220 package was made for the schematic, and the recommended schematic was constructed based on the MIC29512 datasheet. The respective resistors used were calculated to output 6V on the voltage regulator. The batteries were connected to both the microcontroller (which can take inputs up to 12V) and the voltage regulator. The regulator feeds the servos as to get to that optimal voltage of 6V. The Arduino micro is placed directly in the schematic. Lastly, some simple connectors were used for the sensors, actuators, and Bluetooth.

By : Camilla Nelly Jensen (Systems Engineer)

In order to meet the project level 2 requirement 13 for the Velociraptor, the Bluetooth module shall be connected to the microcontroller to establish a wireless communication with the Arxterra application that is installed on an android device. Below are the instructions on how to establish this communication.

1.Download Arxterra application to Android device

• Follow instruction from Getting Started: Arxterra Control Panel & Android Applications powerpoint on how to access to Arxterra Android Application and installing it to the android device.
• Obtain permission to access the Arxterra Application from Mr. Jeff Gomes.

2. Build Bluetooth configuration on breadboard

• According to the System Recourse Map, the HC-06 Bluetooth module is connected to the following pins on the Arduino microcontroller:
  • HC-06 Ground (GND) pin to Arduino ground(GND).
  • HC-06 power (Vcc) pin to Arduino 5 V – To power Bluetooth module.
  • HC-06 TX/TXD pin to Arduino digital pin 0 (RX)- to transmit data from Arxterra app to MCU(Velociraptor).
  • HC-06 RX/RXD pin to Arduino digital pin 1 (TX)- for Arxterra app to receive data from MCU(Velociraptor).
  • The STATE and EN pins will not be used.

Figure 1:

Figure 1 displays the Bluetooth module that will be used to establish the wireless communication to the robot. On the Bluetooth module, the RXD and TXD pins are used for sending and receive data. The module operates at 3.6V to 5V. Receiving data via the RXD pin operate at 3.3V from Arduino.

Receiving data via the RXD line on the Bluetooth module from the Arduino can potentially burn the device. The Arduino transmits a higher voltage than the Bluetooth module can handle, therefore, modification needs to be done. The TX line of the Arduino transmits a voltage at 5V to the receiving pin RXD on the Bluetooth module, however,  the RXD pin can only be operated  at 3.3V. By implementing a voltage divider, this will drop the voltage to 3.3V coming into the RXD pin of the Bluetooth device and thus, protects it from being damaged.

Figure 2: Voltage Divider Equation – credit to 42 Bots

Figure 2 displays how the voltage divider is set up in order reduce the voltage from TX to the RXD pin of the Module.

The Bluetooth module uses  3 x  10k ohm resistors in series and the calculations are shown below in figure 3:

 

Figure 3: Voltage divider calculations for Velociraptor

3. Establish Bluetooth connection between Android device and Arduino

The Bluetooth schematic on the breadboard is shown in figure 4.

 

Figure 4: Bluetooth setup

• The Bluetooth module uses the Arduino IDE code which has the latest version of the Arxrobot firmware. The codes can be downloaded from Github.
• Under the Arxrobot_firmware tab, you find the code to upload to the Arduino, to be able to connect to the Bluetooth module that is connected to the Arduino Microcontroller

Figure 5

• The Bluetooth module utilizes the code above in figure 5 to connect to the Arduino. This code is uploaded to the Arduino microcontroller to recognize the serial communication with the Bluetooth module.
• The Bluetooth is connected to the Arduino which is powered by a computer. The Bluetooth module’s red LED blinks when the module is turned on, the LED stop blinking when communication with Android device is established.
• To pair the module with the Android device go to Settings >> Bluetooth and select the HC-06 module and type in the default pairing code: 1234.
• In the Arxterra application, ARXROBOT the Bluetooth symbol on the top right corner is selected and tap connect to the paired device the Bluetooth module HC-06.
• Once the communication is successfully established; the Bluetooth symbol on the top right corner of the ARXROBOT app turned blue (white when there is no connection) and the red status LED on the Bluetooth module stop blinking. The established communication is shown in figure 4 where the red status LED is on.

