PIC微控制器驱动直流伺服电机设计及源码

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"这篇文档是关于使用PIC18CXXX系列微控制器进行直流伺服电机设计的教程，包含源代码，适用于学习和参考。" 在本文档中，作者介绍了如何利用Microchip Technology Inc.的PICmicro®微控制器，特别是PIC18CXXX和PIC16CXXX系列芯片来设计直流伺服电机系统。直流伺服电机广泛应用于需要精确位置控制和速度控制的领域，如机器人、自动化设备和精密定位等。文档的初步介绍指出，PICmicro家族提供了多种器件和选项供嵌入式设计者选择，其中PIC16CXXX和PIC18CXXX家族的引脚兼容性使得在同一硬件设计中可以灵活选用不同器件，为设计者提供了平滑的升级路径。文中具体以PIC18C452和PIC16F877为例，探讨了这些微控制器在伺服电机应用中的资源需求。在直流伺服电机的应用中，PICmicro MCUs执行的关键任务包括： 1. **用户控制接口**：允许用户通过各种输入（如按钮、旋钮或传感器）设定电机的动作指令。 2. **电机位置测量**：通过霍尔效应传感器、编码器或其他位置检测装置来实时监测电机的位置，确保准确控制。 3. **运动轮廓计算**：根据用户输入或预设的运动轨迹，计算电机应遵循的速度和位置变化。 4. **误差信号计算与PID控制**：微控制器会计算实际位置与目标位置之间的误差，并通过比例积分（PI）控制器来调整电机的控制信号，以减小误差并达到期望的性能。通过理解这些基本功能，读者可以根据提供的设计基础，结合具体的项目需求，选择合适的PICmicro器件来定制自己的直流伺服电机控制系统。源代码的提供有助于读者深入理解控制逻辑和实现细节，加快学习和开发进程。这份资料对于想要学习和实践微控制器驱动直流伺服电机控制技术的人来说是一份宝贵的资源，不仅提供了理论知识，还包含了实际操作的源代码，能够帮助开发者快速掌握相关技能。

 2000 Microchip Technology Inc. Preliminary DS00696A-page 9

AN696

that you wish to change the acceleration value for seg-

ment 2 to 1000. To do this, you would send the follow-

ing ASCII string to the servomotor:

A,2,1000 <CR>

This syntax can be used to change all motion profile

segment parameters.

After all motion profile data has been entered, a single

motion profile segment, or range of segments, may be

executed using the ’G’ or ’L’ command. To execute seg-

ments 1 through 4, for example, you would send the fol-

lowing ASCII string to the servomotor:

G,1,4 <CR>

If you only want to run one motion segment, the desired

segment number is entered twice as shown:

G,1,1 <CR>

The ’L’ command is used the same way as the ’G’ com-

mand, except that the range of motion segments is

executed repeatedly. This command is useful for creat-

ing repetitive motions with the servomotor. The ’S’ com-

mand stops the motion profile after the presently

executing motion segment has completed.

Three commands are available to change the PID gain

constants. With these commands, you can manually

tune the PID algorithm to obtain the best performance

from the motor in your application.

The ’W’ command turns the motor driver IC on or off.

A summary of all servomotor commands and their syn-

tax is given in Table 1.

Two status flags, stat.run and stat.loop, are

used to control execution of the motion profile. If a ’G’

command is entered to run a series of motion seg-

ments, the stat.run flag is set. If a ’L’ command is

entered, the stat.run and the stat.loop flags are

set. When the SetupMove() function determines that

the last segment in the motion profile has executed, the

stat.loop flag is checked. If stat.loop is set, the

motion profile segment data for the first segment in the

sequence is loaded and execution continues. If

stat.loop is clear, then the stat.run flag is cleared

and motion stops.

Operation With ASCII Terminal

You can use a PC terminal program, such as

PROCOMM

or HyperTerminal

, to control the servo-

motor. The terminal program should be configured for

19.2 kBaud, no parity, 8 data bits, and 1 stop bit. When

the servomotor is reset, you will see an introduction

message and a ’READY>’ prompt. You should now be

able to enter any of the commands shown in Table 1.

TABLE 1: SERVOMOTOR COMMAND

SUMMARY

Note: You may find the ‘W’ command to be

extremely useful if the PID gain constants

you've chosen cause the servomotor to

become unstable.

Command: X,seg#,data <CR>

Sets the distance to be travelled for the specified

motion profile segment. Data is provided in encoder

counts relative to the present position.

0 ≤ seg# ≤ 23

-32768 ≤ data ≤ 32767

Command: A,seg#,data <CR>

Sets the acceleration for the specified motion profile

segment. Data is provided in encoder

counts/T

SERVO

/65536.

0 ≤ seg# ≤ 23

1 ≤ data ≤ 32767

Command: V,seg#,data <CR>

Sets the velocity limit for the specified motion profile

segment. Data is provided in encoder

counts/T

SERVO

/256.

