CdS photocell. They utilize resistors that are sensitive to force (bending, touch, gravity), temperature,

Transcription

1 More About Sensors temperature touch switch bend mercury switch resistor CdS photocell rotation light sonar Passive (resistive) Sensors Active (powered) Sensors Passive sensors operate as simple resistances and require no power from the RCX. They utilize resistors that are sensitive to force (bending, touch, gravity), temperature, or visible light. Active sensors (also known as powered sensors) require power from the RCX and involve slightly more sophisticated electronics.

2 The NQC program must tell the RCX which sensor(s) is connected to which input port. i.e., the program needs to configure the sensor port. NQC allows the user to specify the "SENSOR_TYPE" (as _TOUCH, _TEMPERATURE, _LIGHT, or _ROTATION). It also allows the user to configure the reading mode of the sensor by setting SENSOR_MODE (as _RAW, _BOOL, _PERCENT, _FAHRENHEIT, _CELSIUS, _ROTATION, _EDGE, or _PULSE). The quick way to declare a sensor and configure it (to its default mode) is to use the SetSensor ("sensor port", "sensor type") instruction. Here, sensor port = SENSOR_1, SENSOR_2, or SENSOR_ 3 (or 0, 1, 2, respectively). sensor type = SENSOR_TOUCH, SENSOR_LIGHT, SENSOR_CELSIUS, SENSOR_FAHRENHEIT, SENSOR_ROTATION, SENSOR_PULSE, SENSOR_EDGE. The default reading mode is as follows: Touch sensor: BOOL (reads 0 or 1) (other useful modes: RAW, EDGE, PULSE) Light sensor: PERCENT (0-100) (other useful modes: RAW, EDGE, PULSE) Temperature sensor: CELSIUS (-20 to 70 operational range) FAHRENHEIT (-4 to 158 operational range) Rotation sensor: ROTATION (16 ticks per rotation; range: -32,768 to +32,767) Temperature sensor values are scaled by 10 inside the RCX; for example, when the LCD displays a temperature sensor reading of 22.5 degrees, the RCX represent this value as integer 225. This should be kept in mind when writing NQC code. ClearSensor ("sensor port") is useful in resetting sensor readings in ROTATION, EDGE and PULSE modes. Here is an example of configuring sensor input #2 as a touch sensor in RAW mode: SetSensorType(SENSOR_2, SENSOR_TYPE_TOUCH) SetSensorMode(SENSOR_2, SENSOR_MODE_RAW) do in class

3 Sensor Configuration Shortcuts [works with SetSensor("sensor_#", "sensor_configuration") ] Sensor Configuration Implied Type Default Mode SENSOR_TOUCH SENSOR_TYPE_TOUCH SENSOR_MODE_BOOL SENSOR_LIGHT SENSOR_TYPE_LIGHT SENSOR_MODE_PERCENT SENSOR_ROTATION SENSOR_TYPE_ROTATION SENSOR_MODE_ROTATION SENSOR_CELSIUS SENSOR_TYPE_TEMPERATURE SENSOR_MODE_CELSIUS SENSOR_FAHRENHEIT SENSOR_TYPE_TEMPERATURE SENSOR_MODE_FAHRENHEIT SENSOR_PULSE SENSOR_TYPE_TOUCH SENSOR_MODE_PULSE SENSOR_EDGE SENSOR_TYPE_TOUCH SENSOR_MODE_EDGE

4 Raw to Boolean Conversion The Boolean Mode SetSensorMode(SENSOR_1, SENSOR_MODE_BOOL + "slope"); where slope ranges from 0 to 31. Touch Sensor with slope = 0: (This is the default for touch sensors) Condition Boolean Conversion Algorithm raw_value > (not pressed) raw_value < (pressed) 460 < raw_value < 562 unchanged (dead zone or hysteresis that reduces jitter) Note: The RCX first reads a raw_value for all sensors regardless of the MODE setting. Then, the raw_value is converted automatically based on the default or user specified mode of operation. The range of raw values is: 0 < raw_value < 1023 Slope > 0 (useful for Light (and other "fast-response" sensors) If the sensor's value changes more than the slope value during a certain time (3ms), then the sensor's boolean state will change. This allows the boolean state to reflect rapid changes in the raw value. A rapid increase (or a rapid decrease) in the raw value will cause the boolean reading to flip (0 to 1, or 1 to 0). See the section on light sensor below for details

