Robotic Systems ECE 401RB Fall 2006

Transcription

1 The following notes are from: Robotic Systems ECE 401RB Fall 2006 Lecture 15: Processors Part 3 Chapter 14, G. McComb, and M. Predko, Robot Builder's Bonanza, Third Edition, Mc- Graw Hill, I. Peripherals The usefulness of robots. A robot is not intelligent without inputs. Gives the robot the ability to understand its environment. A robot is useless without outputs. Motors to make the robot move. Arms and grippers to pick up things. Lights and displays to let you know what is going on. Sound outputs for alarms or indications. Voice (synthesized or recorded) messages as well. Input data passed from the environment to the robot s computer. Output commands to devices to do something. Intelligent Having intelligence; having a high degree of intelligence. Showing sound judgment and rationality. Intelligence The capacity to acquire and apply knowledge. I/O devices are called. Usually require some kind of conditioning or programming. For the computer or microcontroller to be able to process information. Lecture 15, Page 1 of 16

2 II. Sensors Goal: Provide the robot with data it can use to make intelligent decisions. Can be super simple. Can be amazingly complex. Two types of sensors 1. Digital A finite set of possible values. That finite set of values is mapped to a binary representation. For example - 0 Volts 00-2 Volts 01-4 Volts 10-5 Volts 11 Two types of outputs On/off binary results - i.e., a digital value of 0 or 1. - A switch is a good example. - Either open or closed. - Binary digital sensors sometimes be directly connected to the computer. - Without a need for additional interfacing electronics. Stepped binary results - A numeric value in discrete steps. - That can be represented by a number of bits. - Values 0 to 255 for eight bits, for example. Lecture 15, Page 2 of 16

3 - Example: An ultrasonic ranger that provides a digital output. - Note: The digitized value is not exact, but rounded to the most appropriate digital value. - Rounded to the nearest value, rounded down, or rounded up. - This is called. - These values need not correspond to a uniform scale. Diagram Diagram Diagram Diagram Diagram Diagram The actual input to the robot computer is a low or high voltage. Or a sequence of low and high voltages. - A serial input where multiple high/low values are input over time. 2. Analog Provides a continuous range of values. Usually a voltage. The sensor itself may provide a varying resistance or current. Which is then converted by an external circuit into a voltage. For example, a CdS (cadmium sulfide) cell which changes resistance based on its exposure to light. - Then, when built into a simple voltage divider, provides a voltage output. Lecture 15, Page 3 of 16

4 Analog sensors need additional electronics. To convert varying voltage levels into a form a computer can use. Can use a variety of analog-to-digital converters. - From comparators that convert into a binary 0 or 1 in reference to a setpoint. - To multi-bit serial inputs. Examples of sensors Range finders and proximity detectors A few inches to 40 feet. Light sensors Presence or absence of light. Arrays of thousands of elements CCD cameras. Pyroelectric infrared Detects changes in heat patterns. Often used in motion detectors. Speech input or speech recognition Sound Tuned to various frequencies. Or just sense volume levels. Contact switches Touch sensors. Accelerometer Detects changes in speed. Or changes in the pull of gravity. - To sense tilting or vertical orientation. Gas or smoke Toxic fumes. Temperature Ambient heat of the surrounding room or air. Applied heat from some hot or cold source. Etcetera III. Input and Output Methodologies The most basic input and output of a microcontroller are two-state binary levels. Off and on. Usually between 0 V and 5 V. Lecture 15, Page 4 of 16

5 Parallel interfacing Multiple bits of data are transferred. One at a time. But many at the same time. This increases the speed. A common parallel interface is the old parallel port on PCs. Sends data entire bytes (eight bits) at a time. But this consumes many I/O lines for the microcontroller. There are a limited number of I/O lines. - Typically 16 or fewer. Two eight-bit parallel ports might leave no more I/O lines for anything else. Serial interfacing Conserves I/O lines. Separates bytes of information into constituent bits. Sends each bit down the wire one-at-a-time. Lecture 15, Page 5 of 16

6 There are a variety of serial interface schemes. Using one, two, three, or four I/O lines. Additional I/O lines are used for timing and coordination. - Between sender and data recipient. RS-232 and USB are common serial interfaces. These may seem more difficult to implement than parallel interfaces. But not with the right combination of hardware and software. And the software on a microcontroller will typically take care of most of the details of using the interface. IV. Outputs LED Driver This is the most simple kind of output circuit. The controller applies a high voltage to turn on the LED. Or a low voltage to turn off the LED. The 470 Ω resistor limits the amount of current passing through the LED to 5 to 6 ma. Power Outputs Outputs typically drive heavy loads. Motors, solenoids, pumps. Even high-volume sound can demand a lot of current. Lecture 15, Page 6 of 16

