GB2185785A

GB2185785A - Gear pump type liquid flow meter

Info

Publication number: GB2185785A
Application number: GB08601852A
Authority: GB
Inventors: Victor Humberstone; Dexter Plummer
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1986-01-25
Filing date: 1986-01-25
Publication date: 1987-07-29
Also published as: GB2185785B; GB8601852D0

Abstract

The rotation of the pump gears 12, 14 is monitored to give a reading of liquid flow. The pump is maintained with a zero pressure drop across it, and in order to maintain this zero pressure drop in spite of possibly increasing or decreasing liquid demands by the liquid consumer, a sensing piston 32 is arranged in parallel with the pump. The sensing piston will move when a pressure difference occurs across the pump, and a signal is then produced representative of the sensing piston position which alters the speed of revolution of the gears in the pump, and which is also summed with the signal denoting the rotation or speed of the gears in order to produce a corrected total figure for liquid flow. <IMAGE>

Description

SPECIFICATION Liquid flow meter This invention relates to a liquid flow meter, and in particular to a positive displacement volume flow meter of the gear pump type, which is servoed to give a zero pressure drop across the pump, thus minimising leakage losses across the meter.

In such a pump, it is known to use a sensing piston to indicate the pressure differential across the gear pump and to use a photosensor to detect movement of the sensing piston.

When movement of the sensing piston is detected, a signal is sent to increase or decrease the speed of the pump, so as to bring the piston back to a central position.

Movement of the sensing piston however involves the introduction of an error into the meter, as a small volume of liquid is not passed to the consumer but instead is stored in the apparatus and is then reintroduced into the output in an uncontrolled manner.

According to the present invention there is provided a positive displacement liquid flow meter comprising a gear pump and means for measuring the angular rotation of the pump gears so as to measure the volume flow across the pump, the meter also having means for maintaining a constant pressure across the pump, which means include a cylinder connected in parallel with the pump with its ends in fluid communication with opposite sides of the pump, a piston freely reciprocable in the cyclinder and a piston position detector which provides a signal indicative of the piston position, which signal is used to make a correction in the flow rate measured by the rotation measuring means.

Preferably the piston position detector is a Moire transducer which permits high resolution measurement.

In conventional gear pumps, backlash in the gears will result in errors in the final volume flow measurement, and it is therefore preferred to use anti-backlash gears.

In order to provide a high level of resolution on the reading of the rotary movement of the gears, a rotary Moire encoder is preferred.

The invention will now be further described, by way of example, with reference to the accompanying drawing which is a schematic diagram of a meter in accordance with the invention.

The meter comprises a gear pump 10 with two inter-meshing gears 12 and 14. The gear 12 is driven by a shaft 16, and drives the gear 14. The pump has an inlet 18 and an outlet 20, and liquid to be metered enters the system at 22 and leaves at 24. The shaft 16 carries a disk 26 which rotates with the shaft whilst an encoder head 28 measures the degree of annular rotation of the shaft. The angular rotation of the shaft and thus of the gears 12 and 14 is proportional to, and by suitable calibration, can represent the volume flow of liquid through the pump.

In parallel with the pump 10 is a cylinder 30 containing a piston 32. A piston position sensor 34 monitors the position of the piston 32 in the cylinder 30.

In operation, liquid entering at the inlet 22 is passed through the pump 10 to the outlet 24.

At the same time the liquid pressure in the inlet and in the outlet act respectively on opposite ends of the piston 32. The meter is normally set up so that there is zero pressure across the pump 10, but should this change as a result of increased or reduced demand at the outlet 24, then the pressures acting on the opposite ends of the piston 32 will be out of balance and the piston 32 will move. This movement is detected by the sensor 34, and a signal is fed to the motor driving the drive shaft 16 to either speed up or slow down the pump 10 in order to remove the pressure difference across the pump.

The gears 12 and 14 are anti-backlash gears.

The encoder 26, 28 is a rotary Moire encoder with a resolution of 4000 pulses/revolutions. This can achieve an accuracy of +/- 0.25%.

The position of the piston 32 is monitored by the sensor 34 which uses a Moire transducer which can give a signal denoting the instantaneous position of the piston. This allows an accurate signal to be returned to the drive mechanism so that the drive speed can be changed by the right amount and in the right direction.

Furthermore, each time the piston moves (say to the left in the figure) a small amount of additional liquid will flow through the pump which does not flow directly to the output 24.

The movement of the piston in this way will give rise to inaccuracies in transient measurements of liquid consumption, but to avoid these errors occurring the signal from the sensor 34 is electronically combined with the signal from the encoder 28 and these signals are summed to give an accurate reading which takes account of the movement of the piston 32. This has an additional advantage in that there is no longer any need to minimise the amplitude of movement of the piston, as would be the case if no compensation was made for errors arising from puiston movement. A large amplitude of movement allows a high resolution reading of the piston position to be taken and reduces any possibility of the piston sticking.

