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Devices Electronic for the Mechanical Experimenter* By Nathan Delson Purpose of Document In today s day and age, a mechanical engineer is required to be familiar with electronic control. The cost of microprocessors and sensors has dropped to the point, where even the least expensive products often have electronics in them. These electronics often supplement the mechanical performance. Accordingly, the capabilities of both mechanical and electronic components must be considered together when designing a device. Therefore, a mechanical engineer may be called upon to design simple electronic devices, or work closely with electrical engineers during a project. The purpose of this document is to give an overview of how electronics can be incorporated into mechanical devices. This document does not replace fundamental courses in electronics, but rather brings together many practical aspects of electronic circuits that are used in mechanical control. Other texts provide more in-depth coverage, but are often focused on specific elements of electronics. This document brings together material necessary for a mechanical engineer to get started with electronic projects, and hopefully motivate further study. Good Electronic Design vs. Bad Electronic Design It is possible to hook up electronic components in an unplanned fashion and actually get a circuit to work. However, such an approach can lead to intermittent and unpredictable performance. Moreover, it can lead to hours of frustrating debugging. Good Electronic Design includes: Using the specifications of each electrical component to ensure that the circuit is not asking a component to exceed the specifications it is designed for. Clear drawing of all circuit diagrams, accompanied by current and voltage calculations where necessary. Step by step implementation of an electronic circuit, where the performance of each step is verified with a multimeter or oscilloscope before moving on to the next step. Follow the Hands-on Guidelines for Good Circuit Implementation described later in this document. This includes using consistent wire * Title inspired by Britt Rorabaugh s book Mechanical Devices For the Electronics Experimenter colors and separating high power and lower power circuits. Not only will this keep you organized, but will allow others to understand your circuit and help you debug it. Bad Electronic Design includes: - Copying the wiring diagram of a circuit without understanding how it works. - Treating an electronic circuit as a magical black box which sometimes works and sometimes doesn t. If your circuit is not working, then use your multimeter or oscilloscope to figure out why. Not only will you end up with a reliable circuit, but you will gain the satisfaction and knowledge of good electronic design. Components of Mechatronic Devices Terminology is useful for putting different components in context, so we start with some definitions. As you will see below, dual use of terminology is a recurring theme that is indicative of the multi-disciplinary nature of Mechatronics projects. Each discipline typically considers themselves at the center of the project, and uses terminology that may neglect other disciplines. Since mechanical engineering is broadest engineering discipline, it is often the MEs responsibility to bring the different parts of a design together. From a practical perspective, one should emphasize clarity and make sure to include the complete design picture during communication. Electrical vs. Electronic: Any device that uses electricity such a motor can be considered electrical. However, when one adds sensors or control circuitry then it becomes electronic. If you see an Integrated Chip (IC) on a device, then it is electronic. Electromechanical and Mechatronic Devices: An electromechanical device includes both mechanical and electrical or electronic components. In early design of electromechanical devices, the mechanical design was performed separately from the electrical and electronic design. However, as electronics has become more pervasive, the mechanical and electrical design have become more tightly integrated in many products. The term Mechatronics has been introduced to describe products where both the mechanical and electronic design must be considered concurrently for optimal performance. The term electromechanical is more widely used, but the term Mechatronics is gaining recognition. Consider a simple oscillatory fan used to blow a breeze throughout the room. The propeller, housing, and bearing design could all be done by a mechanical engineer. An electrical engineer could then design the motor wiring and on/off N. Delson 2 Updated: 9/25/2008 controls with only general specifications from the mechanical engineer. I would consider such a device as electromechanical. Now consider a fan used for cooling the microprocessor on your PC. Some of these fans have controllers that sense the temperate of the