4. Confirm Bluetooth communication by turning on 4 LEDS on the breadboard via the joystick on the Arxterra app

The Velociraptor shall be able to walk forward and turn when the robot detects obstacles in order to avoid a collision. Therefore, the joystick on the ARXROBOT qualifies as the best solution for control pad as it provides four buttons of control. The joystick that’s made up of four buttons makes its possible to attach one command to each button, ie. four commands: forward, turn left, turn right and backward. However, the 4^th button is not necessary, as we don’t intend to implement a backward walking command. In order to access the four different buttons, a simple code turning on four LEDS was written in the Arduino IDE, so that the forward button in Arxterra app will turn on LED attached to pin 8 and so on. This test was made so that the Electronic engineer decode the signal into subcommands of move forward, turn to either side etc.

Figure 6

Figure 6 displays the Arduino IDE code that will control each LED by one of the four buttons on the joystick on the Arxterra app in the order of:

• Forward button will turn on LED attached to pin 8
• Right button will turn on LED attached to pin 9
• Left button will turn on LED attached to pin 10
• Back button will turn on LED attached to pin 11

Note: Before uploading the code to the Arduino remember to detach the Bluetooth module from the breadboard as it will interfere with the upload and causes error. Re-attach when the code finishes uploading.

Click here for the video showing the communication between the Bluetooth and the Arxterra Application.

References:

• http://www.docfoc.com/getting-started-arxterra-control-panel-android-applications-create-an-account-connect-on-app-log-in-to-control-panel-tommy-sanchez-ee400d
• http://42bots.com/tutorials/hc-06-bluetooth-module-datasheet-and-configuration-with-arduino/
• http://42bots.com/tutorials/how-to-connect-arduino-uno-to-android-phone-via-bluetooth/
• http://www.amazon.com/JBtek-Bluetooth-Converter-Serial-Communication/dp/B00L08GA4Q/ref=sr_1_1?s=pc&ie=UTF8&qid=1456784518&sr=1-1&keywords=Hc.+06+bluetooth+module+for+arduino

 

By Ashlee Chang (E&C)

Fulfilling Requirements

Level 1 requirements #4 is stated as follows:

The Velociraptor shall be able to statically walk on all surfaces of the course.

The Arduino software, which uses much of C++ programming language, will be used to construct and upload the code onto the Arduino microcontroller.

Coding Summary

This blog will focus on the main walking code for the velociraptor. In the future, a collection of codes must be integrated into the final code in order to meet all the level one requirements. Below is a complete list of the main walking code and all future codes with a short description:

1. Main walking code: This is for static walking (i.e. upon telling the velociraptor to stop, the velociraptor will be able to balance in its current state).
2. Walking fast: The main walking code will be modified to have shorter delay times. In addition, the head and tail will not swing in full motion, as the velociraptors momentum will be used for balance.
3. Sensing – objects: Upon close object detection using the ultrasonic sensor, the velociraptor will halt.
4. Turning: The user will be able to tell the velociraptor to turn, whether it be because the velociraptor halted in front of an object or at any given time.
5. Sensing – incline: Upon sensing an incline using an accelerometer, the velociraptor will adapt with a new walking code with its body closer to the floor and smaller steps for balance.

Calibration

Respective servo degrees of freedom

Servos are limited to movement from 0* to 180*, unable to make a full rotation. Thus, the servos had to be calibrated in order to move in the desired directions. In order to calibrate the servos, they were first connected to the Arduino microcontroller and written to 90*. This way, it is certain that any movements 45* in either direction from this initial calibration will be possible. After calibration, the velociraptor legs were fastened on.

Flaws of the MicroBiPed

Below are visualizations of the Japanese velociraptor Titrus III (our team’s goal), the MicroBiPed of last semester, and the preliminary walking code for the velociraptor.

Titrus III walking (https://www.youtube.com/watch?v=GxVv4WNlXMA)

MicroBiPed walking (https://www.youtube.com/watch?v=3sMzl35wO98)

Velociraptor walking (preliminary code)

Observe the Titrus III. As the back leg is taking a step, the front leg is stationary. However, even though the front leg is stationary, the servos are still moving! As noted in the MicroBiPed and the preliminary velociraptor walking code above, the non-walking-leg servos are not moving. What does this in turn cause? As one foot is taking a step forward, the other foot gets dragged behind, hindering the robot from effectively moving forward. This is the biggest flaw of the MicroBiPed in terms of coding. The purpose of the moving servos of the stationary leg is to propel the velociraptor’s body forward, as opposed to a stationary leg without moving servos dragging the velociraptor behind. Although this causes great complexity in the coding for walking as all four servos are moving simultaneously (and seeing that codes are read line by line), implementing this improvement will no doubt result in an actual walking velociraptor.

