PLC and mechanical cams
- Thread starter peterchy
- Start date
Steve Bailey
Lifetime Supporting Member + Moderator
Multiple cam-actuated switches used to be popular, particularly on synchronous machines where a line shaft powered multiple operations. You could adjust each cam for both actuation position and dwell time. These were commonly used to trigger additional operations such as pushers, grippers, etc. As PLCs replaced hard-wired control, these individual switches were wired as inputs to the PLC.
Sometimes, the camswitch assemblies get replaced by an encoder or resolver wired to an interface module in the PLC. Is this what you mean by a digital cam replacing a mechanical cam?
Advantages to the electronic approach:
1) Potentially faster commissioning time, because of less trial-and-error adjustment.
2) Faster replacement of damaged components since the settings are part of the PLC program.
3) More repeatable actuation.
4) Easier to make modifications. Mechanical designers had an annoying tendency to mount the camswitch boxes in inaccessible locations. Typical adjustment involved standing on one's head in a pool of hydraulic oil while clenching a flashlight in ones's teeth.
Disadvantages:
1) If only a few switches are required, the electronic approach is more expensive.
2) The PLC interface is more complicated. You need a High Speed Counter to interface with an encoder, which needs to be properly configured. Not all brands of PLC have resolver interface modules.
Sometimes, the camswitch assemblies get replaced by an encoder or resolver wired to an interface module in the PLC. Is this what you mean by a digital cam replacing a mechanical cam?
Advantages to the electronic approach:
1) Potentially faster commissioning time, because of less trial-and-error adjustment.
2) Faster replacement of damaged components since the settings are part of the PLC program.
3) More repeatable actuation.
4) Easier to make modifications. Mechanical designers had an annoying tendency to mount the camswitch boxes in inaccessible locations. Typical adjustment involved standing on one's head in a pool of hydraulic oil while clenching a flashlight in ones's teeth.
Disadvantages:
1) If only a few switches are required, the electronic approach is more expensive.
2) The PLC interface is more complicated. You need a High Speed Counter to interface with an encoder, which needs to be properly configured. Not all brands of PLC have resolver interface modules.
Goody
Member
Ha ha ha, ah memories......its a torch in the UK by the way.
And what about the cam timers that when you undo the locking nut to adjust the 3rd from the left cam, the whole lot swing to the bottom and you have to readjust the lot.
But as Steve says; plc's do not often take the place of cams, they give timing signals to the plc.
i agree totally
i aggree totaly, we have eight fridge machines with various cam set ups (from 6 to 22 cams) their so awkward to set up mechanically!, very time consuming to get them "right".
i've now replaced 1 of them with a misely slc 500 and and 5 of them with a 5/01. gaffas well chuffed at how quick we can set them up now
i aggree totaly, we have eight fridge machines with various cam set ups (from 6 to 22 cams) their so awkward to set up mechanically!, very time consuming to get them "right".
i've now replaced 1 of them with a misely slc 500 and and 5 of them with a 5/01. gaffas well chuffed at how quick we can set them up now
Jay Anthony
Member
OK, so if a flashlight is a torch, what's a gaffa?
Greg Gauper
Member
Steve listed most of the advantages and disadvantages of the 'traditional' electronic camshaft. One disadvantage that was not brought up is that the ability of a mechanical cam to incorporate some type of speed sensative 'advance' or 'retard' feature to automatically adjust the timing based on chnages in speed or load like a an automotive distributor. Most of the 'off-the-shelf' units are simple digital limit switch replacers with static vs dynamic timing. One advantage that some PLC's offer is the ability to have the digital limit switching logic reside in the I/O module itself i.e. an intellegent module, where it can update much faster than the PLC scan time. The module only needs the setpoints from the main CPU and then does the rest by itself.
