AS5304 magnetic encoder and AS5000-MR20-44 ring magnet for Micromouse Robot - Main Features of AS5304A and AS5000-MR20-44, Design Considerations, Combining AS5304 with STM32 Encoder Mode

The AS5304 is a compact magnetic rotary encoder IC widely used in precision motor control applications. In a Micromouse robot, accurate wheel position and speed measurement are essential for stable movement, wall following, and precise maze navigation. The AS5304 provides a reliable solution for detecting rotational motion without physical contact, making it highly suitable for high-speed autonomous robots.

What is the AS5304 and AS5000-MR20-44?

The AS5304 is a magnetic incremental encoder sensor developed by ams OSRAM. It detects the rotation of a magnetic pole wheel and generates quadrature output signals similar to optical encoders.

Unlike traditional optical encoders, magnetic encoders are:

More resistant to dust and vibration
Smaller in size
Easier to integrate into compact robots
More durable in harsh environments

The AS5000-MR20-44 is a ferrite multipole ring magnet with 44 poles and a pole length of 2.0 mm. It is designed for use with linear position magnetic encoders in off-axis rotary applications. Part Number naming convention breaks down as follows:

M = Magnet
R = Ring
20 = Nominal pole length in 1/10 mm (i.e., 2.0 mm), corresponding to the AS5304 encoder
44 = Number of poles

Why Micromouse Needs an Encoder

A Micromouse robot relies heavily on encoder feedback for:

Speed control
Distance measurement
Accurate turning
Position tracking
PID motor control
Maze mapping algorithms

Without an encoder, the robot cannot determine:

How far it moved
Whether both wheels rotate equally
If the robot drifted during motion
Exact turning angles

The AS5304A allows the robot controller to continuously monitor wheel rotation with high precision.

Operating Principle

The AS5304 works together with a magnetic ring or magnetic pole wheel attached to the motor shaft.

As the shaft rotates:

Magnetic poles pass over the sensor
Internal Hall sensors detect magnetic changes
The IC generates:

Channel A pulse
Channel B pulse

4. Quadrature signals indicate:

Rotation speed
Rotation direction

This method is similar to optical quadrature encoders but uses magnetic sensing instead of light interruption.

Main Features of AS5304

1. Incremental Quadrature Output

The encoder outputs two square wave signals:

A phase
B phase

These signals are 90 degrees out of phase.

The MCU can determine:

Rotation count
Direction
Speed

This is ideal for STM32 timer encoder mode.

2. Compact Size

Micromouse robots require extremely compact electronics.

The AS5304:

Occupies very little PCB area
Fits near micro metal gear motors
Supports lightweight robot designs

3. Non-Contact Sensing

Since there is no physical contact:

Mechanical wear is reduced
Reliability increases
Long-term accuracy improves

This is important for repeated maze competitions.

4. High Speed Capability

Micromouse robots can move at very high speeds.

The AS5304 supports fast rotational detection suitable for:

Speed runs
Acceleration control
High RPM gear motors

Main Features of AS5000-MR20-44

Physical Specifications

Outer Diameter (OD): 32 mm
Inner Diameter (ID): 24 mm
Thickness (T): 1.5 mm
Pole Length: 2.0 mm
Number of Poles: 44
Shape: Ring
Material: Plastic bonded Ferrite
Magnet Type: Permanent magnet

Design & Sensor Placement

The correct measurement radius for the MR20-44 magnet ring is calculated using the formula:

r_m = (pole_length × number_of_poles) / (2π) = (2.0 × 44) / (2π) ≈ 14.01 mm

The measurement radius should coincide with the Hall sensor array on the chip.

For the TSSOP-20 package, the Hall sensor array is located 1.02 mm above the horizontal centerline, or 4.22 mm above the edge of the pins.

Compatible Encoder IC

This magnet is specifically designed to pair with the AMS AS5304 (and related AS5306) linear incremental position sensor ICs, which use Hall-effect sensing for off-axis rotary position detection.

Typical Micromouse Encoder System

A typical encoder assembly contains:

Components

DC gear motor
Magnet ring or pole wheel
AS5304A sensor PCB
STM32 microcontroller

Working Flow

  Motor Shaft Rotation
        ↓
  Magnetic Pole Movement
        ↓
  AS5304A Detection
        ↓
  Quadrature Pulse Output
        ↓
  STM32 Timer Encoder Interface
        ↓
  Speed and Position Calculation

Connection with STM32

The AS5304 is commonly connected to STM32 timers configured in encoder mode.

Example Pins

AS5304	STM32
A Output	TIMx_CH1
B Output	TIMx_CH2
VCC	3.3V
GND	GND

The STM32 hardware timer automatically counts encoder pulses.