microprocessor, and adjust the speed up the fan using Pulse Width Modulation (described later), so that high fan speeds are only used when necessary, thus minimizing power consumption and reducing noise. The design of such a fan would require close coordination between mechanical and electrical engineers. I would consider this a Mechatronic device. Block Diagram of a Mechatronic Devices: The basic components of a typical Mechatronics device are shown below Contoller Driver(s) Actuator(s) Sensor(s) Mechanical Structure Controller: The controller is typically a microprocessor, which could be a Basic Stamp, a Pentium IC, or one of the many custom chips used for control. The controller measures sensor readings, and based upon the software code it uses specifies commands to the drivers. Microprocessors have built in memory, so that one can download a software program to it. Microprocessors usually use low power, such as the ones in digital watches which lasts years on a single battery. However, high-end microprocessors can use substantial power, as attested to the heat built up in a laptop computer. Alternative controllers include analog op-amps or discrete logic chips such as AND and OR gates. However, as the cost of microprocessors drops, most Mechatronics devices include a microprocessor where software can be easily updated to modify controller performance, without changes in the circuit. Sensors: A sensor measures a physical property such as distance, velocity, temperature, or presence of an object. The availability and type of sensors has increased tremendously over recent years. Examples include Microelectromechanical systems (MEMS) to sense acceleration that deploy airbags in automobiles, capacitive sensors that detect whether cornflakes have settled too much in a box, and optical sensors that detect if there is a green blueberry that needs to be N. Delson 3 Updated: 9/25/2008 removed from a canning process. Sensor development is a very active field, and if there is a physical property of importance, there most likely is someone working on a better way to sense that property. Sensors typically use low power. Actuators: An actuator transfers mechanical energy into a system. In a Mechatronic device the energy transfer is typically electrical energy into mechanical energy. Example actuators include: motors, pistons, and solenoids. The power requirements of an actuator can be quite high. Of course, smaller actuators such as the vibrator in a pager has relatively power consumption. Because motors are the most common actuators in Mechatronic devices, many engineers simply use the term motor, rather than the more general term actuator. (Note, in the hydraulics industry some use the term actuator specifically to refer to hydraulic pistons, but this is not relevant to this document). There are many types of actuators. Motors alone include DC brush motors, brushless motors, AC motors, linear motors, and stepper motors. Reduction in cost of rare earth magnets, advances in magnetic modeling, and low cost motor drivers have produced new types of actuators in recent years. A Mechatronics engineer needs to stay abreast of the capabilities of different actuators. Drivers (e.g. motor drivers) : Because actors typically require high power, and microprocessors are low power devices, one usually needs a driver to power an actuator (see block diagram above). The driver receives its input from the microprocessor, and then uses power from an electrical source to operate the actuator. The simplest driver can provide on/off control of an actuator. More sophisticated drivers allow for variable speed or torque control (more on this later). A common beginners mistake is to connect the microprocessor output directly to an actuator, not realizing that the microprocessor can output sufficient power to drive the actuator on its own. To add to the confusion, some driver manufacturers refer to motor drivers as controllers, since from their perspective their device controls the motor. Hardware: The term hardware is used in various fashions. Electrical Engineers use the term hardware for electronic circuits, while using the term software for control code used (often their deliverables do not include moving mechanical parts so a complete EE project can be separated into hardware and software components). On the other hand, Mechanical Engineers often use the term hardware to refer to mechanical components, while considering electrical components and control code as a separate item. Once can avoid confusion by using the terms mechanical hardware and electrical hardware. However, due to the multidisciplinary nature of electromechanical devices, one will encounter this N. Delson 4 Updated: 9/25/2008 ambiguity. Just remember that each engineering discipline comes from a different perspective, but we all have to work together to get our hardware to work! Electronic Background Required This document does not replace a fundamental course in electronics and assumes that the reader