5 Edge Count and Pulse Count Sensor Modes In the edge count mode, the RCX (via a touch or other sensors) counts how many times the Boolean value changes. Count starts at 0 and increments every time the Boolean value changes from 0 to 1 or from 1 to 0. For touch sensors, the RCX ignores edges for 0.3 sec after a transition. This increases reliability of edge counting (solves the de-bouncing problem) but limits the detection to edge transitions that are at least 300msec apart. Pulse count mode is similar to the edge count mode except that the counter increments by one when a full pulse is received by the sensor (i.e., a rising edge followed by a falling edge) task main() { SetSensorType(SENSOR_1,SENSOR_TYPE_TOUCH); SetSensorMode(SENSOR_1,SENSOR_MODE_RAW); } //ClearSensor(SENSOR_1); task main() { SetSensorType(SENSOR_1,SENSOR_TYPE_TOUCH); SetSensorMode(SENSOR_1,SENSOR_MODE_EDGE); } ClearSensor(SENSOR_1);

6 Code Short Cuts: task main() { SetSensor(SENSOR_1,SENSOR_PULSE); } ClearSensor(SENSOR_1); task main() { SetSensor(SENSOR_1,SENSOR_EDGE); } ClearSensor(SENSOR_1);

7 Non-Lego Passive Sensors How to configure a passive (resistive type) sensor (e.g., photocell, bend sensor)? Ans. Treat it as a Touch Sensor and set its mode to RAW. Alternatively, we may use the generic sensor type for resistive sensors: SENSOR_TYPE_NONE and set sensor mode to RAW, as follows: task main () { SetSensorType (SENSOR_1, SENSOR_TYPE_NONE); SetSensorMode (SENSOR_1, SENSOR_MODE_RAW); } Write NQC code to configure and experiment with the following sensors: 1. Mercury Switch 2. Bend Sensor 3. CdS Photocell Determine max & min readings. Use RCX LCD to display sensor readings. Now, attach a touch sensor and record the Min and Max values.

8

9 RCX Reading of Passive Sensors Note: Here, port #1 is configured as TOUCH/RAW. Leaving the input port with no load (or equivalently attaching a resistor with very high resistance, say in the Mega Ohm range) will result in a voltage drop of 5V at the input. This is converted to 1023 by the A/D converter. The LCD displays a reading of input port #1 while a 10K Ohm load resistor is attached. This reading is proportional to the voltage drop across the load resistor. The display range is between 0 and 1023 which correspond to voltage readings of 0V and 5 V, respectively. Therefore, the 512 reading reflects a voltage drop of 2.5 V across the load resistor. Where is that voltage coming from? See next slide. The input port reading is converted to binary by a 10-bit A/D converter which can represent values between 0 and 2^10 1 = 1023.

10

11

12

13

14 Passive Sensor Type Sensor Resistance (Ohms) RCX Reading (RAW) LEGO Touch Sensor 700 (closed) Infinity (open) Mercury Switch Infinity (horizontal) Zero (tipped forward) Photocell (CdS) 65,000 (dark) 2,500 (room light) Bend Sensor 9,500 (no bending) 18,000 (bent 90 degrees) Lego Temp. Sensor 12,500 at room temp RAW value = o C= o F= Useful formulas: RCX reading (in raw value) = (1,023 x Rsensor)/(10,000 + Rsensor) o C reading = (785 raw value)/8 o F reading = 32 + (9/5)x( o C reading) These two last readings are referred to as processed values..

