Brush motor drive application solution based on MM32SPIN motor/power supply chip - robot steering gear

Program features :

• ARM Cortex-M0 high-performance motor driver chip

• Built-in two sets of N- type Gate Drivers

• Speed ​​command input can be analog voltage or PWM signal

• ADC  sampling position feedback

Figure 1: Common 7.5V/10W  brushed servos

2. Technical principle of brushed steering gear

Here we introduce the method to realize the trajectory movement of the brushed servo .

The interior of the brushed servo is divided into three major areas

• control panel

• brushed motor

• position feedback sensor

Figure 2: Brush motor and control board    

Figure 3 : Feedback position sensor               

definition :

LSB: 12 ​​bits ADC  sampling unit

a: Acceleration unit:  (LSB ÷ (ms^2))

t: time unit: (ms)

SP:   Speed ​​unit : ( LSB ÷ ms)

P 1: End position unit: LSB

P0: Starting position unit : LSB

T1: end time unit : ms ( milliseconds )

T0: Starting time unit : ms ( milliseconds )

V1: (P1-P0)  ÷ (T1-T0) Unit : ( LSB ÷ ms) 

V0:  Initial speed unit: ( LSB ÷ ms) 

X: Distance unit : ( LSB )

Th: (T1-T0) ÷ 2 Unit: (ms)

V2:  Speed ​​track maximum speed unit: ( LSB ÷ ms)

Relationship between brush speed and trajectory

Figure 4: Ideal rectangular trajectory motion ( red circle marks a =  ∞ ) 

V1 is constant speed

At this time, the red acceleration motion time is ideally zero

Green is the location track

According to the formula of constant acceleration motion :

V = V0 +(a*t) ;( Formula 1 )

x =  V0 t +((a*t*t)÷2);( Formula 2 )

When the initial velocity is zero, V0  can be ignored

We can know from the acceleration motion ( Formula 1 ) ( Formula 2 )  that it takes time and distance for the motor to go from initial speed to constant speed .

Figure 5: Non-ideal trajectory motion

It is necessary to consider that the process of the object itself from stationary → moving → stationary requires the so-called acceleration movement

CH2  Green : Position track (0~5) Voltage corresponds to (0~4095) LSB

CH3  blue : speed trajectory (0~5) Voltage corresponds to (0~4095) V    

Triangular speed trajectory motion

As shown in Figure 6 

V2 = (P1-P0)÷(Th-T0); 

V2 = 2*V1;  Assumption : the motor speed can reach (2*V1)      

Figure 7:  Actual trajectory movement time

CH2  Green : Position track (0~5) Voltage corresponds to (0~4095) LSB 

CH3  blue : speed trajectory (0~5) Voltage corresponds to (0~4095) V

Figure 9: Actual operation of the motor

CH1  yellow : PWM duty cycle (0~5) Voltage , corresponding to (0~100)% duty cycle

CH2  Green : Position track (0~5) Voltage corresponds to (0~4095) LSB  

CH3  blue : speed trajectory (0~5) Voltage corresponds to (0~4095) V

• Prevent damage to the motor due to excessive speed

• The motor itself can drive the highest speed

Figure 10: Actual operation of the motor        

CH1  yellow : PWM duty cycle (0~5) Voltage , corresponding to (0~100)% duty cycle

CH2  Green : Position track (0~5) Voltage corresponds to (0~4095) LSB

CH3  blue : speed trajectory (0~5) Voltage corresponds to (0~4095) V    

Figure 11: Actual operation of the motor

CH1  yellow : PWM duty cycle (0~5) Voltage , corresponding to (0~100)% duty cycle

CH2  Green : Position track (0~5) Voltage corresponds to (0~4095) LSB

CH3  blue : speed trajectory (0~5) Voltage corresponds to (0~4095) V

From Figure 9 and Figure 11, it can be clearly seen that the duty cycle of the trapezoidal velocity trajectory decreases compared to the triangular velocity trajectory , but the slope increases.

How to choose between triangular speed trajectory and trapezoidal speed trajectory depends on the characteristics of the motor itself and the application adjustment trajectory settings.

3. Hardware design

4 Conclusion

MM32SPIN motor/power supply dedicated chip has a built-in two-phase Gate Driver, which is very suitable for brush motor applications and greatly simplifies the design of the motor driver board. The built-in divider can also quickly respond to the trajectory control algorithm , improving software efficiency . It also brings the benefit of reduced development costs.