is familiar with: Ohm s Law: The voltage drop across a resistor is given by: V=IR Where, V is voltage in volts, R is the resistance in Ohms, and I is current in amps. Power Dissipation: Power dissipated in a resistor or other electrical component is given by: P=VI Where, P is power in Watts, V is voltage drop across the component in volts, and I is current through the component in amps. Kirchhoff's first rule: The sum of all currents entering a branch point of a circuit (where three or more wires merge) must be equal to the sum of the currents leaving the branch point. Kirchhoff's second rule: Around a closed loop in a circuit, the sum of all the voltage drops must equal zero. Circuit Dynamics: We will not cover circuit dynamics in-depth in this document. However, once should have a general understanding of capacitors and inductors. One should be familiar with the behavior of RC circuits. N. Delson 5 Updated: 9/25/2008 Voltage vs Current: Voltage is measured across a device. Voltage can be easily measured with a voltmeter or oscilloscope, and good designers are always measuring voltage at nodes throughout a circuit. In ideal voltmeter (for most circuits typical voltmeters can be considered ideal) draws no current. Current is measured through a device. Current is hard to measure; it requires taking apart the circuit and amp-meters often are limited in the amount of current they can measure. Because current is hard to measure, a beginner mistake is to ignore it in circuit debugging, yet it is one of the largest cause of circuit failure. A good designer estimates the current in a circuit through voltage measurements. An ideal amp-meter has no voltage drop (we typically do not use an amp-meter) Multimeter vs Oscilloscope: Multimeters are inexpensive and easy to use; however, they are slow and average readings over a second or more. In electronic devices small glitches or noise can cause significant havoc with microprocessor logic. To see such glitches, one needs the high speed response of an oscilloscope. So if you cannot figure out why a circuit is not operational after reviewing it with a multimeter, then fire up the scope, and don t forget to ground the probe properly! Importance of Power in Electronic Components The law of energy conservation dictates that energy needed to drive a component cannot come from thin air. Since there is always some energy loss, one needs to supply each component with a minimum amount of energy it needs to operate. Power is the use of energy over time, and thus the power requirements is one the of more important factors needed to size a component. This is true of both mechanical and electrical components. Consider the mechanical properties of a motor and gearbox. Once could find a small low power motor that rotates at very high velocity but at low torque. One could attach this small motor to a gearbox to generate high torque, but the output velocity would be low. It is the power, which is torque times velocity, that is limited by a small motor. If one needs a large power output, then one would have to have a large motor with large magnets and windings. By looking at the size of a mechanical device, one can get a feel for the amount of power it can output. In a similar fashion, one can look at an electronic device to get a feel for the amount of power it can output or can handle. Electronics that handle high power have large heatsinks, thick conductors for wire connections, and high current ratings. Even if an electronic device does not exhibit these high power characteristics, it is important to evaluate its power specifications. N. Delson 6 Updated: 9/25/2008 Every electronic component is limited in terms of power in either: Amount of power it can output. Amount of power that can be transferred through it. Amount of power it needs to operate The reason why circuits fail is often because the power specifications are not being met. For this reason it is important to identify the input and output specifications of each component, which are defined on their specification sheet from the manufacture. These specifications should be used in the circuit calculations for both voltage and current. Low Current Circuits Voltage Divider Objective: Create an adjustable voltage that can be read by a voltagemeasuring device, such as a voltmeter or microprocessor. Applications include testing of an Analog to Digital input, or use with a comparator. A secondary objective is to avoid excessive energy loss. Given: Supply voltage of 5VDC Potentiometer (pot) Ideal voltage-measuring device (i.e. it draws no current) 5VD C poteniometer wiper V1 R1 I1 I2 R2 V2 I3 voltage measuring device N. Delson 7 Updated: 9/25/2008 Kirchhoff's first rule: I1 + I2 = I3 Since voltage-measuring device draws no current, I2 = 0, and thus: I = I1 = I2 Ohm s Law: Voltage drops across each resistors V1 = I * R1 V2 = I * R2 Ohm s Law: Voltage drops across both resisters in series 5 = I * (R1+R2) Kirchhoff's second rule: 5 = V1 + V2 The voltage at the