Linear motor controller

• US 6,812,597 B2
• Filed: 11/13/2002
• Issued: 11/02/2004
• Est. Priority Date: 11/20/2001
• Status: Expired due to Fees

• Current Assignee: Fisher & Paykel Appliances Limited (Fisher & Paykel Appliances Holdings Limited)
• Original Assignee: Fisher & Paykel Appliances Limited (Fisher & Paykel Appliances Holdings Limited)
• Inventors: Tian, Zhuang, McGill, Ian
• Primary Examiner(s): Schuberg, Darren
• Assistant Examiner(s): Mohandesi, Iraj A.
• Application Number:
  US10/293,874
• Publication Number:
  US 20030173834A1
• Time in Patent Office: 720 Days
• Field of Search: 310/12, 310/13, 310/15, 318/135, 417/44.1
• US Class Current: 310/12.19
• CPC Class Codes: F04B 2201/0209 Duration of piston stroke
• CPC Class Codes: F04B 35/045 using solenoids

Abstract

A free piston gas compressor comprising a cylinder, a piston reciprocable within the cylinder and a reciprocating linear electric motor derivably coupled to the piston having at least one excitation winding. A measure of the reciprocation time of the piston is obtained, any change in the reciprocation time is detected and the power input to said excitation winding is adjusted in response to any detected change in reciprocation time.

Claims

1. A free piston gas compressor comprising:

a cylinder,

a piston,

said piston reciprocable within said cylinder,

a reciprocating linear electric motor having at least one excitation winding, said motor drivably coupled to said piston and having a reciprocation period,

means for obtaining an indicative measure of the reciprocation period of said piston,

means for detecting any sudden change in said reciprocation period, a reduction indicative of a piston collision with the cylinder head, and

means for reducing the power input to said excitation winding in response to any sudden change in reciprocation period.

2. A free piston gas compressor as claimed in claim 1 wherein said motor is an electronically commutated permanent magnet DC motor.

3. A free piston gas compressor as claimed in either of claim 1 or 2 wherein said means for obtaining a measure of the reciprocation period of said piston comprise:

a back EMF detection means for sampling the back EMF induced in said at least one excitation winding when exciting current is not flowing,

zero crossing detection means connected to the output of said back EMF detection means and

timing means which determine the time interval between zero crossing detection means to thereby determine the time of each half cycle of the reciprocation of said piston, and means for summing two successive half cycle times to provide said reciprocation period.

4. A free piston gas compressor as claimed in claim 3 wherein said means for detecting any sudden change in reciprocation period includes means to produce separate filtered or smoothed values of the times of alternate half cycles, means for summing the two smoothed values of alternate half cycle times to produce a smoothed value of reciprocation period, means to compare the most recent measured reciprocation period with said smoothed value of reciprocation period, to provide a difference value and means to determine if said difference value is above a predetermined threshold for a predetermined period.

11. A free piston gas compressor as claimed in claim 1 or 2, further including means for incrementally increasing the power input to said motor over a period of time in response to a reduction in power input.

12. A free piston gas compressor as claimed in claim 3, further including means for incrementally increasing the power input to said motor over a period of time in response to a reduction in power input.

13. A free piston gas compressor as claimed in claim 4, further including means for incrementally increasing the power input to said motor over a period of time in response to a reduction in power input.

5. A controller for a linear motor including a reciprocating portion and having a reciprocation period, the improvement comprising said controller being configured to:

determine the reciprocation period of said reciprocating portion,

detect any change in said reciprocation period, and

adjust the power input to said linear motor in response to any detected reduction in reciprocation period.

6. A controller as claimed in claim 5 wherein said reciprocating portion comprises the armature of a linear motor.

7. A controller as claimed in claim 5 or 6 wherein said step of determining said reciprocation period includes the step of detecting zero crossings of the current in a linear motor and determining said reciprocation period from the time interval there between.

8. A controller as claimed in claim 5 wherein said step of detecting any change in said reciprocation period includes the step of deducting said reciprocation period from a filtered or smoothed value, to provide a difference value and if said difference value is above a predetermined threshold for a predetermined period, reducing the power input to said linear motor.

14. A controller as claimed in claim 5 wherein said controller is configured to detect any sudden change in said reciprocation period.

9. A controller as claimed in 6 wherein said step of detecting any change in said reciprocation period includes the step of deducting said reciprocation period from a filtered or smoothed value, to provide a difference value and if said difference value is above a predetermined threshold for a predetermined period, reducing the power input to said linear motor.

10. A controller as claimed in 7 wherein said step of detecting any change in said reciprocation period includes the step of deducting said reciprocation period from a filtered or smoothed value, to provide a difference value and if said difference value is above a predetermined threshold for a predetermined period, reducing the power input to said linear motor.