Back-EMF Detection in Two-Terminal Actuators
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Solution Overview
Problem
Existing methods for measuring back-emf (BEMF) in actuators are inaccurate, often requiring tri-stated actuator terminals or sense resistors, leading to errors and inefficiencies, especially in two-terminal actuators where direct sensing is challenging and current/voltage measurements are prone to errors due to noise and temperature variations.
Innovation Solution
Measuring BEMF by detecting the voltage across the input terminals of a two-terminal actuator during a high frequency driver's recirculation phase when the instantaneous voltage is zero, and determining current crossings near zero to minimize interference from the driver input, allowing for precise BEMF measurement without additional sense components or complex digital logic.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If direct sensing of BEMF is used by tri-stating actuator terminals, then BEMF measurement can be obtained, but the motor driver must be cut off causing ringing and measurement errors, and 3-Terminal motors are required
Solution Approach 1:
The patent applies preliminary action by pre-establishing a recirculation path for motor current before the measurement phase. The body diode of the low-side switch is configured to conduct motor current during the recirculation phase, ensuring continuous current flow and preventing voltage spikes or ringing that would occur if the driver were simply cut off. This preliminary current path setup enables stable BEMF measurement without disrupting the motor's electrical state.
Solution Approach 2:
The patent introduces an intermediary measurement approach by measuring BEMF indirectly through the voltage across the low-side switch during its off-state, rather than directly sensing the back-emf voltage. The control circuit determines BEMF by monitoring the voltage waveform characteristics during the recirculation phase, using the switch voltage as an intermediary signal that contains BEMF information without requiring direct BEMF sensing that would disrupt motor operation.
2Measurement precision
If voltage and current measurements are used to estimate BEMF, then BEMF can be derived, but sense resistors are required reducing efficiency and area, and calculation errors accumulate
Solution Approach 1:
The patent extracts and eliminates the need for separate sense resistors by utilizing the existing low-side switch and its inherent voltage measurement capability. Instead of adding external sensing components, the invention uses the voltage across the low-side switch during its off-state as the measurement signal, which naturally contains BEMF information. This extraction approach removes unnecessary components while maintaining measurement functionality.
Solution Approach 2:
The patent implements self-service by enabling the motor driver's own components (the low-side switch and control circuit) to perform the BEMF measurement function. The driver circuit uses its existing voltage measurement capability to monitor the switch voltage and derive BEMF information, making the driver self-sufficient for measurement purposes without requiring external sense resistors or separate measurement hardware.
3Measurement precision
If voltage and current measurements are used to estimate BEMF, then BEMF can be calculated, but measurement accuracy is limited to 10-20% due to noise and temperature variations
Solution Approach 1:
The patent applies periodic action by measuring BEMF at specific periodic intervals during the motor's operating cycle - specifically during the recirculation phase when the low-side switch is turned off. This periodic measurement approach captures BEMF information at optimal moments when the electrical conditions are most favorable for accurate measurement, avoiding periods with high noise or transient effects that would degrade accuracy.
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 achieves BEMF measurement accuracy of less than 2% with reduced error and increased efficiency, eliminating the need for additional sense components and simplifying signal processing, while maintaining continuous actuator operation without disrupting the driver input.
Implementation Method 1
When an electric actuator or motor is in motion, there is relative motion between the armature of the motor and a magnetic field in the motor. This relative motion generates a voltage within the motor that opposes the applied drive voltage. This motor-generated voltage is known as the Back-emf (BEMF)
Data Source
AI summary
Back-emf for a motor is measured by measuring a voltage across the input terminals for a two terminal input actuator or motor when a high frequency driver having a recirculation phase for the motor applies an instantaneous high frequency zero voltage during normal operation of the driver. By a further approach, current across the two terminal input is measured such that the measurement can be taken when the current crosses a given threshold near zero.