So far the only discussion has been about 'bang-bang' systems i.e. turning outputs on and off with respect to camshaft rotation. Let's not forget that there are motion control applications that duplicate the action of a mechanical camshaft as well! In other words, an axis is 'slaved' to a master axis, and the slave axis has a unique motion profile with respect to the master axis. The master axis can be a true physical axis i.e. feedback from an absolute encoder, or it can be a 'virtual-time' axis. Our company has done a number of control upgrades to hydraulic systems that perviously used a mechanical servo valve to control a linear axis with respect to a rotating master axis. The linear axis had a very specific up and down profile that had to be obeyed. We eliminated the mechanical camshaft and motor. The mechanical servovalve was replaced with a traditional electro-hydraulic unit. A high-resolution, non-contacting, linear, absolute feedback transducer was added to the slave axis. Now the customer has the best of both worlds. The system features motion control and electronic limit switching based on a master time axis.
Advantages:
1) Elimination of a mechanical maintenance nightmare (the old system had lots of mechanical slop due to all of the linkages involved).
2) No mechanical cams to wear out. The motion profile remains exact and repeatable no matter how long the unit runs. This particular unit runs 24/7.
3) Ability to adjust the profile dynamically while the unit is running. The dynamic adjustments include amplitude and offset. These adjustments are required by the eqipment operator for changes in the process, and duplicate the mechanical 'vernier' adjustments that the operator had with the older system.
4) The biggest advantage - Ability to change cam profiles on the fly. The customer can experiment with different profiles without ever stopping the machine. The control compares the old profile with the new profile and internally generates an intermediate 'transition' profile so the change over is seamless in one cycle.
So far the only discussion has been about 'bang-bang' systems i.e. turning outputs on and off with respect to camshaft rotation. Let's not forget that there are motion control applications that duplicate the action of a mechanical camshaft as well! In other words, an axis is 'slaved' to a master axis, and the slave axis has a unique motion profile with respect to the master axis. The master axis can be a true physical axis i.e. feedback from an absolute encoder, or it can be a 'virtual-time' axis. Our company has done a number of control upgrades to hydraulic systems that perviously used a mechanical servo valve to control a linear axis with respect to a rotating master axis. The linear axis had a very specific up and down profile that had to be obeyed. We eliminated the mechanical camshaft and motor. The mechanical servovalve was replaced with a traditional electro-hydraulic unit. A high-resolution, non-contacting, linear, absolute feedback transducer was added to the slave axis. Now the customer has the best of both worlds. The system features motion control and electronic limit switching based on a master time axis.
Advantages:
1) Elimination of a mechanical maintenance nightmare (the old system had lots of mechanical slop due to all of the linkages involved).
2) No mechanical cams to wear out. The motion profile remains exact and repeatable no matter how long the unit runs. This particular unit runs 24/7.
3) Ability to adjust the profile dynamically while the unit is running. The dynamic adjustments include amplitude and offset. These adjustments are required by the eqipment operator for changes in the process, and duplicate the mechanical 'vernier' adjustments that the operator had with the older system.
4) The biggest advantage - Ability to change cam profiles on the fly. The customer can experiment with different profiles without ever stopping the machine. The control compares the old profile with the new profile and internally generates an intermediate 'transition' profile so the change over is seamless in one cycle.
jdbrandt
Lifetime Supporting Member
PLS, not PLC
Lineshaft applications like this call for a targeted and
specific solution.
How about a Programmable Limit Switch? There are a number
of good suppliers (fill in your own here) who can tackle
applications like this.
The PLS does away with external software, 'modules', mechanical
mounting of a third-party encoder, etc. After a while, the
benefits become self-evident.
I'd only apply a PLC if there was other logic or control, no
directly related to the line shaft.
Lineshaft applications like this call for a targeted and
specific solution.
How about a Programmable Limit Switch? There are a number
of good suppliers (fill in your own here) who can tackle
applications like this.
The PLS does away with external software, 'modules', mechanical
mounting of a third-party encoder, etc. After a while, the
benefits become self-evident.
I'd only apply a PLC if there was other logic or control, no
directly related to the line shaft.
rogerhollingsworth
Member
peterchy
I worked in a steel mill for a good number of years and I replaced a good number of cam switchs with a Programmable limit switch and it worked great. I used a PLS that was an in-slot module that fit into a Allen- Bradley (1771-)chassis.
One cam switch that I replaced was on a skip car in the blast furnace, this cam switch controlled the the points of acceleration, deceleration, final stop and emergency overtravel stop of the skip car.
