Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F04—POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
  • F04B—POSITIVE-DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS
  • F04B51/00—Testing machines, pumps, or pumping installations

Definitions

• the invention relates generally to the field of measurement of output of electric motors.
• the invention relates more specifically to measurement of output of stepper motors driven by H-bridge circuitry.
• Fig. 1 is a schematic diagram of a prior art stepper motor H-bridge driver circuit
• Fig. 2 is a block diagram of a circuit for measuring motor current from H-bridge sense resistors
• Fig. 3 is a schematic diagram of a circuit for measuring motor current from H- bridge sense resistors
• Fig. 4 is a flow diagram for deriving pump pressure from motor current
• Fig. 5 is a flow diagram for deriving gain tables used to calculate pump pressure from motor current
• Fig. 6 is a flow diagram for deriving scale factors used to calculate pump pressure from motor current
• Fig. 7 is an exemplary process flow diagram for generating a 0 psi base line reference vector as applied in Figs. 4-6;
• Fig. 8 is an exemplary process flow diagram for generating correction factors.
• the embodiment described below is for determining pump pressure based on motor current in an H-bridge driver circuit for a stepper motor.
• the invention is not limited to motor-driven pumps, however.
• the invention is applicable to any motor-driven device whose mechanical output is related to torque driven by the motor.
• An example of another application is determination of weight of a load lifted by a motor driven shaft.
• FIG 1 is a schematic for a prior art H-bridge two phase stepper motor drive circuit 100, noting the Phase 1 and Phase 2sense resistors 10, 20 in the ground path of the H-bridge DMOS FETS 30, 40. Motor current is sensed by the voltage drop across these resistors. This technique is known in the art. Publications in this area include: U.S. Patent nos. 4,710,686; 5,646,520; 5,703,490 and 5,874,818.
• Figure 2 is a block diagram of an embodiment of the invention showing a signal conditioning circuit for the measurement of the motor current from the sense resistors in Figure 1 by an analog to digital converter. As shown in Figure 2, each motor drive phase is rectified by a half wave active rectifier 40, 41.
• the rectified signals are summed and integrated by a two input integrator 42.
• An envelope detector 43 is used to remove signal noise.
• the signal is then DC amplified and voltage translated 44 to maximize the signal level that is read by the A/D converter. In other words, the signal is level shifted and amplified so that the expected dynamic range is commensurate with the input range of the A/D converter to allow use of the maximum resolution of the converter.
• the signal is buffered by a buffer amplifier 45 before driving the A/D converter.
• Figure 3 is a detailed circuit implementation of the conditioning circuit of Fig. 2. Derivation of Pump Pressure Model from Motor Current
• the relationship between pump pressure and motor current is established through a look-up table.
• the look-up table is used to expedite data processing and because the relationship between the pressure and current is not a continuous function.
• a calibration process is performed whereby for a predetermined pump flow rate, a data set of motor current values is measured and stored for a discrete number of sample periods during the pumping process.
• 1250 current measurements are made for each pump flow rate.
• Figure 4 shows a flow diagram for scaling motor current data with gain corrected scaling factor for a given pump rate to produce a pressure profile for a single dispense rate.
• a pump dispenses fluid in a predetermined process having a predetermined time frame.
• 1250 current measurements are made in the dispense time frame.
• each of the current measurements is scaled by the gain-corrected scaling factor and 1250 corresponding pump pressures are generated.
• a table of scale factors is used to determine the proper scale factor for the pump flow rate.
• each of the scaled current measurements is loaded into a dispense profile buffer. All of the calculations in Figures 4-7 are performed in real time and are based on the dispense rate for each individual data point.
• Figures 5 and 6 describe the process for generating the gain-corrected scaling factors used in scaling the motor current to pump pressure, and for creating a calibrated gain table with values for each of the 1250 sample measurements.
• the current values measured are scaled and compared to the calibrated table for the applicable flow rate. In this manner, it is possible to determine how the production process compares to the calibrated table values and whether the production process is sufficiently close to the calibrated values or if there are deviations from the calibrated values. Such deviations could indicate equipment failure or other system anomalies and if large enough would result in halting further processing of the materials in the particular production process for which the deviations occurred.
• Figure 5 is a flow diagram for creating a gain table that covers all of the applicable flow rates.
