ANALOG CONTROL PROJECT

DC Motor Speed Controller

Closed-loop analog PI control that holds a brushed DC motor at commanded speed under changing load.

For this electronics design project I designed, simulated, and built an electronic speed controller for a small brushed DC motor. A Hall-effect sensor on the shaft acts as a tachometer, a frequency-to-voltage converter turns its pulses into a feedback voltage, and an op-amp PI controller drives a MOSFET output stage from a 15 V supply so the shaft speed tracks the setpoint. The finished loop held steady-state speed within the ±10% requirement and switched cleanly between 2000 and 2400 rpm.

PI Control

Op-Amp Design

LTspice Simulation

Oscilloscope Testing

Sensor Feedback

LTspice schematic of the closed-loop DC motor speed controller with difference amplifier, PI controller, and MOSFET driver

The control loop, end to end

Difference amplifier, PI controller, MOSFET driver, and a simulated motor + tachometer model in LTspice.

View figures & test data ↓

How the Control Loop Works

Six stages take the loop from a voltage command to a regulated shaft speed. Every stage was designed at the component level and verified in simulation before the build.

Setpoint Command

An analog voltage Vset commands the target speed. Stepping it between 0.5 V and 0.6 V corresponds to switching the motor between roughly 2000 and 2400 rpm.

Hall-Effect Tachometer

A magnet on the motor shaft sweeps past a Hall-effect sensor once per revolution, producing a pulse train whose frequency is proportional to shaft speed.

Frequency-to-Voltage Converter

Converts the tachometer pulses into the feedback voltage Vref, designed for 1 V per 67 Hz and bench-calibrated at 65.49 Hz per volt.

Error Amplifier

A unity-gain difference amplifier with matched resistors computes the loop error directly: Vdiff = Vref − Vset.

PI Controller

A second op-amp referenced to ground integrates the error. R6/R5 sets a proportional gain of 56, and C1 smooths how fast the loop corrects.

MOSFET Motor Driver

A high-side source follower feeds the motor from the 15 V supply. As the controller output rises, the gate-source voltage self-adjusts and the drive settles at the required level.

Figures & Test Data

Simulation plots and oscilloscope captures from the report, spanning the first LTspice sweep to the measurements that verified the ±10% speed requirement.

LTspice schematic of the complete loop: difference amp, PI controller, MOSFET driver, and behavioral models for the motor and tachometer.

Simulated motor drive voltage step response with proportional gain of 56 showing fast settling

Simulated drive voltage snapping to the new operating point at a proportional gain of 56. Lowering the gain to 5 made settling visibly slower.

Simulation of feedback voltage Vref tracking the setpoint Vset between 500 mV and 600 mV

Vref locking onto Vset for 500 mV to 600 mV setpoint steps, confirming the loop model reaches steady state with matching voltages.

Measured feedback voltage Vref alternating between the 0.5 V and 0.6 V setpoints on the bench

Bench measurement of Vref alternating between the 0.5 V and 0.6 V setpoints, the basis for the ±10% verification statistics.

Oscilloscope capture of the frequency-to-voltage converter output during calibration

Oscilloscope capture of the frequency-to-voltage converter output; a 67 Hz input measured 1.023 V, a 2.3% deviation from the design target.

Oscilloscope capture of Vref and Vset while stepping the speed setpoint

Vref and Vset captured together while stepping the speed command, showing the feedback following each transition.

Design & Verification Process

Requirements & Targets

Defined the control goal: accept an analog speed command, hold steady-state speed within ±10% of the setpoint across the load range, and alternate cleanly between 2000 and 2400 rpm.

Controller Design

Built a unity-gain difference amplifier to compute the speed error and paired it with an op-amp PI stage — R6/R5 proportional gain plus an integrating capacitor — forming the complete analog controller.

Simulation in LTspice

Modeled the motor dynamics, Hall sensor, and F-to-V converter with behavioral sources, swept the proportional gain (56 vs. 5), and confirmed Vref settles onto Vset at steady state.

