A METHOD FOR NOISE REDUCTION IN STEPPER MOTORS DURING HOMING AND SYSTEM THEREOF

FIELD OF THE INVENTION

The present invention relates to noise reduction in stepper motors during homing without compromising the torque. The proposed method is to drive the pointer (1) at rated voltage during forward direction and then continue to drive at rated voltage for small amount of time during reverse direction till the pointer (1) is accelerated [starting of pointer (1 ) in reverse] and then driving the motor at 10% to 30% lower voltage to reduce the noise and magnitude of bounce. This method ensures the initial torque required for the pointer (1), reduces the noise and bounce of pointer (1). During homing process, homing position is determined by the system by sensing the bounce back voltage using ADC (Analog to Digital Conversion) device.

BACKGROUND OF THE INVENTION AND PRIOR ARTS

In the existing trends of the stepper motor noise reduction includes an embodiment with non-volatile memory. Stepper motor used in the instrument has got a mechanical stopper for the homing (reset or zero) position of the pointer. Stepper motor has got single/multiple pole magnet inside for better resolution and there are two coils separated by phase shift of 30 or 40 or 60 or 90 degrees.

To detect the homing position the pointer is moved opposite to mechanical stopper (forward direction) for some degrees then driven in reverse direction towards mechanical stopper to find the homing position.

During homing, the pointer bounces front and back when the pointer contacts mechanical stopper. The magnitude of the bounce of a pointer attached to the motor and the magnitude of the noise generated as the pointer contacts a mechanical stopper are both directly related to the applied voltage of the homing.

Normally this type of stepper motors are driven at rated voltage like 5 V for the normal operation and homing. To reduce the bounce of the pointer and the noise generated to

the acceptable level, the applied voltage can be reduced. However this will reduce the operating torque required for the pointer initial movement.

One way of re-zeroing of a stepper motor without noise is provided in US Patent document 6,853, 162 wherein the stepper motor without noise is provided by pulse width modulated (PWM) voltage signals which are used to drive the coils of the stepper motor. The phases between the driving signals are 90° out of phase. Thus, a sine /cosine methodology is used to drive the motor. Using micro-programmable pulse width modulation (PWM) levels involves the microprocessor reading stored voltage levels from a table stored in memory which corresponds to the amount of angular displacement desired by the motor. These voltage levels are then applied to the motor's coils. The microprocessor performs these operations by executing software instructions stored in memory. The software can be stored in memory located in the controller or in a separate logic block or logic chip. The magnitude of the bounce of a pointer attached to the output shaft of the motor and the magnitude of the noise generated as the pointer contacts a mechanical stopper are both directly related to the applied voltage and speed or frequency of the homing strategy. To reduce the bounce of the pointer and the generated noise to barely discernable levels, the applied voltage is reduced to between 15% and 30% of the normal driving voltage (approximately 1 volt for a 5 volt system). Additionally, the speed of homing is set to a value below the new start-stop frequency of the motor.

However, the above US 6, 853, 162 document doesn't talk about retaining initial torque required for the pointer. This is a huge drawback in this system. The document also provides for the use of microprocessor, and software instructions which increase the number of components.

Yet another way reducing noise in stepper motor is provided in US Patent 5,665,897. It provides a rotary gauge driven by a stepper motor has a stop near the zero point of the gauge graphics. A factory calibration procedure rotates the gauge field in reverse direction to move the gauge pointer against the stop and continues field rotation until the rotor becomes unstable and flips back in the forward direction, and then moved to a rotor rest position. The angle of flip back, the rest position and the number of steps to the zero position are stored in non-volatile memory. In the vehicle, when the ignition is

off, the microprocessor or the stepper motor wakes periodically and exercises the gauge, using the stored values to reset the gauge to the stable position. When ignition is turned on, the microprocessor moves the gauge directly to the zero position. However the above document US 5, 665, 897 doesn't provide for initial torque required for the pointer which is the main aspect of the instant invention.

