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Electronic Building Blocks By Julian Edgar Quick and easy construction Great results on a low budget Controlling RC servos with the Pololu Micro Maestro Quick and simple programming for automatic servo movements, or manual control with a pot or switch I n previous articles, we’ve looked at selecting actuators (PE, October/ November 2022); controlling stepper motors using an eBay pushbutton-programmable standalone module (PE, December 2021); and controlling linear actuators using the Pololu Jrk 21v3 PC-programmable module (PE, June/ July 2022). In both the linear actuator and stepper motor articles, set-up and wiring were straightforward, putting control of these actuators into the hands of people who may not want to learn to code. This month, we look at the Pololu Micro Maestro board, a PC programmable device than can simultaneously control up to six RC servos. Again, we’re going to take the simplest approach: using a pot (a manual control knob) or a switch to control servo position. However, in addition, we’re also going to look at how a sequence of instructions can be used to automatically cycle the servos through a series of movements. Pololu Micro Maestro The Pololu Micro Maestro is a tiny (just 28 x 22mm), light weight (3g) board that packs a lot of capability into a small footprint. All connections are by header pins – there are no screw terminals. It costs around £40 and is available from a wide range of sources. Part of a family of different Maestro servo motors controllers, the Maestro has six channels (other Maestros have up to 24 channels). They’re called ‘channels’ rather than ‘outputs’ because each channel on the Micro can be used as an RC servo drive or as an analogue input channel. The channel function is selected in the PC configuration software – more on the software in a moment. Each servo control channel can be set for maximum speed and acceleration, and if an error is detected, channels can be configured to go to a specified servo position or turn off. The servo(s) can be controlled from a PC via a USB cable, by a serial connection or by an internal program (called a script) that also allows user control via external pots or switches. It’s the ‘script’ approach that we will be covering here. the board can be powered by the USB connection and the servos powered by a power supply (or batteries, of course). The servo power supply connection (power and ground) makes available power to all the servo channels, so only one pair of connections is needed. See Fig.1 for these connections. With 4.8-6V connected to the module’s servo power feed, the next step is to connect a servo itself. The three-pin female plug provided on RC servos plugs straight into the board header pins at any of the six channel positions (labelled 0-5). However – and this is important – the servo must be connected with the correct polarity. Servo wiring is typically: Ground black brown black Power red red red Signal white orange blue Initial set-up and testing The Pololu Micro Maestro board is a tiny – just 28 x 22mm and weighing 3g – but it can control up to 6 RC servos. The included software gives full control over servo position, and maximum servo speed and acceleration. Practical Electronics | May | 2023 To get the Micro Maestro up and running, you will need a USB mini-B cable (not included), an RC servo and a power supply suitable for the operating voltage of the servo (4.8 – 6V). The power supply connection for the servos is separate to the power supply for the Micro Maestro, but since the Maestro can work on 5-16V, the servo power supply can also supply power to the board. Alternatively, RC servos are versatile controllable actuators that are ideal for operating arms and legs in hobby robots, moving items in animated model layouts, opening and closing vents and many other uses. They’re low in cost and widely available. 63 +5V regulated Servo power +V +V –V –V Channels 0-5 USB connector +V –V Board power 5-16V Signal +V –V Channels Fig.1. These are the connections that you’ll need to make to do all that is covered in this article. The two-pin header for the regulated 5V supply and associated ground connection will need to be provided and soldered into place – it’s not included. The table is read from left to right – that is, one common plug is black-red-white, for example. See Fig.1 or the underside of the Maestro for these servo connections. The next step is to load the Windows drivers and software on to the PC. This is best done by clicking on the download link in the excellent Micro Maestro PDF user’s guide. (The guide is also downloadable from Pololu and its dealers.) There is a large section in the manual on installing the software, but with Windows 10 Pro, I just followed the onscreen prompts and had no difficulties. Now it’s time to control the servo. With the Maestro connected to the PC, servo power switched on and the servo connected, open the software. Software connection with the module is established in the Connected to pulldown. Once the PC is talking to the Maestro, you can then tick the box to Enable the channel to which you wired the servo. Then, without doing anything further, when you move the relevant on-screen slider back and forth, the servo will rotate accordingly – see: Fig.2. Note that quick movements of the slider will give equally quick movements of the servo. To produce smoother movements, change the value in the Speed box. For example, setting this to ‘5’ meant it took the test servo about 8 seconds to go from one extreme of movement to the other. In addition to maximum speed, you can also set maximum acceleration. Fig.2. The Status tab allows you to easily control the movement of the servo by dragging the slider back and forth. Five different servos can be controlled but here only one output channel has been activated. Fig.3. In Channel Settings the speed, acceleration, minimum and maximum positions of each servo can be set. Each channel can also be named (not done here). 