7 Typical robotic control computers can only provide 15 to 22 ma of current on any output. This can drive at most two LEDs. But not much else. A power element is needed to provide adequate current. Using power transistors, MOSFETs, H-bridges (discrete component or packaged), etc. - We studied these in the lectures on motors. Lecture 15, Page 7 of 16

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9 Options for outputs from the robot computer to motors On/off signals To turn continuous DC motors on and off. To turn on phases of a stepper motor. Variable speed control How did we provide this? PWM signals For what other purpose was this type of signal used? PWM for servo motors. V. Digital Inputs Strictly digital inputs These are binary (on/off) sensors. They can be readily connected to control electronics. Options Direct connections With LED high/low voltage indicator. Through a switch debouncer. Through a buffer. - A buffer is recommended so as to isolate the source of the input from the control electronics. Lecture 15, Page 9 of 16

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11 Interfacing from different voltage levels. Some digital devices may operate at voltages that differ from the control electronics. This can cause erratic behavior. Or damage one or both of the devices being interfaced. Logic-transfer voltage conversion circuits are needed. Several integrated circuits provide these functions in off-the-shelf solutions. Or the interface could be custom created using standard logic chips. Optical Isolation You may wish to keep the input and control circuits isolated. With different power sources. Why? Noise This is most easily done with opto-isolators. - Readily available in IC-like packages. The figure below shows the basic concept. - The source controls an LED. - The input to the control electronics is a photodetector. Since both have different power supplies, this also can be used to perform the voltage conversion discussed above. - +V in the figure can be whatever is required. Lecture 15, Page 11 of 16

12 Zener diode input protection Zener diodes can be used to clamp the voltage for an input. To keep the voltage from going above a certain value. Recall the circuit from Lecture 12. From Wikipedia: A Zener diode is a type of diode that permits current to flow in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the rated breakdown voltage known as "Zener knee voltage" or "Zener voltage". Use a resistor to limit the current through the zener diodes. Lecture 15, Page 12 of 16

13 How does the setup below clamp both the positive and negative voltages? High positive top zener turns on, bottom zener limits voltage. High negative bottom zener turns on, top zener limits voltage. VI. Interfacing Analog Inputs These cannot typically be directly connected to the robot. Need to use some form of analog-to-digital conversion (ADC). May also need buffering and signal amplification. Buffering keeps the impedance of the control circuit from affecting the sensor. Lecture 15, Page 13 of 16

14 Common input interfaces CdS (cadmium sulfide) cells are, in essence variable resistors. Controlled by light. Putting this in series with a resistor creates a voltage divider to match the required input voltage. If a potentiometer is the input, it forms a voltage divider all by itself. No amplification is necessary. Phototransistors will output varying levels of current. Can be converted into a voltage using a resistor. Photodiodes also output varying current. But the output tends to be weak. Amplification is usually required. Lecture 15, Page 14 of 16

15 Voltage Comparators The input voltage can be compared against a reference voltage. The output is high or low (1-bit binary). The potentiometer is used to determine the trip point (reference voltage) for the comparator. Analog-to-Digital Converters (ADCs) Translate an analog input value into digital form, which is necessary for using many types of sensors. Temperature Microphones and other audio transducers. Variable output touch sensors. Position potentiometers (for example, for the angle of an elbow joint). Light detectors. Many pre-packaged ADCs are available. Need to be able to read the data sheets. And understand how they interface. And understand how long the conversion process takes. - There can be significant time delays between when an analog value is sampled and the digital output is sent out. - Time to convert the value into digital. Lecture 15, Page 15 of 16

16 - Time to send out over a (typically) serial interface. - Hopefully the analog value does not change during this time. - Or else the result might be unreliable. - There might need to be some sort of sample and hold mechanism. The digital output depends on the following. The range of possible inputs The number of output bits. Example: 0 to 10 V range for 8 bits. 10/255 = V 255 gaps so the output can have 256 values. Max can be 10 V for (255), min can be 0 V for binary Special purpose ADC integrated circuits specifications. Single input or multiple inputs. Bit resolution - Number of bits for the output. - Normally 8 bits. Two possible forms of output - Parallel - Serial Some microcontrollers and single-board computers come already equipped with ADCs. This is very helpful, since a parallel or serial connection does not need to be implemented. Since the connections are already implemented internally. Lecture 15, Page 16 of 16