OBJECTS OF THE INVENTION

The primary object of the present invention is to provide a method for noise reduction in stepper motors during homing without compromising the torque.

Still another object of the present invention is to ensure the initial torque required for the pointer (1) and also reduces the noise and bounce of pointer (1).

Still another object of the present invention is to drive the motor at predetermined lower voltage of rated voltage to reduce the noise.

Yet another object of the present invention is to develop a system to reduce noise in stepper motors during homing without compromising torque.

Still another object of the present invention is to detect back emf produced in the motor using an Analog to Digital Converter (ADC) placed inside the microcontroller.

STATEMENT OF THE INVENTION

Accordingly, the present invention provides for a method for noise reduction in stepper motors during homing without compromising the torque, said method comprising steps of; determining homing position of pointer (1) by sensing the bounce back voltage; driving the pointer (1) at rated voltage in forward direction or direction opposite to the homing position and thereafter driving the pointer (1) in reverse direction at the rated voltage till the pointer (1) is accelerated; and driving the motor with reduced voltage till the pointer reaches the homing position, to reduce the noise in the stepper motor without compromising initial torque, and a system for noise reduction in stepper motors during homing without compromising torque, said system comprises a stepper motor connected to PWM output of microcontroller having high current buffer, an Analog to

Digital Converter (ADC) inside the microcontroller to detect back emf produced in the motor.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIGURE 1: shows the top view of the pointer (1) movement in forward, backward, and towards zero position.

FIGURE 2: shows the top view of the pointer (1) hitting the mechanical stopper (2) when the pointer (1) is driven with the rated voltage.

FIGURE 3: shows the top view of the pointer (1) hitting the mechanical stopper (2) when the pointer (1) is driven in the reduced voltage.

FIGURE 4: shows the profile view of the sound proof chamber with pointer (1) connected to stepper and DB meter placed near the stepper motor.

FIGURE 5: shows block diagram with connections between microcontroller having high current buffer, Analog to Digital Converter (ADC) inside the microcontroller and a stepper motor.

FIGURE 6: shows the graph of voltage v/s angle indicates that as the motor attains the acceleration when the voltage is reduced by 25%.

FIGURE 7: shows the graph of voltage v/s angle indicates that as the motor attains the acceleration when the voltage is not reduced.

DETAILED DESCRIPTION OF THE INVENTION The present invention is in relation to a method for noise reduction in stepper motors during homing without compromising initial torque, said method comprising steps of; determining homing position of pointer (1) by sensing the bounce back voltage; driving the pointer (1) at rated voltage in forward direction or direction opposite to the homing position and thereafter driving the pointer (1) in reverse direction at the rated voltage till the pointer (1) is accelerated; and driving the motor with reduced voltage till the pointer reaches the homing position, to reduce the noise in the stepper motor without compromising initial torque.

In yet another embodiment of the present invention the homing position of the pointer (1) by sensing the bounce back voltage is determined by Analog to Digital Converter (ADC).

In still another embodiment of the present invention the noise generated by the pointer in contacting a mechanical stopper (2) is directly proportional to the bounce of the pointer (1) attached to stepper motor.

In still another embodiment of the present invention the homing position is detected by driving the pointer (1) in the direction opposite to the mechanical stopper (2) or homing position for predetermined degrees, followed by driving the pointer (1) in reverse direction towards the mechanical stopper (2).

In still another embodiment of the present invention the rated voltage is about 5V.

In still another embodiment of the present invention the motor is driven at 10% to 30% lower voltage of the rated voltage preferably the rated voltage is reduced by 25%.

In still another embodiment of the present invention the method reduces bounce of the pointer (1) and reduces the noise level by 30%.

The present invention is in relation to a system for noise reduction in stepper motors during homing without compromising torque, said system comprises a stepper motor connected to PWM output of microcontroller having high current buffer, an Analog to Digital Converter (ADC) inside the microcontroller to detect back emf produced in the motor.

