Hydraulic press (Distance Measurement?)

Jairvargas

Member
Join Date
Jul 2011
Location
Santa Cruz
Posts
10
This is my first post, I'm trying to measure the distance traveled by a hydraulic cilinder to stop it at a determined distance, around 500mm long, and +/-1mm accuracy, it's basically a vertical hydraulic press, I would be comfortable with a 0-10v sensor. do you have any experience with similar systems? what kind of linear sensors would you recommend? any other thoughts? Thanks :geek: .
28l9gte.jpg
 
The MTS or Balluff MDT transducer would work but the problem is mounting it so that it doesn't vibrate. Most of the time the MDT transducer is mounted inside the cylinder but this often can't be done on existing presses.

I don't like analog MDT rods. First, they are noisy. Second they drift. Third they don't run synchronously to the motion controller. If you are using a PLC you probably don't care too much about precision.
SSI rods are very precise but they require SSI interfaces.
 
I've had good luck with the TD580M series from TransducersDirect and have put those on a lot of hydraulic presses. They're 0-10v by default but you can get a plug that converts the signal to 4-20mA.

In my experience they are very reliable and there is no signal bounce or noise. I'm actually surprised when I look in the program and don't see the raw counts change at all when it isn't moving.
 
As i see it needs to be cheap, so try a multiturn potmeter of 10 kiloohm.
The resolution will be enough, however if possible you only use a part of the way, yoy can get better results.
 
I agree with Peter's assessment above. +/- 1 mm resolution is begging for SSI rather than analog. If you must go with analog, I'd recommend a mA signal rather than VDC.
 
A good tempo rod alone won't help you at all unless you have a good variable displacement pump with LVDT feedback, a good servo valve, and some sort of signal conditioner.

What kind of press is it? What kind of pumps does it have? Any servo valves to control cylinder or pumps?
 
Yikes that looks like one scary hydraulic cylinder, from the shine on them, I would guess those are grade 5 nuts holding the gland cap onto the barrel. And the welds where the small pipes are welded around the circumference of the cylinder look pretty wide. Hopefully you are not trying to lift anything too heavy or compress anything with this cylinder on the retraction stroke.

I wouldnt want to be too close to this one for very long when its operating. There are lots of off the shelf cylinder out there, but I guess you can build your own like this if time and money are an issue.

As for feedback, I would go with MTS Tempo's, thousands of systems out there running in applications such as this, but it can be tricky with externally mounted sensors, you dont want to bend the tempo rod, I did once, it didnt turn out well.

Hope it works out for you.
 
+1 for what Peter said. Position control over hydraulics, with any degree of finesse, is not a trivial task for a PLC.

Nick
Yes, in fact I was going to start a thread to that effect on a Chinese hydraulic forum. We see a lot of people try and give up after wasting much time and money. Many can't even figure out the basic like how to control the speed when the actual position is at 100mm and the command position is 200mm. The actuator jumps quickly to the new command position shaking the machine and over shoots the command position. The PID in most PLCs is not suitable for these simple motion applications.

PLCs shouldn’t be used to directly control closed loop hydraulic servos systems.
1. PLC scan times are too slow
2. PLC scan times are not consistent. Adding more code to the PLC makes the scan times slower and more inconsistent.
3. PLC analogue inputs and outputs are slow and may not have enough resolution.
4. PLCs do not have a trajectory planner. It must be written.
5. Diagnostics and debugging tools are lacking.
6. The PLC PID is written for process control applications. Time constants are in minutes instead of seconds or milliseconds.
7. PLC's do not have feed forward control to reduce following error.
8. Most important! Writing optimal hydraulic servo control code requires understanding hydraulics, control theory and programming.

PLC不应用来直接控制闭环液压伺服系统。
1. PLC的扫描时间太慢。
2. PLC的扫描时间不具备一致性。加入更多的代码使PLC扫描时间更慢,更加不一致。
3. PLC的模拟输入和输出缓慢,没有足够的分辨率。
4. PLC没有运动轨迹规划功能,必须人为编写。
5. PLC缺乏诊断和调试工具。
6. PLC的PID是为过程控制应用而编写的,其时间常数是分钟级,而非秒或毫秒级。
7. PLC的不具备前馈控制以减少跟踪误差。
8. 最重要的!编写出最佳的液压伺服运动控制程序,须要具备对液压系统和控制理论的理解,以及编程优化的综合能力。
 
I've used many lasers for hydraulic positioning in the past with no issues. Vibration and dust may occasionally cause you to have to fiddle with the laser but they work in cold, heat and dusty environment's.
 
So I decided not to begin this project, because the customer didn't want to spend money on a linear position trasmitter (4-20mA/Hart), The customer wanted to go cheap, but he asked me for more precision. Anyway thanks guys, i've learned a lot of these days about this subject.
 

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