Encoder Resolution

Encoder resolution determines how accurately wheel movement can be measured.

Higher resolution provides:

Encoder pulse : 3420
Better distance calculation
Smoother speed control
More accurate turning

However:

CPU interrupt load may increase
Signal quality becomes more important

Micromouse designers must balance:

Speed
Resolution
Processing capability

Using Encoder Feedback in Micromouse

1. Speed PID Control

Encoder pulses allow real-time speed measurement.

The PID controller compares:

  Target Speed - Actual Speed

Then motor PWM is adjusted automatically.

Benefits:

Stable movement
Straight driving
Better acceleration control

2. Accurate Turning

During a 90° turn:

Encoder counts measure wheel rotation
Robot stops exactly at target angle

This improves:

Corner precision
Maze alignment
Path consistency

3. Distance Measurement

The robot calculates travel distance using:

  Distance = Wheel Circumference × Rotation Count

Accurate distance measurement is critical for:

Cell-to-cell movement
Flood-fill algorithms
Position estimation

Advantages over Optical Encoders

Magnetic Encoder	Optical Encoder
Dust resistant	Sensitive to dust
Compact	Larger structure
Durable	More fragile
Lower maintenance	Requires cleaner environment
Easier assembly	More alignment sensitive

For compact robots like Micromouse, magnetic encoders are often preferred.

Design Considerations

Magnet Alignment

The magnet must align correctly with the sensor.

Poor alignment can cause:

Missed pulses
Signal instability
Position errors

PCB Placement

Keep:

Stable spacing
Minimal vibration
Proper shielding from motor noise

Good PCB layout improves signal reliability.

Signal Filtering

Motor noise can introduce encoder errors.

Common solutions:

Pull-up resistors
Capacitors
Software debouncing
Hardware filtering

Summary Table

Parameter	Value
Part Number	AS5000-MR20-44
Shape	Ring
OD × ID × Thickness	32 mm × 24 mm × 1.5 mm
Number of Poles	44
Pole Length	2.0 mm
Material	Plastic Bonded Ferrite
Magnet Type	Permanent
Compatible IC	AS5304 / AS5306
Application	Encoder / Position Sensor

Combining AS5304 with STM32 Encoder Mode

The STM32 timer encoder interface works exceptionally well with the AS5304.

Advantages include:

Hardware pulse counting
Reduced CPU usage
Accurate direction detection
High-speed operation

Popular STM32 MCUs used in Micromouse:

STM32F411CEU6 ->
STM32F405RG
STM32G431CBU6

Micromouse encoder coding :

// units are all in counts and counts per second
void CDriveEncoders::update_encoders() {

	int32_t left_delta;
	int32_t right_delta;

	float left_change ;
	float right_change ;

	// write code here what you need

	left_total  =  LEnc();
	right_total =  REnc();

	left_delta  =  LEnc()- prevLEnc;
	right_delta =  REnc()- prevREnc;

	prevLEnc = left_total ;
	prevREnc = right_total;

	left_change  = left_delta  * MM_PER_COUNT_LEFT;
	right_change = right_delta * MM_PER_COUNT_RIGHT;

	s_robot_fwd_increment = 0.5f * (right_change + left_change);
	s_robot_rot_increment = (right_change - left_change) * DEG_PER_MM_DIFFERENCE;

	s_robot_position += s_robot_fwd_increment;
	s_robot_angle    += s_robot_rot_increment;

}

PID Controll coding :

float CBody::position_controller() {

	objMindForward.update();

	// write code here what you need
	fwd_error += objMindForward.increment() - objArms.robot_fwd_increment();
	float diff = fwd_error - old_fwd_error;
	old_fwd_error = fwd_error;
	float output = FWD_KP * fwd_error + FWD_KD * diff;

	return output;
}

float CBody::angle_controller() {

	objMindRotation.update();

	// write code here what you need
	rot_error += objMindRotation.increment() - objArms.robot_rot_increment();
	if (objEyes.steering_enabled) {
		rot_error += steering_adjustment;
	}
	float diff = rot_error - old_rot_error;
	old_rot_error = rot_error;
	float output =ROT_KP * rot_error + ROT_KD * diff;

	return output;
}

Conclusion

The AS5304A magnetic encoder is an excellent choice for Micromouse robots requiring precise motor feedback in a compact form factor. Its magnetic sensing method provides strong resistance to dust and vibration while delivering reliable quadrature signals for accurate motion control.

When combined with an STM32 microcontroller and proper PID algorithms, the AS5304 enables:

Stable high-speed driving
Precise turning
Accurate odometry
Improved maze-solving performance

For modern Micromouse designs focused on speed, precision, and reliability, magnetic encoders like the AS5304 are becoming increasingly popular.

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