In yet another embodiment of the present invention the high current buffer and the Analog to Digital Converter (ADC) are connected to the stepper motor through coil A and coil B.

In still another embodiment of the present invention the high current buffer is preferably inbuilt of the microcontroller and drives 25mA current to stepper motor.

In still another embodiment of the present invention the PWM output signal has a frequency range of 15 KHz to 16 KHz.

Figure 1 illustrates the top view of the pointer (1) of the stepper motor. The pointer (1) movement is in forward, backward, and towards zero position. The pointer (1) is started in the forward direction with the rated voltage and moved back till the pointer (1) gets accelerated in the rated voltage. The pointer (1) is slowly accelerated by applying the rated voltage; after the pointer (1) gets accelerated, the pointer (1) is driven with the reduced voltage till the zero position. The voltage is reduced by 25%.

Stepper motor used in the instrument has got a mechanical stopper (2) for the Zero (reset or homing) position of the pointer (1) (1). It has got single/multiple pole magnet inside for better resolution and there are two coils separated by phase shift of 30 or 40 or 60 or 90 degrees.

To detect the homing position the pointer (1) is moved opposite to mechanical stopper (2) (forward direction) for some degrees then driven in reverse direction towards mechanical stopper (2) to find the homing position.

Figure 2 and 3 illustrates the top view of the pointer (1) hitting the mechanical stopper (2) when the pointer (1) is driven with the rated voltage and when the pointer (1) is driven in the reduced voltage respectively. During homing, the pointer (1) bounces front and back when the pointer (1) contacts mechanical stopper (2). The magnitude of the bounce of a pointer (1) attached to the motor and the magnitude of the noise generated as the pointer (1) contacts a mechanical stopper (2) are both directly related to the applied voltage of the homing.

Normally this type of stepper motors are driven at rated voltage like 5 V for the normal operation and homing. To reduce the bounce of the pointer (1) and the noise generated to the acceptable level, the applied voltage can be reduced. However this will reduce the operating torque required for the pointer (1) initial movement.

The proposed method is to drive the pointer (1) at rated voltage during forward direction and then continue to drive at rated voltage for small amount of time during reverse direction till the pointer (1) is accelerated (starting of pointer (1) in reverse) and

then drive the motor at 10% to 30% lower voltage to reduce the noise and magnitude of bounce.

This method ensures the initial torque required for the pointer (1) and also reduces the noise and bounce of pointer (1).

During homing process, homing position is determined by the system by sensing the bounce back voltage using ADC (Analog to Digital Conversion) method.

Figure 4 illustrates the profile view of the sound proof chamber with pointer (1) connected to stepper and DB meter placed near the stepper motor. The DB meter is used to measure the noise level of the pointer (1) hitting the mechanical stopper (2). The sound proof chamber prevents the system from sound from external agents into the system.

The application of the present invention is used in automobile industries, aerospace industries, medical field and mechanical Applications.

Figure 5 illustrates the block diagram with connections between microcontroller having high current buffer, Analog to Digital Converter (ADC) inside the microcontroller and a stepper motor The stepper motor is connected directly to the PWM output of the microcontroller. The micro controller has an inbuilt high current buffer which can drive 25mA current which is required for the stepper motor. The back emf produced in the motor is detected using Analog to digital converter inside the microcontroller.

The voltage output to the stepper motor can be adjusted by varying the duty cycle of the

PWM signal. The frequency used for PWM signal is 15 to 16 KHz.

Figure 6 and Figure 7: illustrates the graph of voltage v/s angle indicates that as the motor attains the acceleration when the voltage is reduced by 25% and illustrates the graph of voltage v/s angle indicates that as the motor attains the acceleration when the voltage is not reduced wherein we can see the value of Vcc remains 0.75 Vcc and Vcc respectively.