Approach

Process for coding the walking

Above shows a block diagram used in the process of compiling the walking code. The velociraptor servos are initialized at [110* 70* 70* 110*] for [front-left back-left front-right back-right]. It is equally important to keep the velociraptor tall enough as to take larger steps and short enough as to ensure the velociraptor keeps balance. From this initial position, a succession of degrees were written for the velociraptor to take a step. The front servo ensures the foot will rise in the air, and the back servo ensures the foot will push out for a large step. After a simple stepping code, a code had to be made for moving the servos of the other leg while the first leg was taking its step. With such complex math due to the different leg sizes and multiple moving pieces, this part was carried out by trial and error. The front and back servos had to complement each other as to keep the foot completely parallel to the floor. Implementing both the step and the perpendicular leg movement in one leg was quite simple, but integrating this motion in both legs at the same time proved to be quite difficult. Thus, a timing diagram was made. Observing that the Titrus III takes about 3.2 seconds for one full walking loop, the velociraptor’s walking loop was set to be close at 3.6 seconds. Initially, the walking code loop conditions were based on a certain servo’s angle. However, as the looping became more complex with multiple servos moving at once, the looping condition variable was changed to one that kept track of time (i.e. the delays), thus utilizing the timing diagram. Lastly, each servo within a loop had to move at a different pace, whether it be double, triple, or even 1.5 times the rate of another servo in its loop. Thus, loops had to be nested within each other, or some servos had to increase or decrease by more than just +1 or -1 degree at a time, in order to achieve the desired degree changes per section of time.

Timing Diagram

Timing diagram

Because all four leg servos will be moving simultaneously, a timing diagram was made as to allow a timing variable be the looping condition in the velociraptor code. A full looping cycle was set to be 3600 ms, broken into 9 segments of 400 ms (meaning there are 9 total main loops in the walking code). The diagram lists the total time elapsed for a full walking loop (which is used as the timing variable in the code), delay in between each segment of code, the degrees of each servo at a certain time, and the change in degrees of each servo over the period of 400 ms. The yellow and blue font colored numbers indicate when that leg is taking a step (when the foot is on the floor, when the foot is in the air, and when the foot is on the floor again). Note there is 400*3=1200 ms in between the stepping of one foot and the other, which will be saved later for the movement of the head and tail responsible for relocating the center of mass on the stationary foot.

Debugging

Although the servos have the capability to rotate from 0* to 180*, the velociraptor leg joints are connected in such a way that limits the servos degree of freedom even more-so. Any slight error in the coding that moves the servo to an angle that the leg joints aren’t meant to handle can cause damage to the servo gears, or if the torque is powerful enough, could potentially snap the legs of the velociraptor. That is why debugging the program on the Arduino without physically connecting power to the servos is important. Serial.print( ) was frequently used to test the program and ensure all variables were as expected.

Debugging the program and checking the serial monitor

Code Structure

The first section of the code includes the servo library and creates (currently) four servo objects to control each servo. The second section of the code (void setup) initializes variables, attaches the servos to the Arduino pins, writes the initialized angles tot he servo, and includes a Serial.begin(57600) for debugging purposes. Also, the first segment of the walking code is placed here. This is because the velociraptor will initially be in a stance where it is balanced on two feet with the head and tail both facing forward. It is coded so the right foot will take a step first while the front-left and back-left servos are kept at 110* and 70*. Finally when the front-right and back-right servos reach 85* and 140* (at total time = 0), the loop will begin. In the third section of the code (void loop), the left foot will take a step while the right foot re-initializes while keeping perpendicular to the floor. Then, the right foot will take a step while the left foot re-initializes while keeping perpendicular to the floor. This segment of the code repeats.

Walking Code NotePad File

Results

Compare the preliminary walking code versus the improved walking code (see first GIF of the velociraptor). The first prototype has some joint dimensions that are slightly off, and some imperfections on connections and drill hole sizes, thus the velociraptor appears to walk lopsided. Rest assured, the angle movements on each leg mirror each other exactly in the code, which will reflect on the next prototype and/or final product. The prototype must be held as the walking takes place, as there is a head and tail installed for balance. Examine how the stationary foot’s moving servos push the velociraptor forward.