15 Active (powered) Sensors When the RCX is programmed to expect an active sensor (e.g., a light sensor) at one of its input ports, then the RCX input interface must supply power to the sensor and read its value using the same pair of wires. It accomplishes this by rapidly alternating between these two functions: First the RCX supplies 8 V for a period of 3 ms and then it measures the sensor value the - same way it does for a passive sensor - for a time of 0.1 ms during which the RCX applied voltage is 5 V. The trace shown in the figure depicts this behavior (setting: 1 ms/div and 2V/div). Note how the RCX makes a sensor reading every 3 ms. This is also the case for passive sensors.

16 Lego Light Sensor PERCENT Mode (for light sensors) The default reading mode for a Lego light sensor is PERCENT. Here, RAW readings are converted into percentages through the use of the following equation: Percentage = (1023 raw_value) / 7 = 146 raw_value/7 (Note: RCX truncates every arithmetic calculations to its integer value. Therefore, only integer results are generated using this formula) Examples: raw_value = 322 when sensor is placed very close to a 100W light bulb (corresponds to a 100% reading). raw_value = 610 when a mirror is used to maximize reflected sensor light (corresponds to a 59% reading). raw_value = 715 when sensor light is reflected back by a regular white printer paper (corresponds to a 44 percent reading) raw_value = 825 when sensor light is reflected back by a (flat) black project paper (corresponds to a 28 percent reading) raw_value = 915 in a dark room (minimal reflectance and ambient light), which corresponds to a percentage value of 16. Note how the first example justifies the division by 7 in the above formula.

17 SetSensorMode(SENSOR_1, SENSOR_MODE_BOOL + "slope"); where slope ranges from 0 to 31. If Slope is between 1 and 31 you get what is called Dynamic Measurement. The value of the Slope parameter sets the size of the Dynamic Measurement i.e. the necessary change of "raw" sensor reading between two consecutive samples, to get a change in the Boolean state. Test the above program by moving the light sensor across the edge of a closely placed piece of white paper. Use the view button to display the light sensor (Boolean) value.

18 Recall that the inputs are read every 3ms or seconds, and that the full-scale range of an input is 0 to 1023 for 0 to 5v. So a raw value of 1 is equal to 5v/1023 or about 5mv or.005v. The slope parameter sets the amount of raw value change on an input from one reading to the next to consider that the input's value has changed state. For example: for a Slope of 10, the raw sensor reading must change by at least 10 in sec. (or equivialently, the reading of the input voltage must change by at least 0.05v in.003s). If the raw value change is (+ or -) 10 per 3msec the corresponding Boolean sensor value becomes TRUE (or 1) and stays TRUE until there is at least a 10 per 3msec change. Then it changes to FALSE (or 0) and stays FALSE. The following plot shows an example voltage input and the state of the input over time.

19 The Lego Rotation Sensor (active sensor)

20 Rotation Sensor Example //Connect a rotation sensor to sensor#1 //Connect a motor to output A and use it to drive the rotation sensor int x, x1; task main() { x=0; SetSensor(SENSOR_1,SENSOR_ROTATION); ClearSensor(SENSOR_1); start display; OnFwd(OUT_A); Wait(6000); Off(OUT_A); } task display() { SetUserDisplay (x1, 0); while(true) { x = SENSOR_1/16; x1=x; } } Note: with 16 ticks per turn, the rotation sensor can resolve 22.5 degrees. Gear reduction can be used to increase this resolution. Suggest a gearing configuration to increase accuracy to 1 degree. (see Baum's text, p. 29). The above program works beacuse the RCX can reliably (no counts are missed) measure rotations between 50 and 300 rpm. However, if missing few counts (ticks) is not critical, then the rotation sensor can be safely used over the range 12 rpm to 1250 rpm. Also, keep in mind that you can often gear your sensor up or down to put it in the proper range.

21 Bonus Earning Team Mini-Projects (Each project earns you bonus points toward Test 1) Each team should complete at least one mini-project and demonstrate it in class (give brief informal presentation) I. (5 points) Use Tankbot with a bend sensor and program it to track (smoothly) a wall. II. (5 points) Use Tankbot with a rotation sensor (no other sensors) and program it to backup when it hits an obstacle (i.e., when motors stall).

22

23