Methods and apparatus for robust and efficient stepper motor BEMF measurement

Patent metadata

Field	Value
Publication number	US-10063170-B2
Application number	US-201715619128-A
Country	US
Kind code	B2
Filing date	Jun 9, 2017
Priority date	Jun 15, 2016
Publication date	Aug 28, 2018
Grant date	Aug 28, 2018

Abstract

In a described example an apparatus includes: an FET driver circuit configured to supply current to a coil in a stepper motor, the FET driver circuit configured to regulate the current to the coil using a fixed delta current; a current chopper pulse width modulated circuit coupled to the FET driver circuit configured to supply pulses corresponding to a step control signal and a direction control signal; a back electromotive force (BEMF) monitor coupled to the current chopper circuit configured to measure an off time pulse and to output a BEMF monitor signal; and a controller coupled to the current chopper pulse width modulated circuit to supply the step and direction control signals and coupled to receive the BEMF monitor signal.

First claim

What is claimed is:

1. A method comprising:
supplying a pulse width modulated signal with an on time and an off time to a coil in a stepper motor in cycles, the on time being proportional to a time to increase a current to the coil from a predetermined minimum current to a predetermined maximum current, the off time being proportional to a time to decrease the current in the coil from the predetermined maximum current to a predetermined minimum current;
in a first cycle supplying the pulse width modulated signal, measuring a first off time;
in a second cycle supplying the pulse width modulated signal, measuring a second off time; and
using a difference between the first off time and the second off time, determining a change in a voltage due to back electromotive force (BEMF) for the stepper motor.

2. The method of claim 1 , in which measuring the first off time includes counting pulses of a known frequency during the first off time, and storing a first count that is proportional to the first off time.

3. The method of claim 2 , in which measuring the second off time includes counting pulses of a known frequency during the second off time, and storing a second count that is proportional to the second off time.

4. The method of claim 3 , in which the first count and the second count are formed using a counter in a back electromotive force monitor coupled to a drive circuit supplying the current to the coil.

5. The method of claim 1 in which using a difference between the first off time and the second off time further includes subtracting the second off time from the first off time.

6. The method of claim 1 in which determining the change in a voltage due to back electromotive force ΔVbemf for the stepper motor includes calculating:

.
Δ

⁢

⁢
Vbemf

=

L
⁢

⁢
Δ
⁢

⁢

I
⁡

(

1

Toff
⁢

⁢
2

-

1

Toff
⁢

⁢
1

)

where Toff 1 is the first off time, Toff 2 is the second off time, AI is the difference between the predetermined maximum current and the predetermined minimum current, and L is an inductance of the coil.

7. A method for determining a back electromotive force (BEMF) voltage for a stepper motor, comprising:
supplying a voltage to a coil in a stepper motor for an on time that corresponds to increasing a current in the coil to a predetermined maximum current;
removing the voltage to the coil in the stepper motor for an off time that corresponds to allow the current in the coil to decrease to a predetermined minimum current;
counting a number of pulses of a known frequency to form a count corresponding to a duration of the off time; and
determining a voltage due to the BEMF using the change in the current between the predetermined minimum current and the predetermined maximum current, the off time, an inductance of the coil, a current in the coil, and a resistance of the coil.

8. The method of claim 7 , in which determining the voltage due to the BEMF includes calculating:

1

Toff
⁢

⁢
1

=

1

L
⁢

⁢
Δ
⁢

⁢
I

⁢

(

IR
-

Vbemf
⁢

⁢
1

)

where Toff 1 is the off time, AI is a fixed delta current that is a difference between the predetermined maximum current and the predetermined minimum current, L is the inductance of the coil, I is a constant current to the coil, R is the resistance of the coil, and Vbemf 1 is the voltage due to the BEMF.

9. The method of claim 8 , and further including using the Vbemf 1 voltage to determine a position of the stepper motor.

10. An apparatus, comprising:
an FET driver circuit configured to supply current to a coil in a stepper motor, the FET driver circuit configured to regulate the current to the coil using a fixed delta current ΔI;
a current chopper pulse width modulated circuit coupled to the FET driver circuit configured to supply pulses corresponding to a step control signal and a direction control signal;
a back electromotive force (BEMF) monitor coupled to the current chopper circuit configured to measure an off time pulse and to output a BEMF monitor signal;
a controller coupled to the current chopper pulse width modulated circuit to supply the step and direction control signals and coupled to receive the BEMF monitor signal;
in which the BEMF monitor includes a counter that counts during an off time pulse from the current chopper pulse width modulated circuit to form a count corresponding to the off time duration; and
in which the controller receives the count corresponding to the off time Toff 1 for the off time pulse, and the controller is configured to determine a voltage due to the BEMF by calculating:

1

Toff
⁢

⁢
1

=

1

L
⁢

⁢
Δ
⁢

Assignees

• Texas Instruments Inc

Inventors

• Saw Sooping
• Raja Rakesh
• Jain Anuj
• Hein Matthew

Classifications

• H02P8/14 Primary

  Arrangements for controlling speed or speed and torque (H02P8/12, H02P8/22 take precedence) · CPC title

• H02P6/182

  using back-emf in windings · CPC title