Improved walking algorithm

By Ashlee Chang (E&C)

Updated: 03/30/16

Fulfilling Requirements

Level 2 requirements #7 is stated as follows:

7. For the Velociraptor to perform dynamic walking servos moving at a speed of 0.101 sec/10° shall be implemented to the chassis and thus meet the Project Level 1, requirement 5.

The servos will be the motor responsible for the walking and movement of the velociraptor in static walking, dynamic walking, turning, and walking up an incline. Below are the trade-off studies conducted, analyses on the best option, and the team’s final decision.

Servos Introduction

A servo is a handy device in robotics that converts electrical energy to mechanical energy. The device contains a two wire DC motor, a gear train, a potentiometer, an integrated circuit, and an output shaft. There are usually three wires attached to this device: a ground wire (which is usually black), an input voltage wire (which is usually red), and a control wire (which is usually yellow). Using the control wire, a coded signal is transmitted to the servo, which instructs the shaft to rotate to a specific angular position. The potentiometer can be thought of a variable resistor; its role in the servo is to monitor the angle of the shaft. If the shaft is not oriented at the correct angle, it will turn the motor to correct the direction. If the shaft is oriented at the correct angle, it will shut the motor off. A normal servo usually has the capability of positioning between the range of 0* and 180*.

Anatomy of a servo

The velociraptor will need eight servos: two for the head, two for the tail, and two for each leg. The two each on the head and tail will be for balancing purposes; left and right movements will be for balancing as each foot takes a step. The two per leg will be for not only walking, but a turning mechanism as well.

Calculations

Calculating servo torque

Torque is basically a “twisting force” and is measured in kg*cm. The larger this specification, the more force the servo can exert. If a servo has a power rating of 1 kg*cm, the maximum amount of power it will be able to apply with a 1 cm arm will be 1 kg.

In application to the velociraptor, an estimation for total weight must be made in order to justify the purchase of the servo. For the MicroBiPed, the servo power rating was 3.5 kg*cm at 6.0 V. Thus, for an arm of 1 cm, the servo would be able to produce a 3.5 kg push or pull force to the right angle of the servo arm before stalling. Due to the torque dependency on arm length, the joints of the velociraptor should be kept close to the body to maximize force capabilities. Below is the Titrus III legs showing the arm lengths.

Titrus III arm lengths

The first prototype of the servo has a front arm length of 5.2 cm and back arm length of 2.0 cm. Using these numbers, calculations are made in the “Update” section of this blog post to calculate exactly how much weight each arm alone can handle. The Titrus III is credited and can be seen in action here: https://www.youtube.com/watch?v=GxVv4WNlXMA.

Trade-off Studies

In order to better assess the appropriate servo to use for the velociraptor, a comparison chart was made below of servos in the marketplace.

Servos comparison table

All servos, as per order in the comparison table

* The difference between the two Futaba servos is one has a bearing type of dual bronze brushings, and the latter has a signal ball bearing system.

The first listed servo is the one used by last semester, and the second is one with a lot of torque but Dr. Hill suggested not to go with. 11 different servos were compared, listing some of the most important characteristics. The velociraptor will need eight, so it is important to keep the cost of individual servos at a minimum. The operating speed shows how long it takes for the servo shaft to rotate 60*. It can be seen from the table that all operating speeds are relatively close in value, thus it will not be a major deciding factor. The stall torque is also an important parameter, which shows how much force is needed to move the shaft over a certain distance. In the case of a heavy velociraptor, the stall torque becomes a very important parameter as more torque is needed. The dimensions will affect how large the body encasing them will be. This information is important to M&D division for the size and shape of the body. Lastly, the weight is also significant, considering there are many servos to be accounted for.

Observing the MicroBiPed, weight was undoubtedly the biggest problem. The choice of material used to construct the shell of the robot, PLA, made the overall “micro” robot very heavy as a result. The torque of the servos was not sufficient enough to power through the weight of the MicroBiPed. Thus, it is important that the velociraptor team is weight-conscious in selecting not only the material used to construct the shell, but also the individual component pieces. Below is a table comparing the stall torque to weight ratio of each component. Also below is the price to torque ratio.

