Potentiometer VS Encoder

[Pulsar2003 0]

Hello, I decided to open a new topic as I didn't wan this suggestion to go unnoticed in another topic I brought up about hydraulics. I gentlemen pointed out that a potentiometer is not as precise and reliable as an encoder. This makes allot of sens. The potentiometer currently is hooked up to the PLC via an analogue input for 0 to 10 volts. It's electronic is functioning with +/- 15 dc volts while the reference voltage is provided by a 10 dc volts power supply. There is quite a distance for the signal to go to the potentiometer and back to the input. The wire is a 22 AWG and the signal is also in the same shielded cable. I've reason to believe that their is a serious variation in the real value of the signal added to the hysteresis and resilience of the potentiometer itself. This can explain why I get many problems. Ok now I wish to know how it works and how I can make the replacement. From the studies I briefly made, I understand that an electrical value is replaced by a numeric value (this is how I can put it from French to English) we would say digital. Is a 1747-SN the proper device? This machine has 3 potentiometers but only one need accuracy. So lets take it from there. Thanks again.

[MikeSmith 0]

Salut, I'm not following 100% what you're proposing, do you wish to replace the potentiometer with 1/ An incremental encoder wired to an encoder card in the plc (1746-HSCE at http://www.ab.com/en/epub/catalogs/12762/2...4063/tab6.html) 2/ An absolute encoder (resolver) wired to an encoder card in the plc (http://www.amci.com/plc-modules/plcmodules-slc500-resolver-interface.asp) 3/ Networked encoder with digital communication back to the PLC (eg. http://www.amci.com/rotary-encoders/absolu...ary-encoder.asp or http://www.amci.com/rotary-encoders/absolu...ry-encoder.asp) 4/ Networked encoder card with digital communication back to the PLC (http://www.ab.com/en/epub/catalogs/12762/2181376/104830/2416243/2877322/tab8.html) 5/ Motion controller with digital communication back to the PLC. 6/ Motion card in the PLC (1746HSRV at (http://www.ab.com/en/epub/catalogs/12762/2181376/2416247/1239758/2554063/tab6.html)

[Pulsar2003 0]

Taken from the topic: RAMP and hydraulics. Quote: 'In addition, you mentioned potentiometer. These are not known for accuracy or reliability in areas where they get used frequently. I suggest you consider an encoder instead. They will last much longer and are much more accurate and reliable. In fact if you use a motion controller (or if you switch to the built-in one in a ControlLogix or CompactLogix PLC), the code is already written for you. You just have to configure it. It will easily allow you to set maximum acceleration/deceleration limits (which is the problem you are fighting). If not, one additional thing you can do with your motion control code is that you can slow down/stop before you hit the mechanical stop so this problem goes away entirely...just issue a stop command in the PLC at some software set points.' I wanted to evaluate the possibilities. When you have an oven of a few tons that tilts and rattles every time it stops well I wan to investigate and find out what pros will suggest. I'm amazed by all that I'm learning here. From my topics and from others. Hopefully these details will be helpful.

[Peter Nachtwey 15]

If this is for a hydraulic system then normally a MDT, magnetostrictive displacement transducer, rod is used. The feed back is start stop, PWM or gated or SSI.

[paulengr 44]

Voltage-based signalling has two advantages and two problems relative to current loop signalling. The advantages are that it is much faster than a current loop (necessary for kilohertz signalling or faster), and that it is somewhat simpler to implement in many cases. The disadvantages are that calibration is always a necessary maintenance function (if your needs are not extreme, current loops don't need calibration at all), and that it is susceptible to noise since the inputs are high impedance in nature. A 1747-SN is a Universal Remote I/O scanner for a SLC. This is a scanner (master) for the distributed I/O network protocol built into many SLC and PLC-5 controllers. It has nothing to do with reading encoders, at least not directly. There are many encoder signalling formats. Three of the most popular are SSI (a serial format), incremental ABZ encoders, and absolute encoders. SSI can use either absolute or incremental systems. You need a special card to read SSI. AMCI (www.amci.com) is one source. It has very few wires. Absolute encoders output a binary signal and require one wire per bit, usually up to about 10 bits total. Often they output gray codes which must be translated by some simple PLC logic into standard form. They require an ordinary digital input card to read them. Incremental encoders output a quadrature pattern of on/off signals and often include a single "zero" reference on the "Z" pin. The disadvantage with these is that if you lose power, then you have to somehow get back to a "zero" reference since the output signals are all relative. The "reader" for this is a high speed counter which can handle quadrature inputs such as the 1746-HSCE2. There are of course distributed I/O alternatives to all of these. If you have an extra analog channel free, I've also used the SG1000C from Electro Sensors (www.electro-sensors.com). Despite what their catalog looks like, it's an absolute encoder that internally converts the absolute encoder signal into a 4-20mA current loop output. If you don't have very high speed requirements, this is often a fairly decent alternative. If you are using a motion controller, the controller itself often supports SSI or ABZ directly. Other formats are available but less popular.

[panic mode 246]

you need to describe actual application before recomendation can be made. potentiometer is so called 'absolute' feedback. even if the machine is powered down and the position of device is changed, potentiometer will 'know' that there was a movement the very instant machine is powered up. in contrast, incremental encoders are used in counting pulses and only while powered up. if the machine is powered down and something was moved, there will be no information as no pulses were counted. some devices use battery backup so that feedback stays alive while machine is powered down. another difference is calibration. analog signal such as from potentiometer, is calibrated once and as long sensor was not moved or replaced you don't have to do anything about it. non-absolute feedback devices (incremental encoders for example) must be homed or referenced every time machine is powered up (unless they use the mentioned battery backup). also potentimeters are using contact between wiper and resistive element. surface of resistive element and wiper degrades with time. in most cases potentimeters are expected to have minimal load (load current is in microamp range) or the degradation will be accelerated. this is why potentiometers are usually connected to high impedance ciruits (voltage followers) and not to analog inputs of PLC (if you expect reasonably long life from it). one way to deal with this is by using wirewound potentiometers but they introduce another set of problems (including output jumps or spikes when wiper moves). back to the problem: what is this potentiometer monitoring (cable lenght, environment, linear or rotary, what sensing range, what accuracy, what speed, and what about vibrations)? what kind of output you need? without knowing more, probably the safest call is to replace potentimeter with an absolute encoder. if the axis is linear and not too long (60-80inches or so) and problems are caused by unreliable reference voltage or wear of the potentimeter, it may be worth looking into linear position transducer (EP2A or similar from Temposonic for example).