Motor Controller for Free Piston Linear Compressor

Computing & Wireless : Application Software

Available for licensing


  • Michael Lewis, B.S. , Center for Electromechanics
  • Clay Hearn, B.S. , Center for Electromechanics
  • Charles Penney, B.S. , Center for Electromechanics
  • Raymond Zowarka, Jr. , Center for Electromechanics

Background/unmet need

Natural gas home refueling is critical to the widespread adoption of natural gas vehicles and presents a unique opportunity for consumers to save significantly on the cost of fuel, with a $2 per gallon equivalent advantage over gasoline and diesel. However, traditional home refueling appliances (HRAs) have relied on multi-stage reciprocating compressors driven by a rotary motor. These systems are complicated, expensive, and have historically suffered poor reliability.

The Free Piston Compressor solves these problems by using a linear motor to drive a single, multi-stage piston. Not only is complexity and part count reduced, which improves overall reliability, but a unique coating and seal material combination results in low friction and wear in natural gas environments. Furthermore, efficiency of the linear motor compressor is improved by operation at a resonant frequency.

Invention Description

The invented software provides motion control for natural gas free piston linear compressor developed under ARPA-E funding. This controller is based on a state-feedback architecture and uses a Luenberger Observer. The software uses motor current and encoder measurements to estimate piston position and velocity. These position and velocity estimates are used by the controller to estimate force and current requirements for the motor coils so that the piston tracks a smooth sinusoidal linear path.


  • No rotary to linear motion conversion reduces part count
  • Use of single piston for multiple stage compression reduces part count
  • Reduces seal wear, increases seal life
  • Reduces maintenance
  • Control algorithm allows removal of mechanical spring typically used in linear motor compressor for resonant frequency operation.
  • Reduces costs and increases overall reliability of natural gas compressor for vehicles


  • Multiple stages of compression (two or more) with a single piston
  • Stage geometry allows for reduced pressure differential across seals.
  • Dual acting compression stroke
  • Vapor deposition carbon coating on piston and/or cylinder allows for ultra-low friction and wear in natural gas when used with Teflon seals with carbon additives.
  • Single free piston
  • Advanced controls allow for operation without mechanical springs.
  • Operates at resonant frequency based on mass and dynamic gas spring to increase system efficiency: no mechanical spring required

Market potential/applications

Other potential markets, besides home refueling include: CNG fleets where a low cost compressor could be paired with each vehicle, hydrogen vehicles (although this is market is less developed than CNG), air compressors for SCBA and SCUBA, and refrigerator compressors.

Development Stage

Beta product/commercial prototype