PWM Waveform Sampling with Low-Pass Filter and ADC

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Solution Overview

Problem

Existing methods for recovering the analog representation of a pulse width modulated (PWM) waveform introduce phase lag, especially when the frequency content approaches the carrier frequency, making them unsuitable for feedback control systems, and require complex circuits for accurate measurement.

Innovation Solution

A device using a low-pass filter and a non-integrating analog-to-digital converter that samples the PWM waveform at the pulse frequency rate, preferably at the center or edges of the pulse, to reject the carrier frequency and minimize phase lag, employing a simple first-order RC circuit or higher-order filters with compensation for phase lag.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a high order low pass linear time invariant filter is used to reject carrier frequency, then filtering accuracy is improved, but phase lag increases

Engineering Contradiction:

Improvefiltering accuracyVSAvoidphase lag

Core Design Contradiction:

Measurement precisionVSLoss of time

Solution Approach 1:

The patent segments the filtering process into two distinct stages: first, a low-order analog low-pass filter removes the carrier frequency components; second, a digital filter processes the sampled signal to achieve the remaining filtering requirements. This segmentation allows each filter to operate at lower orders, reducing cumulative phase lag while maintaining overall filtering accuracy.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent replaces part of the analog filtering mechanism with a digital filtering approach. By sampling the signal after a simple analog low-pass filter and then applying digital filtering in the digital domain, the system achieves high-order filtering effects without the phase lag penalties of high-order analog filters, since digital filters can be designed with linear phase characteristics.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Device complexity

If a simple first order RC circuit is used as low pass filter, then device complexity is reduced, but filtering performance deteriorates

Engineering Contradiction:

Improvefilter circuit complexityVSAvoidfiltering performance

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The patent introduces a digital sampling and processing stage as an intermediary between the simple analog low-pass filter and the final output. The analog filter performs initial carrier rejection, then the digital sampling and filtering stages complete the filtering process. This intermediary digital stage compensates for the limited filtering performance of the simple analog filter while keeping the analog hardware complexity low.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Use of energy by moving object

If PWM waveform is used for high-frequency waveform production, then power efficiency is improved, but recovery filter requirements become stricter

Engineering Contradiction:

Improvepower efficiencyVSAvoidrecovery filter requirements

Core Design Contradiction:

Use of energy by moving objectVSDevice complexity

Solution Approach 1:

The patent replaces complex analog recovery filtering with a hybrid analog-digital approach. A simple analog low-pass filter performs initial carrier rejection, then an analog-to-digital converter samples the signal, and digital signal processing completes the recovery process. This substitution of digital processing for analog filtering reduces the complexity and order requirements of the analog filter while maintaining effective PWM waveform recovery.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This approach effectively converts PWM waveforms to analog signals with reduced phase lag and harmonic attenuation, enabling accurate feedback control in systems like self-balancing personal vehicles by accurately measuring actual load voltages and compensating for errors, thus improving system performance.

Implementation Method 1

The device includes a low-pass filter for converting the PWM waveform into an analog signal

Methodology Applied

Scientific EffectLow-pass filtering: Filter (electronic)

Implementation Method 2

The low pass filter is a simple first order RC circuit

Methodology Applied

Scientific EffectRC filtering: Filter (electronic)

Data Source

PatentUS8072180B1Synchronous sampling of PWM waveforms

Publication Date: 2011.12.06 DEKA PRODUCTS LP

AI summary

A device for filtering a carrier frequency of a PWM waveform. The device includes a low-pass filter for converting the PWM waveform into an analog signal. The device further includes a sampling analog-to-digital converter receiving the analog signal from the low-pass filter and the device also includes a controller for causing the non-integrating analog to digital converter to sample the PWM waveform at the pulse frequency rate thereby rejecting the carrier frequency. As already stated the PWM waveform includes a pulse which has a leading edge, a top portion, and a trailing edge. In one embodiment, the controller causes the A/D converter to sample during the top of the pulse. Preferably, the controller causes the A/D converter to sample at substantially the center of the pulse.