Stall Torque/Weight Ratios

Price/Weight Ratios

Links to all the products:
[Towerpro MG92B] http://www.headsuphobby.com/Towerpro-14g-MG92B-Digital-Metal-Gear-High-Torque-SubMicro-Servo-A-537.htm
[JX PDI-6221MG20KG]  http://www.banggood.com/4X-JX-PDI-6221MG-20KG-Large-Torque-Digital-Coreless-Servo-For-RC-Model-p-1031156.html
[Futaba S3003] http://www3.towerhobbies.com/cgi-bin/WTI0001P?I=LXH288&P=8
[RioRand ES08MA] http://www.riorand.com/toys-hobbies/helicopters/riorandr-es08ma-9g-mini-metal-gear-servo-upgrade-mg90s-for-trex-align-450-rc-helicopter-e.html
[TowerPro SG90] http://www.amazon.com/J-Deal%C2%AE-TowerPro-Helicopter-Airplane-Controls/dp/B00X7CJZWM/ref=sr_1_1?ie=UTF8&qid=1455438532&sr=8-1&keywords=servos
[Hitec HS-311] http://www.headsuphobby.com/Hitec-43g-HS-311-Standard-Servo-B-560.htm?categoryId=-1
[Futaba S148] https://www.servocity.com/html/s148_standard_precision.html#.VsBN_fkrKM-
[Futaba S3004] https://www.servocity.com/html/s3004_standard_ball_bearing.html#.VsBOBPkrKM-
[Power Up AS3513NG] http://www.headsuphobby.com/Power-Up-25g-AS3513NG-Analog-Mini-Servo-W-350.htm?categoryId=-1
[Power Up AS3125NG] http://www.headsuphobby.com/Power-Up-38g-AS3215NG-Analog-Standard-Servo-W-400.htm?categoryId=-1
[Emax ES3051] http://www.headsuphobby.com/Emax-43g-ES3051-Digital-Standard-Servo-G-651.htm?categoryId=

Arduino Application

The servo library can be found on the Arduino website, which supports up to 12 different motors on most boards and up to 48 on the Arduino Mega. The Mega can handle up to 12 servos while the Mini can only handle 6-7, so the Mega might be in this semester’s project’s best interest. Two example codes are listed on the Arduino servo website for controlling the position of a servo with a potentiometer and for sweeping the shaft of the servo motor back and forth.

There are three different output types: analog voltage, PWM, and serial. Servo motors use the communication type of PWM (post width modulation). The length of a pulse in seconds will determine how many degrees the motor will turn, and in turn, dictates the angle of the output shaft.

Pulse duration’s relationship to shaft angle

Conclusion

Seeing as the velociraptor will require eight servos, it’s important to minimize weight as much as possible. The servo choice of the MicroBiPed provides the largest torque to weight ratio compared to all other options listed in the table. It’s even more notable that the dimensions are the smallest as well, which will keep the velociraptors body containing most of the servos sizable. To maximize the power ratings of the servos, the arm lengths should also be designed short.

Update (03/14/16)

After the systems and test engineer estimated the project to be around 350 g, torque calculations were able to be made to see exactly how much weight a single servo can handle. The front and back legs (“arm” joints) of the first prototype are 5.2 cm and 2.0 cm, respectively. Using the torque of the TowerPro MG92B servo at 3.5 kg*cm, it is calculated that the front servo alone can handle a weight of approximately 675 g and 1750 g, respectively. This is a basic calculation; of course there are other things to take into consideration like weight distribution of the velociraptor and the fact that some servos work together at the same time. However, this calculation shows that if a single servo can handle the weight of the entire project, then more than one servo (some working together simultaneously) definitely has enough torque to handle the project and then some. The total weight of the project, as estimated by the S&T engineer, is around 500 g. Thus, the TowerPro MG92B was the chosen servo for the velociraptor. All eight servos together come to a total of $60. This servo, also used by last semester’s MicroBiPed, also has the best torque-to-price and torque-to-weight ratio.

Works Cited

[1] What is a servo?
https://www.servocity.com/html/what_is_a_servo_.html#.VsAzEvkrKM8
[2] What’s a servo?
http://www.seattlerobotics.org/guide/servos.html
[3] How to Arduino #3 – Servos
youtube.com/watch?v=ybV8vitYAWU
[4] Servo Power & Speed
https://www.servocity.com/html/servo_power___speed.html#.VsBO9vkrKM8
[5] Understanding RC Servos: Digital, Analog, Coreless, Brushless
http://www.rchelicopterfun.com/rc-servos.html
[6] Servo library
https://www.arduino.cc/en/reference/servo
[7] Titrus III
youtube.com/watch?v=GxVv4WNlXMA