AN857 查看數據表（PDF） - Microchip Technology

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AN857

Brushless DC Motor Control Made Easy

Microchip Technology 

AN857

Using Open-Loop Control to Determine

Motor Characteristics

You can measure the motor characteristics by operat-

ing the motor in Open-Loop mode, and measuring the

motor current at several applied voltages. You can then

chart the response curve in a spreadsheet, such as

Excel, to determine the slope and offset numbers.

Finally, plug the maximum RPM and offset numbers

back into the table generator spreadsheet to regener-

ate the RPM tables.

To operate the motor in Open-Loop mode:

• Set the manual threshold number (ManThresh)

to 0xFF. This will prevent the Auto mode from tak-

ing over.

• When operating the motor in Open-Loop mode,

start by adjusting the offset control until the motor

starts to move. You may also need to adjust the

PWM control slightly above minimum.

• After the motor starts, you can increase the PWM

control to increase the motor speed. The RPM

and voltage will track, but you will need to adjust

the offset frequently to optimize the voltage for the

selected RPM.

• Optimize the voltage by adjusting the offset for

minimum current.

To obtain the response offset with Excel®, enter the

voltage (left column), and RPM (right column) pairs in

adjacent columns of the spreadsheet. Use the chart

wizard to make an X-Y scatter chart. When the chart is

finished, right click on the response curve and select

the pop-up menu “add trendline. . .” option. Choose the

linear regression type and, in the Options tab, check

the “display equation on chart” option. An example of

the spreadsheet is shown in Figure 14.

FIGURE 14:

MOTOR RESPONSE SCOPE DETERMINATION

DS00857B-page 18

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