12. Mechanical Design, Machine Design

This week our group designed and built a prototype XIAO Vending Machine — a column-based dispenser that releases Seeed Studio XIAO boards when a user taps a pre-programmed RFID card. My individual contribution was to explore the STS3215 serial bus servo: understand how it works, bring up the Waveshare SCServo library on the Wio Terminal, assign unique IDs, tune gate positions, and deliver the servo test code that the team integrated into the final firmware.

Group Assignment — XIAO Vending Machine

The full group documentation covers ideation, planning, mechanical assembly, RFID integration, and the final demo video. The machine flow is:

Week 12 Group Assignment — Mechanical Design / Machine Design (Chaihuo Fab Lab)

My Role — Exploring the STS3215

While other teammates focused on the enclosure, RFID writer, and mechanical column design, I took ownership of the actuator subsystem. Before we could integrate servos into the vending machine, we needed to answer several questions:

• How does a serial bus servo differ from the PWM SG90 I used in Week 11?
• How do we wire and power multiple STS3215 units on a single data line?
• How do we assign unique IDs when every servo ships with ID 1?
• What position values open and close the dispensing gate reliably?

I worked through these step by step on a Wio Terminal, producing three standalone test sketches and the gate-motion parameters (HOME_POS = 1024, TRIGGER_POS = 0, two dispense cycles) that the team merged into vending_machine_final_code.ino.

Task	What I did	Output
Research	Read Waveshare ST3215/STS3215 wiki, protocol manual, and compared bus vs PWM servos	Spec table and wiring notes (below)
ID assignment	Flashed change-ID sketch one servo at a time; assigned IDs 1 and 2	STS3215_change_id.ino
Single-servo demo	Verified UART communication and position feedback on one unit	STS3215_demo_sweep.ino
Dual-servo gate test	Used SyncWritePosEx to move both servos together; tuned speed, acceleration, and delays	STS3215_gate_test.ino
Integration support	Shared position values and timing constants with the team for RFID-triggered dispense	Group final firmware (GitLab link below)

How the STS3215 Works

Why not a normal PWM servo?

In Week 11 I controlled an SG90 with a 50 Hz PWM pulse — one signal wire per servo, no feedback, and no way to know the actual horn angle. The STS3215 is a smart serial bus servo (Feetech / Waveshare ST series). Instead of PWM, it speaks a structured UART protocol over a single half-duplex data line. Multiple servos daisy-chain on that one bus, each addressed by a unique ID from 1 to 253.

Feature	SG90 (PWM, Week 11)	STS3215 (Serial Bus)
Control signal	50 Hz PWM pulse width	Half-duplex UART @ 1 Mbps (default)
Wiring	One signal wire per servo	Single daisy-chained data bus (GND + VCC shared)
Addressing	None — wired to a GPIO pin	Unique ID (0–253), set via EEPROM
Position resolution	~180° analog pulse, no readback	360° / 4096 steps (12-bit magnetic encoder)
Feedback	None	Position, load, speed, input voltage
Multi-servo control	Needs one GPIO per servo	SyncWritePosEx — one packet, all servos move together

First principles: half-duplex UART bus

The STS3215 uses half-duplex communication — one data wire carries traffic in both directions, but only one device transmits at a time. The Wio Terminal (master) sends a command packet; every servo on the bus listens; only the servo whose ID matches executes the instruction and optionally replies with feedback.

Wio Terminal TX/RX → single DATA wire → Servo A (port A ↔ port B) → Servo B → …

Each servo has two 3-pin ports (A and B) so the signal passes through the chain. Power (6–12.6 V) and GND run alongside the data line. Because locked-rotor current can reach 2.7 A per servo, the team powered the bus from a dedicated 7.4 V supply rather than the Wio Terminal's 5 V pin.

Packet structure

The protocol follows the Feetech / Waveshare Smart Bus Servo format. Every instruction packet looks like this:

Byte	Field	Meaning
0–1	Header	0xFF 0xFF — start of packet
2	ID	Target servo ID (or 0xFE for broadcast)
3	Length	Number of bytes following
4	Instruction	PING, READ, WRITE, SYNC WRITE, etc.
5…	Parameters	Register address, position, speed, acceleration
last	Checksum	~(ID + Length + Instruction + Params)

Position is a signed 16-bit value from 0 to 4095 representing 360°. The vending machine gate uses a 90° swing: HOME_POS = 1024 (gate closed) and TRIGGER_POS = 0 (gate open). The Waveshare SCServo library wraps all of this into high-level calls like WritePosEx(), ReadPos(), and SyncWritePosEx().

Key specifications

Parameter	Value
Operating voltage	6 – 12.6 V
Torque	30 kg·cm @ 12 V
Position resolution	360° / 4096
Default baud rate	1 000 000 bps
ID range	0 – 253 (factory default: 1)
Max daisy-chain	253 servos (power-limited in practice)
Feedback	Position, load, speed, input voltage
Connector	3-pin: GND, VCC, DATA (per port)

Hardware Setup — Wio Terminal + STS3215

On the Wio Terminal, the Grove UART port maps to Serial1. I connected the servo bus DATA line to the UART pins and kept RFID on the separate I²C Grove port (Wire1), so both subsystems could run simultaneously in the final firmware.

