Supporting > Vehicles & Models
Burrell Single Crank Compound Traction Engines - 4" Scale
jadge:
--- Quote from: Kim on December 14, 2020, 05:43:50 AM ---Boy, seeing those wheels up on your portable bench shure gives me a size reference! They're huge!
--- End quote ---
I was told they'd be surprisingly heavy, and it's true they are much heavier than the wheels with metal strakes. Just as well I have concrete floors in the bungalow.
Andrew
jadge:
--- Quote from: Admiral_dk on December 13, 2020, 05:42:40 PM ---... does it make sense at all to use an OpAmp ?
--- End quote ---
The sensor has three pins, a reference electrode (RE), working electrode (WE) and a counter electrode (CE). Internally the RE is between the CE and WE. When the target gas meets the sensing element an oxidising or reducing reaction occurs that generates a current into, or out of, the WE. To keep the sensor in balance the voltage on the RE needs to be kept the same as that on the WE. To do this once the reaction occurs the voltage on the CE needs to change in order to force another reaction that generates a current (through the large capacitor and parallel resistor) to equal that generated by the WE. At least I think that's how it works. :) The amplifier that does this is called a potentiostat. The voltage difference between RE and WE needs to be small, ideally microvolts. So the potentiostat needs very high DC gain, but doesn't need a high bnadwisth as the sensor takes many seconds to react to target gases. So I've got an opamp with capacitive feedback, making the DC gain essentially the same as the open loop gain of the opamp.
Andrew
Admiral_dk:
--- Quote ---To keep the sensor in balance the voltage on the RE needs to be kept the same as that on the WE.
--- End quote ---
Ok that sentence alone kind of explains why one really has to consider an OpAmp - no matter if it's an off the shelf item or one made from discrete parts. This doesn't help your first mentioned concern, and I wouldn't have been surprised if my first solution (if I had that assignment) at least needed tweaking if not a complete redesign .... Modern simulators are great, but they do not always solve all the problems - so you usually need to build a prototype (but I'm sure I don't need to tell you that) ;)
jadge:
Simulators are a mixed blessing. I've used ones ranging from free to the Mentor Graphics offerings. They're useful for trying out ideas and playing with component values. I don't quite agree with the Bob Pease attitude, but I always prototype analogue circuits. I normally build the circuits "dead bug" style on single sided blank PCB material. I used to prototype buck converters as they never did quite what they said on the tin. But the newer ICs are much better. I still buy dev kits where needed - just ordered dev kits for the selected ADC and DAC on the gas sensor project. They should allow me to check a few things on the analogue side and then let the software writer get a head start.
Andrew
jadge:
Despite working on schematic and PCB design over the last few months I've been able to make the button valves for my traction engines. These divert high pressure steam from before the regulator to the low pressure valve chest, operating the engine as a single using the LP cylinder. The valve is operated by a spring loaded brass button on the regulator bracket and is used for moving in tight areas where one doesn't want the regulator open. Alternatively it can be used to give quick increase in torque when needed. The design is scaled from drawings of the full size valve. I have tried to stay faithful to the drawings. Here is a sectional view of the CAD model assembled on the cylinder:
The single nut on the cylinder cover is to check clearances on the outlet nut. There are a lot of small parts in bronze, stainless steel and brass. Almost all parts were made on my manual machines. The only CNC milling was the elliptical outline of the flanges. Most threads were screwcut on the lathe. Shown is one assembled valve and the parts for a second:
The body is silver soldered from three parts, held in a simple fixture to ensure alignment of the body and mounting flange. The U-shaped bracket started as a brass block. All holes were drilled/tapped including those needed to form the internal radii. A radius cutter was used to form the external radius at the back. Material was then milled away, mostly by eye and the shape finished by filing and filing buttons where needed. The mounting studs were made on the repetition lathe:
In the background is the regulator rod and gland with studs as per full size with split pins. Attention to detail included the hole and slot in the small end of the valve rod, even though it will never be seen. The hole is 1mm diameter and the slot is 20 thou wide:
For interest this shot shows a sectioned body, complete with internal counterbore, that was a failure due to holes not lining up during drilling and reaming:
I've no idea why the holes went askew as the other two bodies were fine. When making the oilers previously shown I used a HSS boring bar to machine the internal counterbore. This worked well on brass but unfortunately it didn't work on bronze, it just deflected and skated over the surface without forming a counterbore. In order to screwcut the 7/16" 32tpi internal threads on the gland nuts I searched high and low for a preformed threading bar that would fit into the 5/16" starting hole without breaking the bank. I eventually found some solid carbide bars made in Israel but sold by a distributor in Kent. They also sold small boring bars which I used to rebore the internals of the body, working through the 1/4" hole that forms the valve seat. Here are the threading tool and a boring bar:
I broke the first boring bar by being over ambitious, resulting in a sulk and damage to the bank account, assuaged by the application of beer. The second time around I made the cut in several steps rather than trying to do it in one go.
Andrew
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version