To hoist these skip cars of about 50 ton each up a 38deg incline for about 100 feet were two 1000HP motors hooked to a common gear reducer, two inputs shafts from the motors and one output shaft to the cable drum. On that drum was a steel cable that ran from the drum through sheeves at the top of the encline back to ground level where it attached to the skip cars. The reason I say skip (cars) is because there were two skip cars for each blast furnace and both skip cars were hooked to the same steel cable, while one skip care was being hoisted the other was being lowered.
The shaft of the cable drum was exposed where it came through the bearing housing, I welded a small shaft to the center of the brum shaft, mounted a small gear reducer to that shaft then mounted a resolver to the output of that small gear reducer, hooked the cable to the resolver the other end to the pls, programmed in my points, all done.
I monitored the program function 24-7 for a few weeks while it wasn,t actually doing any real function before I put it in use. It took another two years to remove the cam switches because higher management wanted them for BACK-UP.
I hope this helps rather than confuse.
Good luck
Roger
I worked in a steel mill for a good number of years and I replaced a good number of cam switchs with a Programmable limit switch and it worked great. I used a PLS that was an in-slot module that fit into a Allen- Bradley (1771-)chassis.
One cam switch that I replaced was on a skip car in the blast furnace, this cam switch controlled the the points of acceleration, deceleration, final stop and emergency overtravel stop of the skip car.
To hoist these skip cars of about 50 ton each up a 38deg incline for about 100 feet were two 1000HP motors hooked to a common gear reducer, two inputs shafts from the motors and one output shaft to the cable drum. On that drum was a steel cable that ran from the drum through sheeves at the top of the encline back to ground level where it attached to the skip cars. The reason I say skip (cars) is because there were two skip cars for each blast furnace and both skip cars were hooked to the same steel cable, while one skip care was being hoisted the other was being lowered.
The shaft of the cable drum was exposed where it came through the bearing housing, I welded a small shaft to the center of the brum shaft, mounted a small gear reducer to that shaft then mounted a resolver to the output of that small gear reducer, hooked the cable to the resolver the other end to the pls, programmed in my points, all done.
I monitored the program function 24-7 for a few weeks while it wasn,t actually doing any real function before I put it in use. It took another two years to remove the cam switches because higher management wanted them for BACK-UP.
I hope this helps rather than confuse.
Good luck
Roger
rogerhollingsworth
Member
oops!
peterchy
Instead of 100ft. it is closer to 1000ft.
And instead of brum it is drum.
oops!
peterchy
Instead of 100ft. it is closer to 1000ft.
And instead of brum it is drum.
oops!
Terry Woods
Member
- Join Date
- Apr 2002
- Posts
- 3,170
"...could anyone pls give me some examples about simple PLC applications that would previously have been carried out using mechanical cams?"
When I was an Electrician in the Navy, the washing machines in the Laundry were controlled by "Linear Cam Cards". Each card was about 6-inches wide, about 18-inches long and made of plastic. The card would be inserted into a "Reader Slot". In the slot was a drive roller. The drive roller would feed the card through a "reader". The "reader" consisted of a set of small limit-switches. It might take as long as 30-minutes to run the card through the "reader". The actual cycle-time could be over much sooner. In that case, the card would be pulled out of the bottom of the "reader".
Shown below is an End-On View, and a Side View of one of these cards. Each of the 6 ribs, shown on the top-side of the card, represents a "Cam". These are "Linear Cams".
.
Each rib consisted of a series of segments. Each segment in each rib was aligned with timing-marks. The operator could "build" a custom wash cycle by breaking off various segments at the appropriate timing-marks. As the card was driven through the "reader", the switches in the "reader" would respond to the presence or absence of each segment.
In its day, these were pretty cool! They did, however, wear out and sometimes they broke - they were, after all, just plastic.
These days, the washing machine is controlled by a low-level PLC. That is, these particular PLC's are only switch handlers - very little "smarts" are needed. Operation is defined and controlled through an Operator Interface. All timing issues are selectable. Any particular set of settings can be saved as a "Recipe" and recalled as needed.
