Valve PID Control Unit

This box currently called the FACU will close the loop on the four actuators of each leg. The FACU takes four encoder inputs one for each leg, and sends 4-20ma signals down to the VAPS unit for distribution to the appropriate valve (See my earlier post for further info on the VAPS).

As mentioned previously, this design is probably going to change, but this configuration and hardware should surface for our initial test leg.



The FACU has a dsPIC5011 micro controller, and has been programmed in C using the Hitech compiler. The 4-20ma signals are generated using a four channel 10bit DAC, with a 4-20ma loop drivers on each of the outputs. Although the DAC is only 10bits, this only effects the flow rate resolution of the valve, the actual loop resolution is 16bit, effectively reduced to 15bit with out 15bit absolute encoders.

If higher resolution flow control is required, there is a 12bit version of the DAC.

Femur box section almost complete

We welded the femur box section together this morning, it went together really well, the curved steel slots & tabs all fitted neatly together.. which is a good thing, otherwise there would be something seriously wrong with Alibre Design.



We did find out that the metatarsus ram mount brackets are wrong, this was due to an incorrect sized parker ram we had used for our 3D model, so this will need to be re-cut, and our first mistake takes up its position on the shelf of shame...

Engine Hoist.

Great bit of kit for £160, even if its just to move the work benches around!

The Boxford Lathe Arrived!

Having turned up yesterday on the back of a transit van, and no way for us to get it off... the boxford made its way back to us, this time on a lorry with a tail lift!



Now we have an excuse to try out our new engine hoist :)



Engine hoist worked great for moving the lathe into position, I think it's going to come in very handy during the build! We have yet to get the boxford running, its 3 phase, and currently we don't have a 3 phase socket in the unit. We have borrowed a 3 phase converter from a frined, so will give it a go on that at some point.

Hydraulic Power Pack

We have to get our test leg running within the next couple of months, this means we will need a hydraulic power pack. The pack will need to run one leg, which should require up to 160bar at 25lpm. We have made some enquiries to hire a power pack, which would cost about £100 per month for the size of unit we need. We will need the pack for at least 4 weeks, probably more like 8, so I have been looking in to the cost of buying a pack. To my surprise we can purchase a 7.5Kw power pack with 27 lpm @ 150 bar for about £800.

http://www.hydraproducts.co.uk/TI-050-10%20Standard%20Power%20Pack.pdf

Purchasing a pack will mean we can use it whenever we need it and for as long as we need, and no doubt it will be a great asset to have n the company.

Valve Distribution Unit (VAPS)

This box of tricks distributes the 4-20ma valve control signals from the Feedback Acquisition & Control Unit (FACU). The box takes a 24V feed in, and distributes the power and signals to one of four hydraulic valves. There is an interlock to switch the valves on, that also goes through the FACU box, so if there is a control problem or the signal cable is unplugged, the valve power is removed. There is also an output for the Manifold isolator valve.



I'm considering changing the VAPS / FACU design. Currently the FACU runs the four PID loops, and sends the 4-20ma signal for each valve down a multicore cable to the VAPS. The FACU has two connectors for the daisy chain of the data bus, and four M12 8 way connectors for each of the encoders. The idea is that the VAPS sits next to each 4 way manifold block, and the FACU is mounted on the femur of each leg, with an interconnecting signal cable between the two.

My thinking is that I should try and fit the PID control system into the VAPS unit, so it is local to the valves, then the FACU would simply be an encoder junction box. Doing this simplifies wiring between the two units, and keeps the 4-20ma signal cables very short.

Sick Encoders!

These encoders really are sick! I'm loving them, 15bit absolute rotary encoders, these will provide position feedback on all of our leg joints to close our PID servo loops.



We have four of these to use on our test leg.. once were happy with the control loop, we will order up the remaining 20... kerching!

The first laser cut steel parts arrive!

Our first pallet of laser cut mild steel arrived today. I looks like it has all been cut correctly, lets hope it all goes together properly. I think Josh will finally be able to start welding next week.. he's dying to get off the computer and get his hands dirty :)



These parts make up the steel box sections for one of the legs femur and tibia.

Bearings

Early on in the project it was decided that we would like to use maintenance free bearings for the main leg axis. Usually these would be phosphor bronze or oillite type bearings. However, we have long been fans of Igus igular type bearings, so we had the rep come in and run some of our load calculations through his bearing calculator. According to the rep our loads are well within the limits for the bearings we need, so Igus bearings and Hard anodized bearing shaft it is!



However, now that the bearings have arrived, Josh seems a little sceptical about the loads they can take... I'm sure it will be fine... what could possibly go wrong! :)

Mantisfoot - ball joint

An idea of scale, this is the ball joint we will be using for the Mantis foot next to the V5 hexapod