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A drive unit for a carrier that includes a chassis with a drive wheel that is pivotable with an air spring in and out of contact with a surface over which the carrier is propelled. The air spring is in a pneumatic system including an air compressor which is connected to a processor in an electrical

A drive unit for a carrier that includes a chassis with a drive wheel that is pivotable with an air spring in and out of contact with a surface over which the carrier is propelled. The air spring is in a pneumatic system including an air compressor which is connected to a processor in an electrical system. The compressor is programmed to provide a fair safe mode for the drive wheel, a runtime control for the air compressor and a battery voltage sensing circuit to shut down the electrical system during periods of inactivity.

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1. A drive unit for propelling a carrier across a supporting surface, said carrier having a chassis on casters, said drive unit comprising a drive wheel connected to an electric motor for rotating the wheel,said drive wheel mounted for rotation on a pivot arm attached to the chassis,said pivot arm r

1. A drive unit for propelling a carrier across a supporting surface, said carrier having a chassis on casters, said drive unit comprising a drive wheel connected to an electric motor for rotating the wheel,said drive wheel mounted for rotation on a pivot arm attached to the chassis,said pivot arm resiliently biased by an extension spring such that the drive wheel is out of contact with the supporting surface,an air spring for counterbalancing the extension spring and biasing the drive wheel into contact with the supporting surface,said air spring connected to an air compressor in a pneumatic system,said air compressor in an electrical system with a battery power source and a processor,said processor having a logic board with a compressor timer programmed with a compressor timer to provide runtime control of the compressor, said compressor timer keeping track of period of time the compressor has run in a duty cycle and the period of time that the compressor has been off between duty cycles. 2. The drive unit of claim 1 wherein the pneumatic system includes a pressure switch which sends a signal to the processor if the pressure in the pneumatic system falls below a predetermined pressure, said processor starting the air compressor in response to said signal to bring the pressure in the pneumatic system back to the predetermined pressure under control of the compressor timer. 3. A drive unit for propelling a carrier across a supporting surface, said carrier having a chassis on casters, said drive unit comprising a drive wheel connected to an electric motor for rotating the wheel,said drive wheel mounted for rotation on a pivot arm attached to the chassis,said pivot arm resiliently biased by an extension spring such that the drive wheel is out of contact with the supporting surface,an air spring for counterbalancing the extension spring and biasing the drive wheel into contact with the supporting surface,said air spring connected to an air compressor in a pneumatic system,said air compressor in an electrical system with the electric motor for the drive wheel, a battery power source and a processor,said processor including a logic board with a compressor timer programmed to provide run time control of the compressor and programmed with a motor timer to open the electrical system shutting down the unit when the electric motor for the drive wheel has not operated for a predetermined period of time. 4. The drive unit of claim 3 wherein the a voltage sensor is connected to the battery, said processor programmed to set the motor timer to open the electrical system when the compressor is not running and no voltage change is sensed above a threshold level in predetermined period of time. 5. The drive unit of claim 3 wherein a pair of minor image drive units are provided on the carrier, the electric motor of each drive wheel under independent motor control through the processor. 6. A carrier with a chassis on casters and a pair of mirror image drive units for propelling the carrier across a supporting surface, each of said drive units comprising: a drive wheel connected to an electric motor for rotating the wheel,said drive wheel mounted for rotation on a pivot arm attached to the chassis,said pivot arm resiliently biased by an extension spring such that the drive wheel is out of contact with the supporting surface,an air spring for counterbalancing the extension spring and biasing the drive wheel into contact with the supporting surface,said air spring connected to an air compressor in a pneumatic system including a normally closed air relief valve under control of a processor,said processor in a battery powered electrical system with an on/off switch, said processor having a logic board programmed with a compressor timer to provide runtime control of the compressor and to send a signal opening the normally closed air relief valve when the switch is moved from on to off position, said compressor timer keeping track of period of time the compressor has run in a duty cycle and the period of time that the compressor has been off between duty cycles,said processor further programmed to open the electrical system shutting down the unit when the electric motors for the drive wheel have not operated for a predetermined period of time. 7. The carrier of claim 6 wherein the normally closed relief valve remains closed if there is an unintended loss of power in the electrical system thereby maintaining pressure in the pneumatic system and the traction of the drive wheel against the supporting surface. 8. The drive unit of claim 6 wherein the pneumatic system includes a pressure switch which sends a signal to the processor if the pressure in the pneumatic system falls below a predetermined pressure, said processor starting the air compressor in response to said signal to bring the pressure in the pneumatic system back to the predetermined pressure under control of the compressor timer. 9. The drive unit of claim 6 wherein the processor includes a logic board with a motor timer. 10. The drive unit of claim 9 wherein the a voltage sensor is connected to the battery, said processor programmed to set the motor timer to open the electrical system when the compressor is not running and no voltage change is sensed above a threshold level in predetermined period of time.