measuring device is equal to V2, which is given by combining the above equations as follows: V2 = 5 * R2 / (R1+R2) Thus, as the pot wiper is moved up the measured voltage approaches 5VDC, and as it is moved down the measured voltage approaches 0. The question remains, what should the total value of the pot be (RT=R1+R2). The current through the pot is given by: I = V/RT = 5/RT The amount of power used by the pot is given by: Ppot = V * I = V * V / I = 5^2/RT One easy way to select the pot value, is to arbitrarily set the current through the pot to a low value of 1 mAmp ( a good number for a sensor level current draw). In this case: RT = V/I = 5/(1e-3) = 5,000 = 5K N. Delson 8 Updated: 9/25/2008 In this case the total power sued by the voltage divider would be: Ppot = V * I = 5^2/ 5000 = 0.005 Watts A more systematic method of determining the value of the pot, would be to consider that the voltage-measuring device is not ideal. Lets assume we use voltage divider as an input into a Microprocessor. The spec sheet indicates that the Basic Stamp s input impedance is 1 Meg Ohm. The maximum amount of current flowing into the Basic Stamp would be: I2 = 5/1e6 = 0.5e-6 Amp As long as I2<<I1 then our assumptions of an ideal voltage measuring device remain valid. Since the value of I2 calculated above is 2000 times smaller than the 1mAmp selected for I initially, we see that our initial selection of a 5K pot will work. Indeed, we could select a larger pot and reduce our energy loss even more, while still maintaining a valid voltage divider. Some questions: Could we use a voltage divider to provide a reliable adjustable voltage source for a motor? What type of problems might we encounter? Hooking up a Switch as a Digital Input Even a simple application as an on/off switch requires proper consideration of voltages and currents. A key consideration is to avoid floating input which are not ties to a specifc voltage. In such a case, any stray noise could cause a logical values to switch form high to low. A proper way of hooking up a switch is: N. Delson 9 Updated: 9/25/2008 A switch hooked up To generate 0V input when open and 5V input when closed (copied from Chris Cassidy s Mechatronics Page) When the switch is not pressed the Lab-X2 will measure 0 volts. This is logic level false or logic zero. This is because the voltage drop across the resistor is zero volts. You can verify this by measuring it with a multimeter. When the switch is pressed down the Lab-X2 will measure 5 volts. This is logic level true or logic one. Now if you use a multimeter you will measure a 5 volt drop across the resistor. The 4.7K resistor is there to prevent the input from floating and to create a very simple ciruit. The value of the resistor is chosen so that only a small amount of current ( I = V/R = 5/4700 = 1 mA) will flow from the Lab-X2 (not too much to drain the it, but enough to keep the input from floating). Hooking up a Light Emitting Diode from a Digital Output Pin Hooking up an LED properly requires that one become familiar with the product spec. sheet. A key specification is that the maximum continuous forward current is 40mA, and that they typical voltage drop across the LED is 1.7V. The circuit below achieves this. N. Delson 10 Updated: 9/25/2008 An LED hooked up within spec from a 5V digital output (copied from Chris Cassidy s Mechatronics Page) Can you answer? What will happen if you use a lower value resistor? What will happen if you use a higher value resistor? Controllers and Digital Logic A microprocessor is a digital device, meaning that all internal values are represented as 0s (logic level low) or 1s (logic level high). To communicate with the rest of the world, one needs to specify corresponding voltage levels. The Basic Stamp is a 5V Transistor-Transistor-Logic (TTL) device, which uses the follow guidelines for logical inputs: Low: 0 to 0.8V High: 2.0-5.0V Note, that voltage values of 0.8 to 2 V are not defined. Indeed the Stamp will work with those inputs (usually reading high above 1.3V), but the manufacturer does not guarantee this. The most robust designs will avoid inputs in this undefined range. If there is an analog value for a sensor, one can convert it to a digital value with an Analog to Digital Converter (see Chapter 3 of Basic Analog and Digital by Parallax). In a corresponding fashion, a microprocessor can only output logic levels high or low. One can convert these to analog values if desired. One approach is to use a N. Delson 11 Updated: 9/25/2008 Digital to Analog converter, and another approach is to use PWM out as described below. Controlling Actuators (higher current devices) To control an actuator one needs to convert the logic from the microprocessor into a signal tot he actuator. Since a microprocessor is a low power device, its output s rarely have enough current to directly drive actuators. One should note that turning an actuator on and off, is much simpler than providing adjustable velocity or torque control. As one