May 2004

Saturday 1^st May 2004

I hadn't done a lot to the Hog since the Brighton Modelworld event, but if I was going to enter the forthcoming Portsmouth event, I had to do some repairs and modifications to the Hog.

I located the reason why one drive would only go forward, which was a blown clamping diode on the power MOSFET brick. Effectively it was clamping the gate inputs of one branch of the H bridge thereby preventing the MOSFETs turning on. I simply replace the diode and full capability was returned to the drive.

The next task I tackled was to replace a number of the armour panel mounting brackets. At Brighton many of them had bent in battle so I decided to toughen them up by replacing the current ones with 3mm thick angle iron, and increase the bolt sizes from 5mm to 6mm Allen screws. For each bracket I had to cut it to length, drill it, weld a 6 mm nut on it, tap the threads out after the welding had distorted them, hacksaw the old bracket off the chassis, and finally weld the new one in its place. Neither of these individual tasks was particularly difficult, but completing one bracket would take between 20 and 30 minutes. The real problem was the number of brackets I had to replace. I counted 50 in all that were weak and needed replacing, which meant it would take 20 to 25 hours to complete the lot. Working on the basis that I could only manage an hour or so in the garage every other day, it was going to take me a month or two to replace all these brackets. In the end I had to force myself into the garage every night during April to keep on schedule for completion before the Portsmouth event on 8th May.

The Hog's tusks had always been more for more show than a serious weapon, but while replacing the mounting brackets I noticed the chassis members the tusks were bolted to were quite bent. It seemed that because the tusks were curved, they would tend to "ride-up" the opposition during collisions and effectively bend the members they were bolted to. Despite them being hardened the tusks had also bent during their life-span and offered no real chance of damaging the opposition apart from maybe scratching their paintwork! After some soul searching, I decided to remove the tusks altogether to stop any further distortion of the chassis. They would also release some weight that I could put to more productive use; each tusk weighing about 1kg.

Thursday 6^th May 2004

After the long and tedious replacement of the mounting bracket, I had to now make a new front panel to replace the severely damaged one that resulted from the Brighton event. I decided to make the new one out of 10mm Polypropylene rather that the 3mm sheet it was made from previously. With the removal of the tusks I had enough spare weight to risk the extra thickness without going overweight. The hardest task of making the panel was to bend it into shape. I marked where the bends should be, and then clamped the panel in the workmate. I then used a hot air paint stripping gun to heat up the panel while continuously leaning on it. It took about 15 minutes of heating before the panel finally yielded and I could bend it into shape. After that, it was simply a case of ligning it up and drilling the mounting holes before screwing it in place.

Saturday 8^th May 2004

The Portsmouth event was by far our most successful days battling. Some battles we won and some we lost, but we kept going right until the end despite the use of copious amounts of gaffer tape. You can read about the battles in the events section.

Friday 28^th May 2004

I came across an interesting device the other day while browsing the Rapid Electronic catalogue that I might use to update my motor controller. It's a "fully auto-protected" power MOSFET (VNP49N04) that has the following characteristics

• Linear currrent limitation of 49Amps
• Thermal shut down
• Short circuit protection
• Integrated clamp
• ESD protection

Although my present controller uses power MOSFETs that can handle more current that these (140 amps each!), I have blown quite a few up in the past. I'm not sure quite why this has happened, but I suspect that with all the nasty voltage spikes that are produced, the microcontroller might have switched both upper and lower MOSFETs on together and effectively shorted the batteries out with them. I don't have any current limiting circuitry to sense if this happens, so the MOSFETs simply self destruct when this situation occurs. However, by using these new "fully auto-protected" devices, I may be able to survive this scenario in the future.

I therefore ordered a couple of these devices to test them out. I planned to do some nasty tests, and if they survived, I might then replace all my existing components with these. Not only would they save me blowing up the power stages, but they would also provide me with a current limiting function should the motors want to draw more that 196 amps. My power stage used four MOSFETs in parallel in each branch, so having 4 x 49 amps means they will limit when I draw more than 196 amps.

The devices arrived today so I wired them up directly across a 12 volt battery and then turned them on by applying a signal to the gate terminal. One of them failed immediately, but I'm not sure that I might have wired it up wrong! When I tested the second one, I couldn't blow it up. It limited the current when I switch it on, and after a while it then thermally shut down for a few seconds. With a reasonable heat sink on it, it then sustained the current load without any problems.

I was quite impressed with its performance so might now order a few more and test them out for real. They cost about 3.30 pounds, which I didn't think was too bad for a component that could handle 49 amps each.