Servo - March 2014

Typical hexapod and octopod robots display incredible agility. When each leg has four DOFs (degrees of freedom) and four servos, remarkably lifelike motions can be achieved. Of course, there is great mechanical, electrical, and software complexity to control 24 to 32 servos. For a less daunting first walker, I chose to hack a simple $8 "Spider Robot" kit from eBay.

This motorized, eight-legged mechanical spider walks in a straight line. A Klann linkage converts the motor's rotary motion into each leg's D-shaped path. There’s a nice kinematic presentation at www.mechanicalspider .com/ comparison.html if you’d like to check it out.

The spider robot consists of identical left and right gear-driven leg assemblies, powered by a central gearmotor through a common steel driveshaft. A single alkaline AA battery drives the spider, but it's a little underpowered. It was "peppier" after I replaced the battery with a four volt

14500 Li-Ion cell. I suggest building the stock kit before hacking it. There's a helpful video at www.youtube.com/ watch?v=aKIbu4pf2zw.

A similar $40 IR RC spider robot kit I found on eBay is fine for basic remote control, but for programmability, expandability, onboard sensors, and autonomous operation, we must get our hands dirty. You'll build a better bot with that $40 than with a kit.

You'll need to order some parts, so plan ahead. I enjoy finding parts bargains and stocking up to build stuff, even if some waiting is involved. Good things come to those who wait ... and hack! Design Considerations

The simplest way to steer the spider chassis is to use differential steering. That means two motors drive the left and right leg assemblies independently — much like tank treads. Servos modified for continuous rotation are easy to use. The plastic spider legs are skinny and somewhat flexible, and there is some slop in the linkage pivots. This eases assembly and improves durability, but it limits the load capacity of the bot. We want to minimize total weight and keep the CG centered to avoid overloading the legs to make the spider walk efficiently. Servo Selection and Modification

Not all nine gram servos are suitable for the well-known continuous rotation hack. Some don't have a complete gear on the output shaft; some have unstable low speed performance. I chose Hobby King HXT-900 nine gram servos. They are inexpensive, strong, and reliable. They must be modified mechanically and electrically.

The feedback potentiometer provides support for the output shaft, so even though we will eliminate its electrical function we must preserve its mechanical integrity. (See my videos at

www.youtube.com/ watch?v=wfmM3cuihJI and www.youtube.com/ watch?v=7pxRhxBPSUM).

Working on a clean surface, remove the four screws and the top of the case where the gears are. Pull the large output gear straight off and remove the two small molded mechanical stops from the back of the gear. Use cutters and a hobby knife to remove them completely. Next, modify the potentiometer internals so the shaft can rotate 360 degrees freely. Two fixed black mechanical stops and some thin rotating metal contacts inside need to be cut back.