C8051F300微控制器驱动步进电机的高性能设计详解

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本文档深入探讨了步进电机驱动的设计方法，包括软硬件方面的详细设计要点。首先，步进电机在众多领域有着广泛的应用，如消费电子（打印机、绘图仪、复印机和扫描仪）、汽车电子（电子节气门控制、仪表盘指示器和气候控制系统）、工业设备（机器人、电子元件处理机、测试设备和自动化生产线）等。这些应用强调了对高性能和精确控制的需求。传统的步进电机控制系统往往依赖专用功能集成电路（Function ICs），这些IC提供了基础的前进和后退控制接口，但可能限制系统的性能和灵活性。为了提高系统性能，特别是在嵌入式系统中，采用微控制器直接控制步进电机是一种更优的选择。例如，像C8051F300这样的小型微控制器就能提供高性能的运动控制解决方案，它具备足够的计算能力和低功耗特性，能够实现精确的步进电机控制算法和实时响应。在硬件设计方面，文档可能会涵盖以下几个关键部分： 1. **驱动电路**：这包括电流斩波器、功率放大器和隔离电路，用于将微控制器的信号转化为步进电机所需的电压和电流脉冲，确保电机顺畅运转。 2. **接口电路**：连接微控制器与步进电机之间的电路，可能包括接口芯片和接口协议，如SPI、UART或I2C，以实现数据交换和命令发送。 3. **保护措施**：如过载保护、过热保护和短路保护，以确保电机和整个系统安全运行。 4. **电源管理**：对于嵌入式系统，高效的电源管理和电池续航能力是必不可少的。软件设计部分则会涉及： - **电机控制算法**：基于脉冲宽度调制（PWM）或微步控制，精确控制电机转速和方向。 - **位置和速度控制**：通过编码器反馈或其他位置传感器实现闭环控制，确保电机按照预设路径移动。 - **错误检测和处理**：软件层面的错误检测机制，如检测丢步、超时和异常情况，并进行相应的处理。 - **通信协议**：如果涉及到与主机PC的交互，可能还会涉及网络协议，如USB、Ethernet或CAN总线的编程。本设计文档为工程师提供了全面的指导，如何通过微控制器驱动设计实现高效、灵活且可靠的步进电机控制，适用于各种复杂和精确控制的场景。通过阅读和学习文档中的电路图、软件代码示例，读者可以深入了解并应用于自己的项目中。

AN155

Rev. 1.1 7

Solving Equation 3 for time gives the results shown in

Equation 4. This is the absolute time required to provide

a linear acceleration profile. This would be useful if we

were working in absolute time and scheduling each

commutation point based on a cumulative count from

the beginning. However, we would like to use a relative

count for each step period.

Equation 4.

The definition of the step period from Equation 1 is used

with the results in Equation 4 to provide an equation for

the step period listed in Equation 5. The constant

acceleration term has been factored out and is called T

as defined in Equation 6. The value of T

will determine

the step period of the initial step with n equal to zero.

Thus, we can use a single table for the relationship and

let T

be a variable. This means one table can be used

with any stepper motor.

Equation 5.

Equation 6.

3.6. Common Mistakes

As demonstrated in the preceding section, the linear

velocity profile is not as simple as it would first appear.

The values stored in the linear velocity table must

closely follow the non-linear equation shown in

Equation 5. Often engineers in a hurry to get hardware

up and running do not use the proper relation for the

stepper motor table. This is a very common mistake that

is very easy to make.

The most common mistake is to have the step period

decrease linearly with the step number. For example,

one might have an initial step period of 256 timer ticks

and decrease the step period by one each time. This

results in a non-linear velocity that is increasing

hyperbolically as the step period approaches zero. Such

a profile will hardly move at first and then the velocity

will increase much too quickly.

The second most common mistake is to have the step

period decrease with the inverse of the step number.

This results in a velocity that is linear with respect to the

step number. But this ignores the fact that the step

period is constantly changing. The velocity should be

plotted against the cumulative time, not the step

number. If the velocity is plotted against the absolute

time, the resulting curve is a second order function. That

is, the velocity is increasing with the square of time. This

profile also starts out too slow and ends up accelerating

too fast.

3.7. Linear-Velocity vs. Linear-Acceleration

Many engineers hold a preconceived notion that a forth-

order linear-acceleration profile will provide much better

dynamic performance than a linear velocity profile. A

linear-acceleration order profile has an acceleration that

is trapezoidal in nature and velocity shaped in an s-

curve.