1. A positive displacement liquid flow meter comprising a gear pump and means for measuring the angular rotation of the pump gears so as to measure the volume flow across the pump, the meter also having

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Liquid flow meter This invention relates to a liquid flow meter, and in particular to a positive displacement volume flow meter of the gear pump type, which is servoed to give a zero pressure drop across the pump, thus minimising leakage losses across the meter. In such a pump, it is known to use a sensing piston to indicate the pressure differential across the gear pump and to use a photosensor to detect movement of the sensing piston. When movement of the sensing piston is detected, a signal is sent to increase or decrease the speed of the pump, so as to bring the piston back to a central position. Movement of the sensing piston however involves the introduction of an error into the meter, as a small volume of liquid is not passed to the consumer but instead is stored in the apparatus and is then reintroduced into the output in an uncontrolled manner. According to the present invention there is provided a positive displacement liquid flow meter comprising a gear pump and means for measuring the angular rotation of the pump gears so as to measure the volume flow across the pump, the meter also having means for maintaining a constant pressure across the pump, which means include a cylinder connected in parallel with the pump with its ends in fluid communication with opposite sides of the pump, a piston freely reciprocable in the cyclinder and a piston position detector which provides a signal indicative of the piston position, which signal is used to make a correction in the flow rate measured by the rotation measuring means. Preferably the piston position detector is a Moire transducer which permits high resolution measurement. In conventional gear pumps, backlash in the gears will result in errors in the final volume flow measurement, and it is therefore preferred to use anti-backlash gears. In order to provide a high level of resolution on the reading of the rotary movement of the gears, a rotary Moire encoder is preferred. The invention will now be further described, by way of example, with reference to the accompanying drawing which is a schematic diagram of a meter in accordance with the invention. The meter comprises a gear pump 10 with two inter-meshing gears 12 and 14. The gear 12 is driven by a shaft 16, and drives the gear 14. The pump has an inlet 18 and an outlet 20, and liquid to be metered enters the system at 22 and leaves at 24. The shaft 16 carries a disk 26 which rotates with the shaft whilst an encoder head 28 measures the degree of annular rotation of the shaft. The angular rotation of the shaft and thus of the gears 12 and 14 is proportional to, and by suitable calibration, can represent the volume flow of liquid through the pump. In parallel with the pump 10 is a cylinder 30 containing a piston 32. A piston position sensor 34 monitors the position of the piston 32 in the cylinder 30. In operation, liquid entering at the inlet 22 is passed through the pump 10 to the outlet 24. At the same time the liquid pressure in the inlet and in the outlet act respectively on opposite ends of the piston 32. The meter is normally set up so that there is zero pressure across the pump 10, but should this change as a result of increased or reduced demand at the outlet 24, then the pressures acting on the opposite ends of the piston 32 will be out of balance and the piston 32 will move. This movement is detected by the sensor 34, and a signal is fed to the motor driving the drive shaft 16 to either speed up or slow down the pump 10 in order to remove the pressure difference across the pump. The gears 12 and 14 are anti-backlash gears. The encoder 26, 28 is a rotary Moire encoder with a resolution of 4000 pulses/revolutions. This can achieve an accuracy of +/- 0.25%. The position of the piston 32 is monitored by the sensor 34 which uses a Moire transducer which can give a signal denoting the instantaneous position of the piston. This allows an accurate signal to be returned to the drive mechanism so that the drive speed can be changed by the right amount and in the right direction. Furthermore, each time the piston moves (say to the left in the figure) a small amount of additional liquid will flow through the pump which does not flow directly to the output 24. The movement of the piston in this way will give rise to inaccuracies in transient measurements of liquid consumption, but to avoid these errors occurring the signal from the sensor 34 is electronically combined with the signal from the encoder 28 and these signals are summed to give an accurate reading which takes account of the movement of the piston 32. This has an additional advantage in that there is no longer any need to minimise the amplitude of movement of the piston, as would be the case if no compensation was made for errors arising from puiston movement. A large amplitude of movement allows a high resolution reading of the piston position to be taken and reduces any possibility of the piston sticking. CLAIMS

1. A positive displacement liquid flow meter comprising a gear pump and means for measuring the angular rotation of the pump gears so as to measure the volume flow across the pump, the meter also having means for maintaining a constant pressure across the pump, which means include a cylinder connected in parallel with the pump with its ends in fluid communication with opposite sides of the pump, a piston freely reciprocable in the cylinder and a piston position detector which provides a signal indicative of the piston position, which signal is used to make a correction in the flow rate measured by the rotation measuring means.

2. A flow meter as claimed in Claim 1, wherein the piston position detector is a Moire transducer which permits high resolution measurement.

3. A flow meter as claimed in Claim 1 or Claim 2, wherein the pump gears are antibacklash gears.

4. A flow meter as claimed in any preceding claim, wherein a rotary Moire encoder is used to measure the angular rotation of the gears.