64 Practical Electronics | May | 2023 Fig.4. In the Sequence tab, the use of frames (think of them as movement steps) allows a sequence of movements to be developed and then uploaded to the module. It’s a quick and easy process, especially for repetitive movements (like making a robot walk). 10kΩ +5V 0V Fig.5. Using the regulated 5V and ground connections, a pot can be energised and its variable voltage output connected to a channel input. This allows manual control over the servo position. By using a combination of these controls, you can have the servo smoothly accelerate to a set maximum speed, and then just as smoothly decelerate to a stop at the setpoint. These effects can be judged not only from the action of the observed servo, but also from watching the on-screen slider, where the setpoint is shown by a marker and the actual servo position by a dot. Now open the Channel Settings tab. For each connected servo, the minimum and maximum rotational positions can be set. In this tab you can also set the servo behaviour on start-up or when an error is detected – off, ignore or go to, with the latter allowing the position to be specified. In this tab you can also see the Speed and Acceleration settings you’ve already configured, and you can name each channel as you wish. Note that any changes need to be uploaded to the Maestro by pressing Apply Settings – further details in Fig.3 You might be wondering what units are used for the servo position. These are in microseconds (µs) pulse width – something that sounds odd until you realise that it is the pulse width of the signal that controls servo position. (See the breakout box for how an RC servo works.) Fig.6. Here, Channel 5 has been altered in function to become an input. Rotation of the added pot moves the Input slider, so that it’s easy to checking that the pot input is working. Practical Electronics | May | 2023 65 Fig.7. This very simple program allows the external pot to control the movement of the servo. A switch can also be used with this program to give full servo movement (for example, to open/ shut a vent). Now that we have the PC controlling the servo, let’s program some sequences of movements. Automatic sequential movements using frames The Pololu software can be used to program a sequence of movements, called ‘frames’. The software can then be used to test and (if necessary) edit that sequence, before uploading the collection of frames (called a script) to the Maestro. If the script is configured to operate as a loop after switch-on, the Maestro will then act as a standalone controller (no PC connection needed), the sequence of servo events repeating until power is cut. To achieve this programmed movement, go to the Sequence tab. The Frame name box will be empty at this stage, but you will be able to see that there are various editing tools and other controls available for manipulating the frames. To create the frames, go back to the Status tab – the one with the sliders. Set the position of the servo(s) for the first of the sequences of movement, then press Save Frame 0. Change the servo position and then press the Save Frame button again. Continue with this process until you have all your movements saved in a sequence of frames. Now return to the Sequence tab and you’ll find the new frames listed there in order. Press Apply Settings and then when you press Play Sequence, the servo will make appropriate movements. If you tick Play in loop, the sequence will run continuously. RC servos RC (radio control) servos were developed for use in, not surprisingly, radio-controlled models. However, their versatility, low price and wide availability means that they’re now used in a broad range of hobby activities. Mechanically, a servo comprises a small DC motor connected to a geartrain, with the output shaft splined to take various levers and disks to which other linkages can be connected. A servo is a ‘smart’ device – it contains control electronics. An internal pot is used as a position sensor, and the motor rotates until the shaft reaches the required angular position. This position is then held – if the shaft is mechanically rotated from its set position, it actively resists. 66 However, you might notice that some movements don’t have enough time to be completed before the next one starts. To fix this, highlight a frame and then click on Frame properties. The time available for each frame can then be changed, with this time expressed in milliseconds, as shown in Fig.4. Pressing Copy sequence to script and then, in the Script tab, Apply Settings lets you both see the code you’ve just created and upload it to the Maestro, respectively. This sequence of servo movements is now programmed into the module and can run continuously once power is applied, even without a PC connection. (Note: the speed and acceleration of all the movements in the sequence can be adjusted in Channel Settings.) For sequences of movement, especially those that are repetitive like making a robot walk, this approach is fast and easy. But what if you don’t want a pre-programmed movement sequence? Then you can use an external pot or switch to control the servo rotation. External manual control Let’s now add a pot to give manual control over the servo position. First, a 2-pin header needs to be soldered into the GND and 5V out PCB holes. These then provide the two end connections to a 10kΩ pot, while the wiper connects to the input of one of the free channels (we will use Channel 5) – see Fig.5. Servos use three connections – power, ground and signal. The control signal comprises a pulse train with a varying pulse width ‘on’ time. It is not a PWM system with varying duty cycle, because the frequency of the pulses can vary over a wide range without affecting the servo’s behaviour. (Standard frequency is about 50Hz – a 20ms period.) The pulse width determines the servo’s position, with 1.5ms corresponding to the servo’s neutral point. That is, shorter pulse widths than 1.5ms causes the servo to rotate one way, and pulse widths longer than 1.5ms cause the servo to rotate the other way. The normal pulse width range is 1.0ms to 2.0ms, which corresponds to an output shaft rotation of up to 180° degrees (maximum – but in practice, often less than this). Centre position VCC 0V 1.5ms pulse width 20ms period VCC 0V 2ms pulse width 20ms period VCC 0V +90° –90° 1ms pulse width 20ms period Servo control signals use pulse width to control servo position. 