ESP32-based Development Board
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The SRA board is a development board based on ESP32 with on-board peripherals like programmable LEDs, switches, sensor ports for Line Sensor Array and MPU-6050, protection circuit for over-current and reverse voltage and motor drivers.
- Frontside
- Backside
In general, every development board has the following basic features:
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- Microcontrollers (MCUs) usually run on 3.3V or 5V logic supply voltage while input to a development board is normally 12V for motor and driving/controlling peripheral devices.
- So, in order to have a single input source, a power section which inter converts this 12V to standard levels like 5V & 3.3V for MCU and sensors is present.This is achieved using a step-down buck regulator.
- Buck Regulator IC LM2576-S-5 is used for stepping down the voltage from 12V to 5V DC. This 5V is further regulated to 3.3V using LDO IC AMS1117-3.3.
- The previous edition of the SRA board (2020) used a similar buck regulator setup.
- The older editions of the SRA board used the LM7805 linear voltage regulator, for stepping down from 12V to 5V; this powered the ESP32. Further, this 5V was converted to 3.3V using the LD33 linear voltage regulator, used by the sensor port.
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- Motors usually run on 12V and MCU output is generally 5V/3.3V. So, an external motor driver circuitry is required to control motors according to the MCU input.
- The current and previous editions of SRA board use the TB6612FNG Motor Driver, which is a MOS-based H-Bridge motor driver.
- The older editions of SRA Board used the L298N IC for motor-control, which is a BJT-based H-Bridge motor driver.
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- According to the external sensor types, usually development boards have onboard sensor ports where the sensors can be connected easily. LSA - Line Sensor Array and MPU- Motion Processing Unit have on-board connection ports.
- The current edition uses easily available and efficient JST XH connectors.
- Previous versions used bulky FRC connectors
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Protection against Reverse Voltage
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Protection against Over Current
- Earlier, for the overcurrent protection of MCU and motor driver circuit, bulky glass fuses of 300mA and 3A were used respectively. After breakdown, they used to be replaced.
- In the recent versions of the board, these were replaced with compact, PTC Resettable Fuses.
- On 12V line - RXEF160 : 1.6A hold current; 3.2A trip current Fuse was used.
- On 5V line - RXEF160 : 0.5A hold current; 1A trip current Fuse was used.
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- Every development board should have some programmable switches and LEDs for testing, control and debugging purposes.
- The current editions has an array of 8 programmable LEDs and switches.
- The previous edition had a pair of programmable switches and programmable LEDs each.
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- All versions have a power switch for the motor driver, using which power supply to the motor driver can be toggled. Similarly, there was a switch for the ESP32 MCU.
Now that we covered basics of development boards, let us talk about the changes made in the new design.
| Feature | SRA Board 2019 | SRA Board 2020 | SRA Board 2022 |
|---|---|---|---|
| 12V to 5V | LM7805 Linear Regulator | LM2596 Buck Convertor | LM2576-S Buck Convertor |
| 5V to 3.3V | LD33 | AMS1117-3.3 | AMS1117-3.3 |
| Reverse Voltage Protection | Diodes | P-MOSFET | Diodes |
| Motor Driver | L298N | TB6612FNG | TB6612FNG |
| No. of Motor Channels | 2 | 4 | 4 |
| No. of Switches | 2 | 4 | 4 |
| No. of LEDs | 2 | 8 | 8 |
| Over Current Protection | Bulky Glass Fuses | Bulky Glass Fuses | PTC Resettable Fuses |
| Sensor Port Connectors | FRC (Fused Ribbon Cable) Connectors | JST (Japan Solderless Terminal) Connectors | JST Connectors |
| Component Type and Board Size | THT (Through Hole), Big | THT (Through Hole), Big | SMD(Surface Mount Device), Smaller |
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7805 (5V linear regulator) to LM2576/96 Buck Convertor
- The greater efficiency, output current and reliability of LM2576/96 were the reasons for this change.
