Modifying the Hextronik HX5010 Servo Motors for Babuinobot 1.0 Brett Nelson January 2010 1
Converting the Hextronik HX5010 to a Geared DC Motor The following pages will describe step by step the process of modifying a Hextronik HX5010 from a standard servo motor into a DC motor without any drive circuitry. Most servo modifications show how to convert to a continuous rotation servo, but the Babuino hex file provides no servo control output and instead uses standard H-bridge output controls. For those wanting a continuous rotation servo, there are many servos out there that are easier to hack, but I will provide some (untested) hints on how to do it. Why use the HX5010? Simply because it is the cheapest hobby servo I could find that had decent reviews. At the time of this writing, it is available for $5.95US at hobbyking.com. Note that these ship from China, so for a box of 12, shipping was about $25US and it took about 5 days; if I was willing to wait 4-6 weks, shipping would have only been about $10US. So, for just a few bucks, and some effort, you get a compact, easy to mount gearbox DC motor with ball bearings. Good luck and enjoy! I strongly recommend working on only 1 servo at a time. That way, if all the pieces come flying out, you have another one to look at to see how they fit back together. Unscrew the four case screws that hold the bottom cover on and remove the bottom cover. Then carefully remove the top cover to expose the gearing. Remove the output shaft gear. Underneath will be a small black disk with a rectangular hole in the middle that connects the potentiometer to the gearing. plastic disk output shaft gear Use a thin blade or a small screwdriver to gently pry the plastic disk off and remove it. Do not put the disk back in during reassembly. 2
There is one tab on the output shaft and 2 tabs inside the case that prevent the shaft from rotating in a complete circle. We will remove the tab on the output shaft with a sharp knife. Some people cut the tab off using diagonal cutters, but it puts a lot of internal pressure on the gear and I've had a couple of the gears shatter in the process, so now I always slice it with a sharp blade. tabs Start by making 3-5 vertical slices through the tab just as if you were slicing bread. Make sure that your fingers are always safely out of the path of the blade! Then flip the gear upside down and make a horizontal slice to remove all of the pieces formed with the vertical cuts. Make sure that the tab is removed clear to the output shaft and that none of it sticks out above the level of the gear teeth. Clean out any plastic shavings from the cutting process or they will damage the gears. Reassemble the gearbox, leaving out the plastic disk that was removed earlier. If you don't remember how it goes back together, open another servo and look at it. If there are any globs of grease, you might want to take a clean toothpick and spread it around on the gear teeth. Be patient and make sure that all of the gears and gear shafts line up properly. Once the gearbox is firmly in place on the servo motor, hold it tightly in place as you slide a servo horn over the output shaft and gently turn it around. It should turn without binding or making ugly grinding noise. If there are any problems, open up the gearbox and check everything again. Once the gearbox is in place and running smoothly, just leave it in place while you work on the electronics underneath. Even without the screws, the gearbox will usually stay in place while you work on the bottom. 3
Hints for those wanting to make a continuous rotation servo: (the rest of you making a DC motor for the Babuinobot, skip this part) potentiometer leads The easiest way to use the existing circuitry for a continuous rotation servo is to replace the potentiometer with a 2-resistor network. With many servos, you can lift out the circuit board to gain access to the potentiometer. With this model, the potentiometer is soldered directly to the board and it is also held to the case with a retention screw, so if you try to pry the board up, you will destroy the potentiometer and may damage the board. The potentiometer was screwed into place, then the circuit board was put into place with the long leads from the potentiometer sticking through the board, and finally the leads were soldered to the board. Reverse the process. Desolder the potentiometer leads using a solder sucker or solder braid. Pull the board off carefully; the motor is attached and fits snugly in the case. Remove the retention screw and lift out the potentiometer. I do not know which lead is the swiper of the potentiometer. To find it, rotate the potentiometer so that it is about in the middle of its travel and measure the resistance of the three possible pairings of leads. The pair of leads that has the greatest resistance will not contain the swiper. Find two equal resistors that are each about half of that maximum resistance. Both resistors will be soldered to the board where the swiper was soldered. Solder the other end of the resistors to the two remaining pads where the leads were removed(one resistor for each pad). If you are using through-hole resistors, the resistors will need to be placed on the motor side of the board and soldered on the surface mount side; there is not enough room for the resistors on the surface mount side. Trim the resistors leads, reassemble, and test. If you have surface mount resistors and if the swiper is the middle lead, you should be able to solder surface mount resistors between the pads on the surface mount side. I have used this method on other servos with success, but have not tried it with this model. It should work, but your mileage may vary; proceed with care and caution. Back to converting the servo into a DC motor. Lift the wire strain guard up from the servo case and slide it back down the wire a couple of inches. It fits in there snugly, so it may take some work. Use wire cutters to cut the cable near the circuit board. Desolder and remove the ends of the wires. 4
Use a sharp blade to cut the traces that connect the two power transistors to the motor leads. You will need to make several passes to cut clear through the traces. Test the resistance between the motor pads and the power transistors. There should be at least 10 KΩ; if it is less than that, cut the trace deeper. Cut the yellow wire about ¾ shorter than the others and the orange wire about 1/8 shorter than the brown wire. Strip some of the insulation off of the orange & brown wires and tin them with solder. Then solder them to the two motor pads. Route the wires down the center of the circuit board between the power resistors. Slide the strain relief guard up the wire and back into the case. Put the bottom cover back on and tighten the screws. All done! You now have a DC motor with a gearbox that is easy to mount, has ball bearing construction, and is inexpensive. 5