1.5ms is the neutral position; shorter pulse widths rotate the servo in one direction and longer ones rotate it in the other direction. The full range is typically 1-2ms. The frequency of the pulse train can vary but is typically around 50Hz (a 20ms period). Practical Electronics | May | 2023 Go to the Channel Settings tab and change the function of Channel 5 from Servo to Input. Now back on the Status tab, when you turn the pot, you should see the slider for Channel 5 moving to show the changing input signal (Fig.6). To make the system use the pot to control the position of the servo, some scripting code is needed. However, this is just a few lines as shown below and in Fig.7: 1 – 100kΩ +5V 0V Switch begin 5 get_position 4 times 4000 plus 0 servo repeat Type this directly into the Script tab, tick Run script on startup, run the script and then press Apply Settings and the servo should move as directed by the external pot – and continue to do so, even when disconnected from the PC. Incidentally, if you wish to better understand the code, the PDF manual has a good explanation. For example, if you change the endpoints of the servo movement, you can change the pot scaling to suit, so that there’s no dead travel in the pot’s movement. Even when controlled by a pot in this way, the maximum speed and acceleration of the servo can still be set by the previously described controls. It’s therefore easy to give a progressive and smooth servo movement, even if the pot is turned quickly and/or jerkily. To move over its full rotation, a switch can replace the pot. Pull the input up to 5V via a 1-100kΩ resistor and then use the switch to pull the input to ground – see Fig.8. Operation of the switch will now cause the servo to rotate from one extreme to the other – perfect for things that need to be open or closed, such as a vent. Again, this movement GET T LATES HE T CO OF OU PY R TEACH -IN SE RIES AVAILA BL NOW! E Fig.8. A switch allowing the servo to be set to two positions. The input is pulled up to 5V via the resistor when the switch is open; the input is pulled to ground when the switch is closed. can be made to occur smoothly by using the Acceleration and Speed controls. Conclusion The Pololu Micro Maestro has a lot more capability than has been covered here (the manual is over 100-pages long). However, in this article I have been aiming at giving you fast and practical ways of controlling servos – methods than can be put into immediate effect. Whether that’s opening and closing household vents, building a walking robot or even operating active aerodynamics on a car, the Micro Maestro and RC servos can help achieve all that. Order direct from Electron Publishing PRICE £8.99 (includes P&P to UK if ordered direct from us) EE FR -ROM CD ELECTRONICS TEACH-IN 9 £8.99 FROM THE PUBLISHERS OF GET TESTING! Electronic test equipment and measuring techniques, plus eight projects to build FREE CD-ROM TWO TEACH -INs FOR THE PRICE OF ONE • Multimeters and a multimeter checker • Oscilloscopes plus a scope calibrator • AC Millivoltmeters with a range extender • Digital measurements plus a logic probe • Frequency measurements and a signal generator • Component measurements plus a semiconductor junction tester PIC n’ Mix Including Practical Digital Signal Processing PLUS... YOUR GUIDE TO THE BBC MICROBIT Teach-In 9 – Get Testing! Teach-In 9 A LOW-COST ARM-BASED SINGLE-BOARD COMPUTER Get Testing Three Microchip PICkit 4 Debugger Guides Files for: PIC n’ Mix PLUS Teach-In 2 -Using PIC Microcontrollers. In PDF format This series of articles provides a broad-based introduction to choosing and using a wide range of test gear, how to get the best out of each item and the pitfalls to avoid. It provides hints and tips on using, and – just as importantly – interpreting the results that you get. The series deals with familiar test gear as well as equipment designed for more specialised applications. The articles have been designed to have the broadest possible appeal and are applicable to all branches of electronics. The series crosses the boundaries of analogue and digital electronics with applications that span the full range of electronics – from a single-stage transistor amplifier to the most sophisticated microcontroller system. There really is something for everyone! Each part includes a simple but useful practical test gear project that will build into a handy gadget that will either extend the features, ranges and usability of an existing item of test equipment or that will serve as a stand-alone instrument. We’ve kept the cost of these projects as low as possible, and most of them can be built for less than £10 (including components, enclosure and circuit board). © 2018 Wimborne Publishing Ltd. www.epemag.com Teach In 9 Cover.indd 1 01/08/2018 19:56 PLUS! You will receive the software for the PIC n’ Mix series of articles and the full Teach-In 2 book – Using PIC Microcontrollers – A practical introduction – in PDF format. Also included are Microchip’s MPLAB ICD 4 In-Circuit Debugger User’s Guide; MPLAB PICkit 4 In-Circuit Debugger Quick Start Guide; and MPLAB PICkit4 Debugger User’s Guide. ORDER YOUR COPY TODAY: www.electronpublishing.com Practical Electronics | May | 2023 67