- The efficiency of LM2576 is up to 92% which is significantly better than 7805. The LM2576 can provide current up to 3A, so the MARIO workshop manipulator can now be run using onboard regulator.
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LD33 (3.3V) to AMS1117:
- The older editions used the LD33 IC to step down from 5V to 3.3V; several discussions resulted in the shift to more compact, reliable AMS1117-3.3(SOT-23) linear voltage regulator. (AMS1117 is also used on-board on the ESP32-DevKitC V4 module)
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- Diodes in series to the power line are inefficient as compare to a P-MOSFET. Dut to the usage of high-rated motors, it was difficult to manage the diode size and the current rating. (As the current rating of diode increases, its size also increases.)So, the previous version uses the P-MOSFET instead of a diode, which is more efficient and can handle more current.
- In the current edition, SMD diodes are used, hence rating and size is no longer an issue.
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- In the previous and older editions, THT or PTH(Plated through hole) components were used, which take up a lot of space and their size increases with increasing rating.
- In the current edition, all the components were replaced by SMD components. This reduced cost, size of PCB and helped assembling complex circuitry in a small space and form factor.
- Previous edition PCB dimension - 81 x 112 mm
- Current edition PCB dimension - 81 x 92.68 mm (Nearly 81 x 20 mm area was shaved off)
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L298N to TB6612FNG
- L298N is a BJT-based H-bridge motor driver but it is less efficient as compared to the new MOS-based TB6612FNG.
- The detailed comparison is shown below. As you can see the efficiency of TB6612FNG can reach up to 91-95% which is significantly higher than the 40-70% efficiency of L298N.
- The only drawback of TB6612FNG is the less continuous current which is equal to 1.2A. So, for higher current capacity motors, two TB6612FNG are given on the board, which can be used in parallel mode to double the current capacity to 2.4A.
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The new edition has 2x TB6612FNG motor drivers which allow a maximum of 4 motors to be controlled. This motor driver is characterized by its operation in two modes - Normal mode and Parallel mode:
- Normal Mode
- As discussed earlier, the new design has two motor drivers. Each TB6612FNG can control two motors. Therefore, using two motor driver one can control 4 motors using 8 GPIO's of ESP32.
- E.g.: If pin 32 is HIGH(IN1 = HIHG) and pin 33 is low(IN2 = LOW) then motor 1 moves in the forward direction.
- So in normal mode, 4 motors can be connected to the board, with a per channel/motor current capacity of 1.2A.
- Parallel Mode
- The parallel mode is a special feature, used for high-rated motors, requiring more than the 1.2A current limit.
- In this mode, the channel's directional pins and output pins are shorted; only one motor is connected to a motor driver i.e. two channels, giving a current capacity of 2.4A. Thus, two high rated motors can be controlled using ESP32.
- Note: The directional pin shorting is done by a manual DPDT switch. If the user turns on TB_A switch then the first motor driver goes into the parallel mode and its directional pins are shorted, where GPIO connections are IN1 = IN3 = 25 and IN2 = IN4 = 26. If TB_A switch is off, then the first motor driver goes into normal mode where IN1 = 32: IN2 = 33: IN3 = 25: IN4 = 26. This is all done automatically. Also for parallel mode, the J1, J2, J3 and J4 junctions need to be shorted.
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- The older edition used a pair of programmable switches and LEDs each.
- Since the previous edition, the number of debuggable switches has been increased to 4.
- The previous edition used an LED voltage bar-graph array of 10 LEDs which was very bulky.
- In the current edition, 10 SMD LEDs arranged in an array out of which two are reserved for 5V and 3.3V voltage indication have been used.
- So there are 8 programmable LEDs on the board. These LEDs multiplexed with directional pins of the two motor drivers to save pins.