0 ≤ seg# ≤ 23

1 ≤ data ≤ 32767

Command: T,seg#,data <CR>

Specifies the amount of time to wait before executing

the next motion profile segment. Data is provided in

SERVO

multiples.

0 ≤ seg# ≤ 23

0 ≤ data ≤ 32767

Command: G,startseg,stopseg <CR>

Executes a range of motion profile segments.

0 ≤ startseg ≤ 23

0 ≤ stopseg ≤ 23

Command: S <CR>

Stops execution of a motion profile.

Command: P,data <CR>

Changes the proportional gain for the PID algorithm.

-32768 ≤ data ≤ 32767

Command: I,data <CR>

Changes the integral gain for the PID algorithm.

-32768 ≤ data ≤ 32767

Command: D,data <CR>

Changes the differential gain for the PID algorithm.

-32768 ≤ data ≤ 32767

Command: W <CR>

Enables or disables the PWM driver stage.

AN696

DS00696A-page 10 Preliminary  2000 Microchip Technology Inc.

Stand-alone Operation

The provided application firmware allows the servo-

motor to perform a few basic motions without a PC con-

nected. Specifically, data for three different motion

profiles are stored in the MCU program memory and

are loaded into data memory at start-up. Each profile is

selected by turning on DIP switch #2, #3, or #4, con-

nected to PORTB and pressing the MCLR

button.

The software polls the DIP switches once, at start-up,

to see if a profile should be executed. The selected pro-

file will begin to execute immediately. If DIP switch #1

is turned on in combination with one of the other

switches, the selected profile will execute repeatedly.

PICmicro MCU RESOURCES

There is a broad range of PICmicro devices that can be

used to implement the servomotor application, depend-

ing on the level of performance that you need. To begin

with, let’s consider the processing time needed by the

servo calculations.

A large amount of time is spent in the servo calcula-

tions executing the compensator, which requires one or

more multiplications depending on the type of algorithm

used. Three 16 x 16 signed multiplications are required

by the PID compensator algorithm used here. Since the

servo update calculations must be performed fre-

quently, a hardware multiplier can provide a significant

reduction in the MCU bandwidth. With a 8 x 8 hardware

multiplier, each 16 x 16 multiplication can be performed

in approximately 32 instruction cycles. Without the

hardware multiplier, each multiplication can take 500

instruction cycles or more, depending on the algorithm

that is used.

The servo calculation times were compared for the

PIC16CXXX and PIC18CXXX architectures, using the

same source code. Table 2 shows the performance

results. You can easily see the increase in available

bandwidth gained by the hardware multiplier. For a

given servo update period, the hardware multiplier in

the PIC18CXXX architecture frees a large amount of

MCU bandwidth for performing other tasks. In addition,

extra MCU bandwidth may be obtained from the

PIC18CXXX architecture, since the devices may be

operated up to 40 MHz.

Table 3 and Table 4 show a comparison of memory

usage by the servomotor application, for both the

16F877 and the 18C452. Depending on the memory

requirements for motion profile segment data and other

application functions, the design may be adapted for

other MCUs. As an example of a minimal implementa-

tion, this application could be modified to operate on a

PIC16C73B. The PIC16C73B has 22 I/O pins, 4K x 14

words of program memory, and 192 bytes of data

memory.

The resolution of the available timer resources must be

considered when using the position sensing method

described here. The maximum RPM of the servomotor

is a function of the timer resolution, servo update fre-

quency, and the resolution of the incremental encoder.

Because two’s complement arithmetic is used to find

the motor position, the timers used to accumulate the

encoder pulses should not increment more than 2

N-1

counts during each servo update interval, or position

information will be lost.

When a PIC16CXXX device is used for the servomotor

application, Timer0 and Timer1 are the only timers with

an external clock input and Timer0 has only 8 bits of

resolution. For some cases, this may limit the maxi-

mum motor RPM. A formula that can be used to calcu-

late the maximum RPM is given in Equation 1 below:

EQUATION 1: MAXIMUM RPM

In this equation, N represents the resolution of the timer

in bits, f

, is the servo update frequency, and CPR is

the resolution of the encoder. The incremental encoder

used in this application provides 500 CPR. For the

moment, let’s assume that our servo update frequency

is 1000 Hz. Using 1x decoding, the maximum RPM that

we can permit without a timer overflow, is over 15,000

RPM. This maximum limit is of no concern for us, since

the motor we are using provides a no-load speed of

6000 RPM. Now, let’s assume that a 4x decoding

method was used, so that our encoder now provides

2000 CPR. Now, the maximum motor speed is 3840

RPM, which is definitely a problem! In this case, the

servo update frequency would have to be increased, if

possible, or the encoder resolution decreased.

RPM

MAX

N-1

CPR

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