A linear-acceleration profiler provides only marginally

better dynamic performance in some systems. Only a

few applications can actually benefit from a forth-order

profile. For example, a printer head driven by an elastic

band might benefit from the improved smoothness. The

linear-acceleration profiler will have a smoother

transition between the acceleration and slewing phases.

The maximum step rate is dictated by motor parameters

and will be the same in either case.

The linear velocity profile has several advantages over

the forth-order profile: The linear velocity profile can be

implemented using a single table. The step table is

fixed. It can use a single multiply function to provide a

variable acceleration. Using the table avoids having to

calculate complex functions like a square root.

Calculating the profile is very simple.

------ -=

n1+

– T

n1+ n–()==

---=

AN155

8 Rev. 1.1

In contrast the linear-acceleration profile is much more

complex. A single fixed table cannot be used. The initial

conditions of each acceleration phase depend on the

length of all prior phases.

The recommended solution is to always start out using

a properly implemented linear-velocity profiler. This will

be the best solution for most applications. Verify that the

velocity is actually linear and evaluate the dynamic

response of the system. If the dynamic response of the

system does not meet the requirements, consider using

a linear-acceleration profile.

3.8. Interrupt Based Algorithm

When developing an algorithm to control a stepper

motor using a small microcontroller, it is important to

consider the manner in which the code will be executed.

A simple sequential algorithm could accomplish the

task. The sequential algorithm might calculate the

current step time and figure out what to do next

depending on the acceleration phase. However, such

an algorithm would end up writing to the timer and then

waiting until the timer times out. It would spend most of

the time just waiting on the timer.

Fortunately most MCU timers are capable of generating

interrupts. Thus, we can set up the timer to generate an

interrupt after one step period. When the interrupt

occurs, the MCU should commutate the motor and

update the timer with the next step period.

Now considering that we want to make the stepper

motor control interrupt based, we must use a different

paradigm. The timer interrupt service routine should be

small, fast, robust, and only do what must be done on

each commutation period. Anything that can be

calculated once beforehand will be done outside the

interrupt service routine. Values may be stored in global

variables to be accessed by the interrupt service

routine.

Using this scheme there are two basic pieces of code.

The first is the profiler or the move() function. The

second is the timer interrupt service routine. The profiler

is called from the main loop and is executed in the

foreground. The profiler calculates the global variables

based on the target location and the current position of

the motor. The function is named move() so that the

user code makes sense in plain English.

This reference design uses a simple divide-by-four

profiler. This means that the total number of steps is

divided by four. The motor will accelerate and

decelerate for one forth of the total number of steps.

The remainder of the steps will be at a constant velocity.

Some actual profiles are shown in Figure 5. Note that

the total time accelerating for short profiles is much

more that one-forth the total time. This is due to the

effect of the variable step period.

The constant-acceleration and deceleration phases are

accomplished by incrementing and decrementing an

index for the stepper motor table. Incrementing the table

index by one each step will accelerate the motor.

Decrementing the index each step will decelerate the

motor. The maximum index and the corresponding

minimum period determine the top speed of the motor

for a particular profile.

4. Hardware Design

The Stepper motor reference design hardware consists

of four sections: the C8051F300 microcontroller, the

power electronics, the voltage regulator, and the serial

interface. The full schematic is included in Appendix A.

The Bill of Materials is in Appendix B and the printed

circuit board artwork is in Appendix C.

4.1. Microcontroller

The reference design features the C8051F300

microcontroller. This microcontroller is housed in a tiny

3 mm by 3 mm 11-lead micro lead package (MLP). This

package is small enough to be integrated into the

smallest motor. The C8051F30x family includes five

devices with various options. The ‘F300 and ‘F301

include a calibrated internal oscillator. The internal

oscillator is calibrated to within

±2% at test. This is

close enough to use the internal oscillator for the UART

with baud rates up to 115.2 kbps. The reference design

does not utilize 8-bit 500 ksps ADC in the ‘F300. Thus,

the design could use either the ‘F300 or the ‘F301 which

does not include the ADC. The ADC on the F300 might

prove useful in some designs for monitoring the dc

motor voltage, stepper motor current, or the stepper

motor temperature.

The unipolar stepper motor drive requires four outputs

to drive the transistors. P0.0 through pin P0.3 are used

to drive the power MOSFETs. The pins have been

time

Figure 5. Different Profiles

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