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Transcript
Last week’s project demos • Servo loop with E-Stop and Reset buttons • Demo in class tonight – don’t start on the next project until I’ve seen this one! More circuit design Potentiometers, capacitors, and motion control The Potentiometer • Also known as a “pot” – It’s a dial that can be turned • Nothing more than a variable resistor Schematic • The schematic shows it as a resistor with three leads – One on each end of the internal resistor – One that cuts into the middle of the internal resistor (this is called the “wiper”) What will this circuit do? Capacitor • A capacitor is essentially a rechargeable battery • The [typically] only hold their charge for a short period of time • When coupled with a resistor we create an “RC circuit” of which we can measure the capacitor’s discharge time • With that time measurement, we can do wondrous things Capacitor • A capacitor’s value (capacitance) is measured in farads – Our circuits will use values in microfarads (μF) • Two types – Polarized – these have a +/- side and MUST be put in the circuit in the proper direction or bad, really bad things can happen! – Non-polarized – these are like resistors and can be placed in the circuit either direction Two types of capacitors • Polarized • Non-polarized Capacitor usage – the RC circuit Protective resistor Timing resistor I/O pin How to use it • Send 5V to the circuit via the I/O pin for a set period of time – This will charge the capacitor • We then cut the 5V and monitor the I/O pin – This monitoring removes the I/O pin from the circuit – it becomes a passive device – Thus the voltage stored in the capacitor will drain through the resistor to Vss (ground) • The time it takes the I/O pin to change from a 1 (capacitor charged) to a 0 (capacitor discharged) is the RC time of the circuit Measuring the discharge time • There are 2 ways to do this – Charge up the capacitor then loop on reading the I/O pin until it drops to 0 (counting the time through the loop) – Use the PBASIC RCTIME command RCTIME 0, 1, time • Read pin 0, waiting for it to change from a 1, tell us the time (accuracy is dependent on the model of BasicStamp you are using) To do • Perform the lab (Activity 2) on page 143 of the book • CHECK THE DIRECTION OF YOUR CAPACITOR BEFORE APPLYING POWER TO THE CIRCUIT!!! • AND AFTER YOU’VE CHECKED IT, CHECK IT AGAIN!!! Use of fractions in PBASIC • There are no floating point data types • Anything requiring fractional precision must be done with scaled arithmetic • PBASIC provides some operators to assist Scaled math • We want to do floating point math! value = value * 3.1415926 – The 3.1415926 must be scaled by 256 and the result “unscaled” by 256 value = (int)(value * (3.1415926 * 256)) / 256 – The result is our desired value to a precision of 1/256 Multiply Middle operator • PBASIC provides an operator to do this */ • To use it in our previous example – Pre-multiply (int)(3.1415926 * 256.0) → 804 – Use the Multiply-Middle operator value = value */ 804 You’ll need this… • … as it is typical of low-cost microcontrollers to not support floating point math but it is equally typically of applications that use low-cost microcontrollers to require floating point math • … for the homework assignment Homework • This week’s homework assignment will be controlling the servo motor (from last week) with the potentiometer as described in the book on page 152, Activity 4 with the following modifications… Homework • Add two buttons (ON and OFF) – – – – The ON button places the circuit in the ON state The OFF button places the circuit in the OFF state At power on reset the circuit is in the OFF state The servo motor can only be moved [by the potentiometer] when the circuit is in the ON state • Add two LEDs (ON and OFF) – A green LED indicates that the circuit is in the ON state – A red LED indicates that the circuit is in the OFF state Question • What happens if I move the VSS connection of the circuit from the A (B) side of the potentiometer to the B (A) side? – That is, I move it from the pin to which it is currently connected to the other – Why does this behavior occur? Deliverables • A state-machine diagram depicting the operation of the system • Source code • A schematic diagram of the circuit • A working demonstration on the Basic Stamp development board (in class) • Answer to question on previous slide