Friday, 26 February 2010

Ebay Valve.. all good

I installed the 4WRPE bosch bargain valve found on ebay, along with my 4-20ma to +/-10V converter toady, in place of one of the 4WREE valves used on the tibia axis. The valve and electronics are both working well! I doubt we will use this valve in the final machine, however, with the use of my signal converter it has made a very cheap spare, the valve cost around £140 and excluding my design time the converter probably about £30. To purchase this valve from bosch, the list price would be £1600!

Thursday, 25 February 2010

Hydraulic Valve Signal Converter

I have assembled a simple signal converter to convert 4-20ma to +/-10V in order to test my bargain 4WRPEH proportional valve acquired from Ebay. If this works it would make a good spare for the Mantis. It also means I can keep a look out for other bargains which will hopefully save a bit of the budget!

I will place the unit in a small case tomorrow, with an Hirschmann CM series connector and flying lead to connect to the valve.

Wednesday, 24 February 2010

Standard Ram Seal Kit

At last! our standard seal kit for our hydraulic rams arrived today. Parker seem to take forever from receipt of order/money to get things to the customer! Although having opened the package I find that the 50mm kit is missing, and is apparently coming separately??

Monday, 22 February 2010

PTFE on Steel bearings

As mentioned in my previous post, I have now changed the bearings in the coxa rame to PTFE on steel type. These have a much better fit on the shaft and as the specification suggested have much less play. Hopefully this will improve the overall mechanical play in the leg, which should improve leg control. When we strip down the leg to replace the ram seals, we will replace all the rod end bearings.

Friday, 19 February 2010

Bargan Bearing Press

In order to remove some of the mechanical slop in the leg,I'm going to change the Coxa ram bearings, hopefully next week. Currently we have steel on steel spherical eye bearings, which are generally best suited to our application, however, there is 0.04 to 0.08mm of radial play in each bearing. By changing these to maintenance free steel on PTFE type bearings, the play is reduced: 0.00 to 0.04mm. They are not as suited to the application, but we are well under their rated load.

And hence the purchase of this 1Te arbour press, a bargain at £20.

Friday, 5 February 2010

Improved foot design.

Here is our first finished test foot. The lower half is currently made from MDF, which will ultimately be cut from 25mm 5083 aluminium. The tyre we have been suing has been sliced in two, and I intend on gluing a foot pad to the bottom of the tyre where the rim would usually be. This will create the completed pill shape, however, we are currently slightly concerned about the strength of the glue?

I have managed to get the ball join further down into the foot, I hope this will improve stability and reduce the chance of the ankle buckling over. The ball joint could be taken further into the tyre, but this would reduce the foot compression range, which is currently about 50mm.

Wednesday, 3 February 2010

Borrowed flow meter

Parker have kindly lent us a System 20 hydraulic flow meter. This was a great help and will enable us to establish the flow rate on our test leg, hopefully it will be similar or lower than my simulation!

I have also managed to pick up two brand new 100lpm sensors for £35 each from ebay. What a coincidence that they happened to be listed when we were lent this monitor. I'm told they would cost £150 + each.

Below is one of the smaller 25lpm sensors in place on our test rig pump.

Tuesday, 2 February 2010

Position control improves

I have spent the last two days trying to get to the bottom of our servo loop issues, and at the end of today things were looking up! First of all I added a graphic display to the servo setup uttility which shows the desired and actual position as the ram moves. This highlighted a couple of issues, particularly the fact that there was an offset between desired and actual positions. I tried to remove the offset with the ( I) term, but this produced an overshoots in one direction of travel. Eventually I had the idea that the offset was probably caused by the bore/anulas offset of the ram, and a drive
offset may resolve the issue.

So after adding a drive offset term this morning, I started the loop tuning again. The drive offset worked a treat, and essential removed the need of the (I) term from the loop, which in turn removed the overshoots :).  So having retuned all the servo loops the leg is running much better.

Next on the agenda is to remove as much play as possible room the mechanism whic is also reducing performance. We added a large garage spring to the coxa ram which keeps a constant load on the push stroke, this has the effect of removing play in the mech, which again improved

Seeing as things were going well , I left work early :)