gets more sophisticated control (think synchronization, speed control torque control), then the electrical components can cost even more than the mechanical components. Luckily the cost of control is coming down, especially with PWM circuits. The following types or actuator drivers are described on the Basic Stamp web page: Relay Control Transistor Control Pulse Width Modulation (PWM) Circuits Bi-Directional Control of DC Motors N. Delson 12 Updated: 9/25/2008 Topics Not Covered Here - But You Will Likely Need Them in Your Career Analog Meets Digital Comparators Analog to Digital Conversion Driving Loads from a Digital Output Op-Amps RC circuits DC motor control (brush and brushless) Stepper Motor Control N. Delson 13 Updated: 9/25/2008 KEEP THIS PAGE HANDY WHILE WORKING ON CIRCUITS Hands-on Guidelines for Good Circuit Implementation Building electronics can be a fun experience with frequency joy associated with working circuits and hardware. Or it can be a frustrating experience where it seems like circuits only work intermittently depending on the alignment of the stars. While in reality there is always some frustration in getting hardware to work, developing a systematic approach to building, testing, and debugging will serve you well throughout your career, and eliminate numerous hours of frustration. Below are some guidelines to follow. Voltages Current and Power Voltages can easily be measured with a voltmeter or oscilloscope, but currents are harder to measure (you have to take the circuit apart). Therefore you should measure voltages, but calculate current. Do not ignore current since they indicate power (P=VI). Every electronics or electrical component has power input and output specifications o There is a minimum amount of power (and current) that every component needs to operate. This power is much higher for components that do significant work such as motors and electromagnets, and much smaller for logic components like a microprocessor. o There is a maximum amount of power (and current) that every component can output. This current is generally small for logic chips, and higher for power transistors or some op amps. General Tips for Using Integrated Circuits Do not leave inputs floating (i.e. no specified voltage) o use a pull-up or pull-down resistor for inputs that will change o tie constant inputs with high (Vcc) or low (ground) Use capacitors to filter noise (often one places the capacitors as close as possible to filter location, since long copper wires in a circuit have a small amount of resistance which can reduce the effectiveness of a capacitor. Develop Good Wiring Habits Follow color guidelines, especially for the power supply (Vcc) and ground Keep the wires neat (don t create a nest with too much extra length) Use strain reliefs in any wires that attach to moving parts or may be pulled Avoid shorts. Use electrical tape and shrink wrap N. Delson 14 Updated: 9/25/2008 Debugging Skills Debugging is an Art and skills can be developed Be systematic and isolate components o get each component to work separately before integrating Use a voltmeter (and oscilloscopes for fast changing signals) to measure voltage at all points in circuit o If the voltage out of a logic device is less than the high voltage (typically 5V) then one is trying to draw too much current from device. Electronic Do's and Don'ts Have organized wiring. -While signal wires (blue, green and yellow) can be somewhat interchangeable make sure that RED=+5V; BLACK=Ground; White=7.2V. -If your wiring is messy and you ask for help, we may ask you to rewire it in a more organized fashion. We're not being mean, often the problem is found as the wiring is cleaned up. Have a circuit diagram. -Make it clearly labeled. -If you update your circuit, update the diagram too. -We will also ask for circuit diagrams. Make good connections. -Have strain relief on moving electronics so you do not get pull out. -Solder things together and use heat shrink tubing to avoid shorts. Make electronics accessible to swap out components Test each component in sub-units so that you can be confident it works when thrown together Measure a lot (With a multimeter or the oscilloscope) -Voltmeter will be your friend, and if not it should be. -Measure the across the system to get the voltage. -Make sure you are in DC mode. -Ohmmeter is handy if you don't remember color codes. -DISCONNECT the item in question from the circuit. (If not it won't work and might damage sensitive electronics) -Passive components only please. -Ammeter can measure the current going through components. -You must measure in series to get a useful reading. N. Delson 15 Updated: 9/25/2008 References The Art of Electronics by Horowitz and Hill. Considered by some to be the bible on electronic design. Physics 122 Lecture Notes, by Frank L. H. Wolfs, University of Rochester http://teacher.pas.rochester.edu/phy122/New_Lecture_Notes/ N. Delson 16 Updated: 9/25/2008

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