Monday, 29 March 2010

LED Strip Lighting

The LED strip lighting arrived this morning from China, a nice birthday treat! I found this 5m RGB strip lighting on EBAY, costs around £90 including PSU and controller. I won't use the PSU or controller in the final design, but its good for an initial test. First impressions are good, the LEDS a pretty bright, and the silicone waterproof cover is plenty good enough. We did a quick test with the strip inside the leg cavity, looks really good, especially being able to change the colours! :)

Wednesday, 24 March 2010

Slice valve and pressure releif

Now we have ruled out the use of a counterbalance vale in our system, we need to find a suitable slice valve to lock the leg when the system is switched off. However, with a slice/poppet valve the system could be prone to pressure intensification issues, therefore we would also require and additional dual sandwich relief module per axis, which would be a hefty 3 valve cetop 3 stack!

Fortunately we have found a solution that should incorporate the slice and relief functions into one sandwich valve. This is special manifold from Sun, which uses two T-21 cartridges to control flow through A and B, with a pilot line controlled through a T-9 cartridge. This means we could use counterbalance valves, piloted from P through a solenoid valve. The counterbalance valves will give us our pressure relief function when the system is off, and the solenoid control will allow us to enable one axis at a time, so we can remove the manifold isolation valve.

Tuesday, 23 March 2010

Counterbalance Valve

Having spent some time researching, and asking the opinion of many hydraulics specialists with regards to the use of counterbalance valves in closed loop systems, I have decided that the only way we will find out if it may work for us, is to try one. We ordered a unit from Sun hydraulics, it arrived this morning, very prompt service!

The counterbalance valve sits under our 4WREE proportional valve. We have two different cartridges, one for the bore side with a adjustable setting of 70 to 280bar and one for the annulus side with a setting of 28 to 100 bar. We started with the valves set to their factory defaults of 210 and 70 bar  respectively. The leg seemed to be working pretty well at first, the up stroke was fine, but on the down stroke we started to notice oscillation. We tweaked the pressure setting of the annulus side, reducing it to approximately 50bar, this did help, but we couldn't get rid of the oscillation completely. Therefore we can only assume that we can't use the counterbalance valve in our system. At least now we know!

Thursday, 18 March 2010


We have added an 2.8L accumulator into the system to help with flow fluctuations during heavy demand peaks. The accumulator is fitted to an accumulator safety block, with isolator ball valve, pressure relief and solenoid unload valve. The solenoid will be plumbed in to the E-Stop system, so that when it is de-energized the accumulator dumps to tank.

Tuesday, 16 March 2010

SID & VAPS Pcb's V2

I have started the assembly of the second version of the VAPS, FACU and SID pcb's. The VAPS and SID pcb's were from my cheese pcb manufacturer,, pretty good service, and usually much cheaper and faster than the UK. The FACU pcb is still being manufactured in the UK at spirit circuits using my 10 free pcb's voucher.

I need to get this new version of the control system up and running asap, so we can add the pressure sensors and load pin into the system and graph the output of the leg.

Thursday, 11 March 2010

Pilot operated check valves

We think we are starting to understand the air issue. It seems that when our leg is lowering at speed, the lowering speed can reach a point that the either the flow rate of the pump is exceeded or the flow rate through the valve limits flow to a point that negative pressure is produced. This we believe is releasing air in the hydraulic oil on the bore side of the ram, then when the ram changes direction at the bottom of the stroke, the air is re-compressed back into the oil causing the jolt we are experiencing.

The other issue is that when the leg is switched off, due to the leakage of the fourth position on our proportional valves, the leg slow sinks, as oil is compressed on the annulus side of the ram and squeezed past the valve. This seems to drag air in some where around the manifold and into the bore side of the ram. So what is needed is a zero leakage solution to lock the leg when power/pressure is switched off. For this we can use pilot operated check valves on both the bore and annulus sides of the femur ram. However, in a closed loop system, the AB ports should not be piloted from each other, they must be piloted from the system P port so as not to disrupt the control loop. This was working pretty well, and indeed has stopped ingress of air into the femur ram by not allowing any movement when the system is switched off, however, we found the bore side of the ram was being restricted by the check valve, so we have removed this valve leaving just the one check valve on the annulus side, which still hold the leg still when power is remove, providing of course the load is pulling the leg down.

