Servo - November 2011

PARTS IS PARTS:

Fine-Tuning a Brushless Electronic Speed Controller

● by Pete Smith

My 12 lb Hobbyweight bar spinner Surgical Strike had a problem at the last couple of events. If I tried to spin up quickly, the blade would seem to “stall” at about half speed and stay there unless I backed the throttle off and then backed up again to full speed. It would then run up to its full speed.

In most fights, this wasn’t really a problem since there was time to get it spun back up between hits. However, if the opponent was a good driver they could be attacking when the blade was stalled and perhaps even stop the blade completely, getting

me jammed against the arena bumpers. Surgical Strike’s only defense is its spinning blade and if an opponent could get — and keep — it stopped, they would usually win.

I had noticed this problem first at Franklin in I had three main suspects. The first and second were purely mechanical. First, a loose pulley on the motor or the drive shaft could be slipping under the heavy load at start-up but then stop slipping as the blade got up to speed and the load decreased. The second possibility was the motor’s bell housing slipping on the motor’s shaft. This is a known problem and can usually be fixed by tightening and using Loctite on the screw that secures the bellhousing to the shaft.

The third possibility was that the settings of the Electronic Speed Controller (ESC) were incorrect. I had used a variety of controllers over the last few years but had mostly used the Turnigy Plush 80A. It had worked with few issues. The speed controllers are programmed using a simple

FIGURE 1

FIGURE 2

FIGURE 3

programming card, and you can adjust a variety of settings. I suspected that perhaps I was not using the right settings and it was this that was causing the stalling problem.

The ESCs we use are designed for use in model aircraft or cars, so there are no recommended settings from the manufacturer for quickly and smoothly spinning up heavy metal blades. I decided to methodically try all the options and see which worked best. First, I needed a safe setup so that I could spin the blade up without risking an accident if a tooth or even the whole blade came loose.

I used three sections of my Antweight arena with its 3/8” Polycarbonate (PC) walls as an enclosure, and used another old section of PC and some zip ties to secure the bot in a stationary position (Figure 1). I screwed a square of thin UHMW under the weapon’s axle to reduce friction and to stop the axle from wearing a hole in the PC sheet.