Connection	Wio Terminal	STS3215 bus
Data (RX)	PIN_WIRE_SCL → Serial1 RX	DATA
Data (TX)	PIN_WIRE_SDA → Serial1 TX	DATA (same wire)
Power	External 7.4 V supply	VCC (both servos)
Ground	GND (common with Wio)	GND
RFID reader	Grove I²C (Wire1, addr 0x28)	—

(Add a photo at public/images/week12/wiring.jpg showing the Wio Terminal, daisy-chained STS3215 servos, and RFID reader.)

My Testing Process

1. 1 Install the SCServo library — copied scservo-main from the group repo into Arduino libraries/. This provides the SMS_STS class for the ST/STS series.
2. 2 Assign unique IDs — connected one servo at a time and ran STS3215_change_id.ino. Verified each ID with st.Ping(id) before connecting the next servo to the chain.
3. 3 Single-servo sweep — ran STS3215_demo_sweep.ino to move one servo through positions 0 → 1024 → 2048 → 1024 → 0 and read back actual position with ReadPos(). This confirmed the magnetic encoder feedback loop works.
4. 4 Dual-servo gate motion — daisy-chained both servos (IDs 1 and 2) and ran STS3215_gate_test.ino. Used SyncWritePosEx so both gates open and close in sync. Tuned Speed = 1500, ACC = 50, forward delay 1800 ms, return delay 1200 ms.
5. 5 Hand off to integration — shared the position constants and timing values with teammates who merged them into the RFID-triggered final firmware on the Wio Terminal.

Gate Motion Code

The core dispense function I developed moves both servos together using synchronous write. This is the same pattern used in the group's vending_machine_final_code.ino:

#define SERVO_NUM 2
byte ID[SERVO_NUM]    = {1, 2};
u16  Speed[SERVO_NUM] = {1500, 1500};
byte ACC[SERVO_NUM]   = {50, 50};
s16  Pos[SERVO_NUM]   = {1024, 1024};

const s16 HOME_POS    = 1024;   // gate closed
const s16 TRIGGER_POS = 0;      // gate open

void moveBothTo(s16 target) {
  for (int i = 0; i < SERVO_NUM; i++) Pos[i] = target;
  st.SyncWritePosEx(ID, SERVO_NUM, Pos, Speed, ACC);
}

void runGateCycles(byte cycles) {
  for (byte c = 0; c < cycles; c++) {
    moveBothTo(TRIGGER_POS);
    delay(1800);
    moveBothTo(HOME_POS);
    delay(1200);
  }
}

Integration in the Group Project

Once the gate motion was reliable on the bench, the team mounted each STS3215 to a 3D-printed arm that pushes the column gate. The full system test — RFID card tap → Wio Terminal validates TRUE token → two HOME → TRIGGER → HOME cycles on both servos → one XIAO board drops — is documented on the group page (sections 5–7).

(Add a photo at public/images/week12/gate_test.jpg showing the servo gate mechanism or a screenshot from the group demo video.)

The final firmware reads MIFARE block 4 over I²C and, on a TRUE match, calls the same runDispenseCycles(2) function. Running two cycles ensures the XIAO board clears the gate even if the first push only loosens the stack.

Problems and How I Fixed Them

What I Learned

The biggest takeaway is that serial bus servos change how you think about actuators. With a PWM servo you send a pulse and hope; with the STS3215 you send a structured command, read back the actual position, and can synchronously move an entire chain from one UART port. This is exactly what the vending machine needs — two gates must open and close together, and the team can add more columns later by daisy-chaining additional servos with new IDs.

I also learned that bus servo bring-up has a specific order: correct baud rate → assign IDs → single-servo test → multi-servo sync write → mechanical integration. Skipping the ID step or under-powering the bus causes problems that look like firmware bugs but are really wiring issues.

Design Files & Source Code

My individual sketches

• STS3215_change_id.ino — assign unique bus IDs
• STS3215_demo_sweep.ino — single-servo position sweep + feedback
• STS3215_gate_test.ino — dual-servo gate motion test

Group repository (GitLab)

• code_servo/ — SCServo library and all group servo sketches
• vending_machine_final_code/ — final Wio Terminal firmware
• test_for_assemble/ — first RFID + servo integration sketch

References

• Waveshare ST3215 Servo Wiki
• Seeed Wio Terminal — Getting Started
• Chaihuo Week 12 Group Assignment

Hero Shot

STS3215 servos daisy-chained on the Wio Terminal, driving the vending machine column gates.

(Add your hero photo at public/images/week12/hero.jpg — e.g. a close-up of the servo arm pushing a XIAO board out of the column, or a frame from the group demo video.)

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