When I was an Electrician in the Navy, the washing machines in the Laundry were controlled by "Linear Cam Cards". Each card was about 6-inches wide, about 18-inches long and made of plastic. The card would be inserted into a "Reader Slot". In the slot was a drive roller. The drive roller would feed the card through a "reader". The "reader" consisted of a set of small limit-switches. It might take as long as 30-minutes to run the card through the "reader". The actual cycle-time could be over much sooner. In that case, the card would be pulled out of the bottom of the "reader".
Shown below is an End-On View, and a Side View of one of these cards. Each of the 6 ribs, shown on the top-side of the card, represents a "Cam". These are "Linear Cams".
End-On View
+---+ +---+ +---+ +---+ +---+ +---+
|RIB| |RIB| |RIB| |RIB| |RIB| |RIB|
+---+ +---+ +---+ +---+ +---+ +---+ +---+
| CARD CARD CARD |
+---------------------------------------------------+
Side View
+---------+
<-- FLOW | Switch |
+---+-----+
LS-CLOSED --> ______ / ______________ ________
LS-OPENED --> ____/ / / /__O___/ / / / / / / /_____/ / / /
______________________________________________
.
Each rib consisted of a series of segments. Each segment in each rib was aligned with timing-marks. The operator could "build" a custom wash cycle by breaking off various segments at the appropriate timing-marks. As the card was driven through the "reader", the switches in the "reader" would respond to the presence or absence of each segment.
In its day, these were pretty cool! They did, however, wear out and sometimes they broke - they were, after all, just plastic.
These days, the washing machine is controlled by a low-level PLC. That is, these particular PLC's are only switch handlers - very little "smarts" are needed. Operation is defined and controlled through an Operator Interface. All timing issues are selectable. Any particular set of settings can be saved as a "Recipe" and recalled as needed.
Peter Nachtwey
Member
What is a cam?
Definition of cam
Greg got it right. The rest of you are talking about switches over various types.
I can't think of any place where PLCs do real camming without the aid of a motion control card. Common applications are ladle pouring which is really a cam application in reverse where a linear motion must be mapped to a rotary one. Another application, which is more of a linear camming, is and curve sawing in sawmills.
Definition of cam
Greg got it right. The rest of you are talking about switches over various types.
I can't think of any place where PLCs do real camming without the aid of a motion control card. Common applications are ladle pouring which is really a cam application in reverse where a linear motion must be mapped to a rotary one. Another application, which is more of a linear camming, is and curve sawing in sawmills.
rogerhollingsworth
Member
Cam Switch
Goody got it right, what he is describing is a cam switch, one of many types.
I have replaced so many in so many different applications I can't count them all. Besides the ones I described in my earlier post I have replaced them on old 250vdc control boards, the motor driven cam switches actually did the control function of a program. I have also replaced them in the 2300vac 25hz breakers.
These are multiple contacts (switches) spaced out across a stationary insulating material with cams mounted on a rotating shaft that runs the full length of the switches. As the shaft turns, the various cams come in contact with the contacts (switches) moving them to the open position, spring return. The thing that was so nice about the cams is that they were all held in place with a nut on each end so when you loosened a nut to adjust one cam, the rest could and often did move. At one time these cam switches were widespread and high tech.
There are other types of cam switches that are still widely used in the electrical substations, pistol grip type used for opening and closing breakers, these two were integrated into a PLC system. These pistol grip types are very popular in high voltage distribution and are likely to stay around for a long time.
All the cam (switches) I spoke of were replaced/integrated with some PLC related function, either with I/O cards or an AMCI module w/resolver.
Again, when I say PLC I am referring to the PLC's that I worked with, enhanced PLC-5 family.
Goody got it right, what he is describing is a cam switch, one of many types.
I have replaced so many in so many different applications I can't count them all. Besides the ones I described in my earlier post I have replaced them on old 250vdc control boards, the motor driven cam switches actually did the control function of a program. I have also replaced them in the 2300vac 25hz breakers.
These are multiple contacts (switches) spaced out across a stationary insulating material with cams mounted on a rotating shaft that runs the full length of the switches. As the shaft turns, the various cams come in contact with the contacts (switches) moving them to the open position, spring return. The thing that was so nice about the cams is that they were all held in place with a nut on each end so when you loosened a nut to adjust one cam, the rest could and often did move. At one time these cam switches were widespread and high tech.