Frequency-to-Voltage Calibration

Drove the converter with known sine inputs on the bench; a 67 Hz input produced 1.023 V, giving a measured conversion ratio of 65.49 Hz per volt used to map voltage to rpm.

Bench Verification

Captured Vset and Vref on the oscilloscope while stepping the setpoint. At both operating points, over 97% of samples stayed within the ±10% requirement.

Load Testing

Progressively increased the mechanical load to find the limits of the loop, pushing the motor up to 100 mA of drive current before it could no longer follow the setpoint.

Measured Results

Verification against the project requirements: the steady-state difference between Vset and Vref stayed within ±10% at every tested operating point.

0.5 V SETPOINT

0.4926 V

Mean measured Vref of 0.4926 V with 97.6% of samples inside the ±10% band. Requirement met.

0.6 V SETPOINT

0.5920 V

Mean measured Vref of 0.5920 V with 97.9% of samples inside the ±10% band. Requirement met.

SPEED RANGE

2026–2433 rpm

Using the measured 65.49 Hz/V conversion ratio, the operating points map to shaft speeds satisfying the 2000/2400 rpm targets.

F-TO-V RATIO

65.49 Hz/V

Bench-calibrated conversion ratio, 2.3% from the 67 Hz/V design value, small enough to verify speed directly from Vref.

Possible Improvements

Where the design goes next. Each item comes straight from what the simulation and bench data revealed.

Tighter F-to-V Calibration

Adjust the converter resistor values so the measured conversion ratio lands on the 67 Hz/V design target instead of 2.3% below it.

Full PID Control

Add a differentiator to the existing PI stage to damp transients faster and measure the improvement in setpoint response.

PWM Motor Drive

Replace the linear MOSFET follower with a PWM-based driver for higher efficiency and a wider usable control range.

Output Filtering

Add an RC low-pass filter after the F-to-V converter to deliver a smoother, less noisy Vref into the error amplifier.

Skills Demonstrated

The engineering capabilities this project exercised, from first schematic to verified hardware.

Analog Circuit Design

Designed op-amp difference and PI stages and a high-side MOSFET drive stage running from ±15 V supplies.

Control Systems

Applied closed-loop feedback theory: error amplification, proportional-integral tuning, and steady-state error analysis.

SPICE Simulation

Built behavioral models of motor dynamics and sensor stages in LTspice and swept controller gains before committing to hardware.

Test & Measurement

Captured and interpreted oscilloscope traces, then verified performance statistically against a ±10% specification.

Sensor Integration

Implemented a Hall-effect tachometer with frequency-to-voltage conversion and calibrated it against known inputs.

Data Analysis

Quantified tracking accuracy across operating points, converting raw measurements into requirement-level pass/fail evidence.

What I Learned

This project connected control theory to physical hardware. Concepts like proportional gain and integral action became visible behavior on an oscilloscope: overshoot, settling, and steady-state error I could tune with real resistor values. Just as importantly, it taught me to verify: not to claim the loop works, but to measure how well, against a written requirement.

Practical takeaways

Proportional gain trades response speed against stability. A gain of 56 settled fast where 5 crawled.
Hardware never matches design values exactly; measure the real conversion ratio and use it.
Oscilloscope statistics turn a working demo into quantitative, requirement-level evidence.
Behavioral SPICE models of motors and sensors make simulation predict bench results closely.
Every driver stage has a load limit. Find it deliberately instead of discovering it by accident.

Outcome Summary

The closed-loop controller was tested by measuring the command voltage Vset against the feedback voltage Vref at two operating points, 0.5 V and 0.6 V. Average Vref values of 0.493 V and 0.592 V landed within the ±10% requirement, and applying the measured 65.49 Hz/V conversion ratio put the shaft between 2026 and 2433 rpm, meeting the project requirement for commanded motor speed.

Why This Project Matters

This is a complete feedback system, from sensor and signal conditioning to controller and power stage, designed, simulated, built, and verified against a written specification. The same loop appears in industrial motor drives, robotics joints, cooling systems, and precision actuators. Delivering it end to end demonstrates I can move between control theory, circuit design, and disciplined lab verification on one problem.

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