Файл: Embedded Robotics (Thomas Braunl, 2 ed, 2006).pdf

References

JÖRG, K., BERG, M. Mobile Robot Sonar Sensing with Pseudo-Random Codes, IEEE International Conference on Robotics and Automation 1998 (ICRA ‘98), Leuven Belgium, 16-20 May 1998, pp. 2807-2812 (6)

KIMMEL, R. Demosaicing: Image Reconstruction from Color CCD Samples, IEEE Transactions on Image Processing, vol. 8, no. 9, Sept. 1999, pp. 1221-1228 (8)

KUC, R. Pseudoamplitude scan sonar maps, IEEE Transactions on Robotics and Automation, vol. 17, no. 5, 2001, pp. 767-770

MURESAN, D., PARKS, T. Optimal Recovery Demosaicing, IASTED International Conference on Signal and Image Processing, SIP 2002, Kauai

Hawaii, http://dsplab.ece.cornell.edu/papers/conference/ sip_02_6.pdf, 2002, pp. (6)

PRECISION NAVIGATION, Vector Electronic Modules, Application Notes, Precision Navigation Inc., http://www.precisionnav.com, July 1998

SHARP, Data Sheet GP2D02 - Compact, High Sensitive Distance Measuring Sensor, Sharp Co., data sheet, http://www.sharp.co.jp/ecg/, 2006

SICK, Auto Ident Laser-supported sensor systems, Sick AG, http:// www.sick.de/de/products/categories/auto/en.html, 2006

SMITH, C. Vision Support System for Tracked Vehicle, B.E. Honours Thesis, The Univ. of Western Australia, Electrical and Computer Eng., supervised by T. Bräunl, 2002

STAMATIOU, N. Sensor processing for a tracked vehicle, B.E. Honours Thesis, The Univ. of Western Australia, Electrical and Computer Eng., supervised by T. Bräunl, 2002

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There are many different ways that robotic actuators can be built. Most prominently these are electrical motors or pneumatic actuators with valves. In this chapter we will deal with electrical actuators using direct current (DC) power. These are standard DC motors, stepper motors, and

servos, which are DC motors with encapsulated positioning hardware and are not to be confused with servo motors.

3.1 DC Motors

Figure 3.1: Motor–encoder combination

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3 Actuators

The first step when building robot hardware is to select the appropriate motor system. The best choice is an encapsulated motor combination comprising a:

•DC motor

•Gearbox

•Optical or magnetic encoder

(dual phase-shifted encoders for detection of speed and direction)

Using encapsulated motor systems has the advantage that the solution is much smaller than that using separate modules, plus the system is dust-proof and shielded against stray light (required for optical encoders). The disadvantage of using a fixed assembly like this is that the gear ratio may only be changed with difficulty, or not at all. In the worst case, a new motor/gearbox/ encoder combination has to be used.

A magnetic encoder comprises a disk equipped with a number of magnets and one or two Hall-effect sensors. An optical encoder has a disk with black and white sectors, an LED, and a reflective or transmissive light sensor. If two sensors are positioned with a phase shift, it is possible to detect which one is triggered first (using a magnet for magnetic encoders or a bright sector for optical encoders). This information can be used to determine whether the motor shaft is being turned clockwise or counterclockwise.

A number of companies offer small, powerful precision motors with encapsulated gearboxes and encoders:

• Faulhaber http://www.faulhaber.de

• Minimotor http://www.minimotor.ch

• MicroMotor http://www.micromo.com

They all have a variety of motor and gearbox combinations available, so it is important to do some power-requirement calculations first, in order to select the right motor and gearbox for a new robotics project. For example, there is a Faulhaber motor series with a power range from 2W to 4W, with gear ratios available from approximately 3:1 to 1,000,000:1.

Kf

Figure 3.2: Motor model

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DC Motors

Table 3.1: DC motor variables and constant values

Figure 3.2 illustrates an effective linear model for the DC motor, and Table 3.1 contains a list of all relevant variables and constant values. A voltage Va is applied to the terminals of the motor, which generates a current i in the motor armature. The torque Wm produced by the motor is proportional to the current, and Km is the motor’s torque constant:

Wm Kmi

It is important to select a motor with the right output power for a desired task. The output power Po is defined as the rate of work, which for a rotational DC motor equates to the angular velocity of the shaft Z multiplied by the applied torque Wa (i.e., the torque of the load):

Po WaZ

The input power Pi , supplied to the motor, is equal to the applied voltage multiplied by the current through the motor:

Pi Vai

The motor also generates heat as an effect of the current flowing through the armature. The power lost to thermal effects Pt is equivalent to:

Pt Ri2

The efficiency K of the motor is a measure of how well electrical energy is converted to mechanical energy. This can be defined as the output power produced by the motor divided by the input power required by the motor:

The efficiency is not constant for all speeds, which needs to be kept in mind if the application requires operation at different speed ranges. The electrical system of the motor can be modelled by a resistor-inductor pair in series with a voltage Vemf, which corresponds to the back electromotive force (see Figure 3.2). This voltage is produced because the coils of the motor are moving through a magnetic field, which is the same principle that allows an electric generator to function. The voltage produced can be approximated as a linear function of the shaft velocity; Ke is referred to as the back-emf constant:

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3 Actuators

Figure 3.3 shows the ideal DC motor performance curves. With increasing torque, the motor velocity is reduced linearly, while the current increases linearly. Maximum output power is achieved at a medium torque level, while the highest efficiency is reached for relatively low torque values. For further reading see [Bolton 1995] and [El-Sharkawi 2000].

Figure 3.3: Ideal DC motor performance curve

3.2 H-Bridge

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H-Bridge

Figure 3.4: H-bridge and operation

The way to implement an H-bridge when using a microcontroller is to use a power amplifier chip in combination with the digital output pins of the controller or an additional latch. This is required because the digital outputs of a microcontroller have very severe output power restrictions. They can only be used to drive other logic chips, but never a motor directly. Since a motor can draw a lot of power (for example 1A or more), connecting digital outputs directly to a motor can destroy the microcontroller.

A typical power amplifier chip containing two separate amplifiers is L293D from ST SGS-Thomson. Figure 3.5 demonstrates the schematics. The two inputs x and y are needed to switch the input voltage, so one of them has to be “+”, the other has to be “–”. Since they are electrically decoupled from the motor, x and y can be directly linked to digital outputs of the microcontroller. So the direction of the motor can then be specified by software, for example setting output x to logic 1 and output y to logic 0. Since x and y are always the opposite of each other, they can also be substituted by a single output port and a negator. The rotation speed can be specified by the “speed” input (see the next section on pulse width modulation).

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