We are trying to find a cetop 3 sandwich valve solution, which we will install on every leg proportional valve, so far we can only find pilot operated check/poppet valves that are cross piloted from AB and not from P.

We have found a slice valve that could be installed under the prop valve, but this requires two solenoids to block both A & B, which of course is a little power hungry and seems excessive.

Air still in the system

So, the theory was that one of our air issues was caused by the low friction seals in our rams, hence the strip down, seal re-placement and re-build. Well it wasn't the seals.. the air is still a problem, but the cleg control hasn't been effected by the use of standard friction seals, so I guess we have learned something from the seal kit change.. and wasted time and money!

Next we have a series of possible fixes suggested, we though it best to try one thing at a time:

1) Add a check valve in to the return line back to the pump. This had no effect on the air issue.
2) Lift the pump/tank up above the height of the manifold block. This had no effect on the air issue.

Our new pump configuration, on top of our welding fume extractor :).

Wednesday, 10 March 2010

Leg is back together

The leg is now back in one piece, next job is to re-calibrate the encoders and give it a quick test.

We need to install some spiral wrap on the metatarsus before we can run the leg properly. The metatarsus lines now run outside the knee joint, as we were unable to get a sufficiently good rout through the joint without stressing the hose. Having the lines on the outside of the joint is ok, but it does mean they can come in contact with the sides of the leg, hence the need for additional protection, turns out it looks pretty cool too!

Tuesday, 9 March 2010

Leg rebuild begins..

We started to put the leg back together today. So far we have replaced the rams, including the new red end bearings. Ncxt job is to replace all of the Igus spherical bearings with steel on PTFE type. We have also ordered the new hydraulic hose set, in Flexor 7, this hose is much lighter than standard twin braid, however, it has a lower working pressure of 155 bar.

Monday, 8 March 2010

Ball joint protector

I found a piece of flexible hose large enough to cover the ball joint. It does the job pretty well, that is to give the joint some protection from dust and debris. I would however still like to find a rubber belows type protector large enough for the job, as the belows would act more like a spring to help re-centre the foot.

Wednesday, 3 March 2010

New Hydraulic Ram Seals

Well we finally got all of the new ram seal kits, and started the process of dismantling the rams and re-fitting with the standard seal kits. The image here shows one of the 40mm pistons, with the wear rings removed and the low friction seal still fitted. In order to get the fitted seal off, we had to cut through them, thus destroying them in the process. To get the new seals on wasn't easy.. at least not without the right tools, so we made our own expansion sleeve to fit the end of the piston, then slowly pulled the seals into place over the sleeve. A bit of a mission, but all 3 rams are now done and back together, ready to be refitted to the leg.

Monday, 1 March 2010

Leg strip down begins.

Seems like we are going backwards, but we had to completely strip the leg down in order to make various changes that we hope will improve performance. Both Josh and I found the task of having to strip and re-build the leg somewhat daunting.. but it has to be done!

First we took the three low friction hydraulic rams off the leg, ready to have the selas replaced with the standard frictions versions. According to Parker this should help us with air in the rams, which we have been having problems with. At the same time all the ram rod end spherical eye bearings are being replaced with maintenance free PTFE on steel type, to reduce mechanical play in the system.

We are also replacing the majority of the IGUS bearings on all of the main joints, with maintenance free PTFE on steel bearings. Again this is to get rid of much of the mechanical play caused by the sloppy fit in the IGUS bearings. For this sleeves have to be made in order to get the new smaller bearings to fit into the IGUS bearing holes.

Finally we are replacing our standard two wire braid hydraulic hoses with a lighter weight polymer version called Flexor 7. This is a bit of a gamble, as we don't know anyone with experience of these hoses, but a according to the spec. sheets they are rated for our application, and will save us 45Kg overall in machine weight, which is not to be sniffed at!