- Directional pins? >> Every motor driver channel has two directional pins IN1, IN2. If IN1 is high and IN2 is low then motors move in a clockwise direction and there are 4 channels on the board, so 4 * 2 = 8 directional pins are multiplexed with 8 programmable LEDs.
- With 8 LEDs in hand, debugging get easier. Some examples -
- According to the line sensor array (LSA) data, one can program 4 LEDs to turn on when the line sensor detects white and turn off for black- line following debugging.
- If a motor is moving in a forward direction, the dedicated LEDs will be indicating IN1 is high and IN2 is low - motor control debugging.
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- ESP32 can be power using two ways - one via the USB port given on the ESP32 and two via providing a voltage on VIN pin.
- In the SRA Board 2019, if simultaneous power (on both the above sources) was provided to the ESP32 then it won't work as there was no circuitry for handling such a condition.
- The following disclaimer from the official ESP32 documentation made it essential to design some external circuit to handle this condition and accordingly pass only one signal to the ESP32 board.
The power supply must be provided using one and only one of the above options.Otherwise, the board and/or the power supply source can be damaged.
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- The solution for this issue was Diode Auctioneering.
- There is an inbuilt BAT760 diode on the USB line on ESP32. If different voltages are applied at Vusb and Vin, then the voltage with bigger magnitude will be given to LD1117 (LDO on ESP32); often the voltage will be the same on Vin and Vusb i.e 5V.
- But, the usage of the SS14 (currently, 1N5417 in previous edition) diode on the Vin path, which has more Vf (forward voltage) than the BAT760, will create a voltage indifference and in a simultaneous power supply condition, USB will be selected as its voltage will be more than Vin.
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Currently 300rpm rated BO motors were ultilised for application. These are brushed DC motors functioning currently on 12v working on PWM concept.
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These BO motors drain a lot of current during operation. 90 - 100 mA - during normal operation. But they drain about 600 - 700 mA current - during momentary or forced stalling. Momentary stalling happens by itself.
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For self balancing operation - the MCU communicates with MPU6050 for the current orientational readings and accordingly gives commands to the motor driver and thus the motors.
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When this PWM goes over 50% duty cycle, the error in MPU readings become large and the motor rotate with full speed, thus not self-balancing the setup.
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The following inspections were carried out os far so solve this issue.
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- The static noise or low-frequecy noise generated by BO Motors was eliminated by soldering a 0.1uF ceramic capacitor across its terminals.
- Initially, the longer length of the I2C line route + cable from the MPU6050 to the ESP32 was suspected to be the main culprt. However, the MPU was checked for I2C packet drop, which when observed under the a DSO, seemed pretty fine. While reading the Motion data, both the SDA and SCLK lines go high.
- The dynamic high-frequency noise wasnt getting eliminated which is produced to the brush action of brushed DC motors. Finally adding ferrite beads across the motor terminals and using twisted cable pairs to connect it to the board lessened the RF noise to an extent.
- Although, the above solution only facililated in lessening the frequency of system going into error. Also, the duty cycle was forced to be under 50% for opltimal application.
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- View this issue #14 for more details and advancements on the suggested solutions.
- The above error didnot occur when L298 motor driver was used with the same setup.
- Using N20 geared motors which draw relatively less current did not throw an error.
- Task is to figure out an economical alternative considering it to be a hardware issue.
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3D preview of the SRA Board 2022
- The complete 3D model (.step) file of SRA Board 2020
- The 3D models of motor driver, LEDs, ESP32 etc.: 3d models of other components
- Designing of the prototype board
- Modular testing of the circuit
- Testing of prototype board
- Final version
- Resolving high frequency noise by BO Motors ( TB6612FNG )
- Dhairya Shah: Designer
- Omkar Bhilare: Mentor
- VedantParanjape: Mentor
- Thanks to PCBPower Market for sponsoring the prototype and the final boards.
- Previous Edition: SRA Board 2020
- KiCAD Tutorials
- README Template by roshanlam
- Distributed under the MIT License.