• the cycle test involves running the pump through an entire dispense for a set of 30 rates from 0.1 mL/s to 3.0 mL/s. These data are maintained in the pump memory as tables to be referenced to speed up the calculations.
• the three sets of tabular data are: 1) a zero psi reference baseline vector, 2) a gain table matrix and 3) a gain corrected scaling factor vector. For each of these three sets of data, each row corresponds to a specific dispense rate from 0.1 mL/s to 3.0 mL/s.
• Figure 5 is a flow diagram for creating a gain table for 30 different pump rates.
• the 30 x 1250 matrix of current sense values (each row representing a different pump rate) is summed with a 30 x 1 baseline vector.
• the steady state response is isolated.
• a linear fit is performed for each of the isolated rows of current data.
• the linear fit data is combined with the steady state data from step 320 to produce a row of the gain table. This process is repeated for each of the flow rate rows.
• Figure 6 is a flow diagram for calculating scale factors for each of the dispense rates.
• the stead stat values for each row are isolated.
• the average of each rate vector for each row is calculated at step 420. This produces a 30x1 matrix of values.
• the maximum value of the 30 values is found.
• each of the 30 values in the matrix is divided by the maximum value to normalize the 30 values to a 30x 1 matrix of gain corrected scale factors.
• Figure 7 is a flow diagram for calculating a 0 psi baseline reference vector for each of the dispense rates. Note that this vector is used in the flow diagram shown in Figure 5.
• the pump is set to a predetermined rate, and is unloaded.
• the pump is ran through a dispense cycle at the predetermined rate.
• Step 530 involves recording quantized current readings over time during the pump dispense cycle.
• the current readings from the steady state portion of the dispense cycle are averaged.
• the average number is assigned to be the 0 psi baseline value for that predetermined dispense rate.
• the 0 psi baseline number is looked up based on the dispense rate and is subtracted from the input current values.
• the pump will recharge.
• the raw current output samples are added together.
• this running sum is divided by the number of total recharge current samples to obtain the average recharge current.
• This recharge average is divided by the recharge rate to obtain the normalized recharge average.
• the normalized recharge average is sorted into one of ten correction ranges, corresponding to ten different dispense correction factor indices. This index, added to the rate (.1 though 3.0 ml/sec), comprise an index into the dispense correction table (30 x 10 elements).
• This dispense correction factor is added to every sample in the dispense profile buffer to complete the compensation.
• Figure 8 is a flow diagram of steps to obtain a dispense compensation factor.
• average recharge current is divided by recharge rate to obtain a normalize recharge average.
• the normalized recharge average is sorted into one of ten correction ranges corresponding to ten different dispense correction factor indices.
• a 30x10 dispense correction table is created with 10 possible correction factors for each of 30 dispense rates.
• the appropriate dispense correction factor for each dispense rate is added to the 1250 elements of the dispense profile buffer for that rate.
• One aspect of the invention is to determine if a motor driven process matches a predetermined profile over time by measuring motor current over time and comparing that current to a stored table of values for current in a desired profile for the process. Where there are a number of conditions in which the process can take place, an equivalent number of tables, one for each condition is stored. In a further embodiment, instead of one table for each condition (e.g. 30 tables for 30 flow rates) less tables could be used and interpolated values from two tables used for condition levels between the two tables. For example, if there are tables at 5 ml/s (milliliters per second) increments, and a production run was made at 22 ml/s one would interpolate table entries for the 20 and 25 ml/s tables.
• the invention is not limited to motor-driven pumps.
• the method described herein can be used to characterize any motor-driven process based on motor current and compare an actual production run of that process against a set of calibrated values for a desired result for the process.
• a window comparator may be used to sense H-bridge current.
• the window comparator produces a high level out put when current is above or below predetermined limits.
• This embodiment can be used for lower resolution applications such as detecting when a motor is jammed, broken or overloaded.
• the upper and lower limits may be set to monitor an acceptable operating band and trigger an alarm when the limits are exceeded.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Control Of Positive-Displacement Pumps (AREA)
• Control Of Stepping Motors (AREA)

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• 2010
  • 2010-07-14 JP JP2012520751A patent/JP6062246B2/ja active Active
  • 2010-07-14 TW TW099123133A patent/TWI495889B/zh active
  • 2010-07-14 US US12/836,038 patent/US8441222B2/en active Active
  • 2010-07-14 WO PCT/US2010/042000 patent/WO2011008874A1/en not_active Ceased

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