There are other types of cam switches that are still widely used in the electrical substations, pistol grip type used for opening and closing breakers, these two were integrated into a PLC system. These pistol grip types are very popular in high voltage distribution and are likely to stay around for a long time.
All the cam (switches) I spoke of were replaced/integrated with some PLC related function, either with I/O cards or an AMCI module w/resolver.
Again, when I say PLC I am referring to the PLC's that I worked with, enhanced PLC-5 family.
Last edited:
Terry Woods
Member
- Join Date
- Apr 2002
- Posts
- 3,170
"mechanical cams"... only one interpretation possible ???
I would say, regarding the actual type of cam-action that Peterchy is referring to...
...Only his Hair Dresser Knows For Sure!
...And neither the Hair Dresser, nor Peterchy, himself, appears to be doing any talking!
It's actually a case of "TWO(Pause), TWO(Pause), TWO POSSIBILITIES IN ONE!" Since neither was excluded, and neither was specified, both are equally valid!
In any case, your basic "Cam-Action" is an eccentric revolving about an axis. The term "eccentric" means that the rotated part is off-center. The farther the center of the rotating axis and the center of the rotated body are from each other, the greater the "eccentricity". The greater the eccentricity, the greater the resulting displacement of the driven body.
Now, here's where the apparent dispute occurs... is the driven body another mechanical device which might cause even further mechanical activity? Or, is the driven device something like a limit switch?
You simply don't know, unless you know!
Since Peterchy didn't say, then neither case is confirmed, nor is either case eliminated. So, in this case, for anyone to declare that someone is right and someone else is wrong is logically unjustifiable - at least, up to this point.
By the way, in those cases where mechanical advantage is NOT required, a "lever" is a cam, or, at least, it produces cam-action. Check the axis (fulcrum) and the point of resulting action.
In those cases where mechanical advantage IS required, the lever is a "reverse-cam". Imagine a large, open can, rotating as a cam does (off-center). Then put a limit switch on the INSIDE of the can. Instead of "reading" or reacting to the high-point, the switch responds to the low-point.
Ain't this mechanical, cause & effect stuff interesting???
I would say, regarding the actual type of cam-action that Peterchy is referring to...
...Only his Hair Dresser Knows For Sure!
...And neither the Hair Dresser, nor Peterchy, himself, appears to be doing any talking!
It's actually a case of "TWO(Pause), TWO(Pause), TWO POSSIBILITIES IN ONE!" Since neither was excluded, and neither was specified, both are equally valid!
In any case, your basic "Cam-Action" is an eccentric revolving about an axis. The term "eccentric" means that the rotated part is off-center. The farther the center of the rotating axis and the center of the rotated body are from each other, the greater the "eccentricity". The greater the eccentricity, the greater the resulting displacement of the driven body.
Now, here's where the apparent dispute occurs... is the driven body another mechanical device which might cause even further mechanical activity? Or, is the driven device something like a limit switch?
You simply don't know, unless you know!
Since Peterchy didn't say, then neither case is confirmed, nor is either case eliminated. So, in this case, for anyone to declare that someone is right and someone else is wrong is logically unjustifiable - at least, up to this point.
By the way, in those cases where mechanical advantage is NOT required, a "lever" is a cam, or, at least, it produces cam-action. Check the axis (fulcrum) and the point of resulting action.
In those cases where mechanical advantage IS required, the lever is a "reverse-cam". Imagine a large, open can, rotating as a cam does (off-center). Then put a limit switch on the INSIDE of the can. Instead of "reading" or reacting to the high-point, the switch responds to the low-point.
Ain't this mechanical, cause & effect stuff interesting???
Peter Nachtwey
Member
Marketing hype
If it is a cam then the resulting action should be a function of the cam position. An engine's valve extends as a function of its cam's rotational position now matter what the speed within reason. A cam works in reverse too.
Once the rotary switch makes a contact that action takes place independent of the rotary switches action. This is not camming.
That makes it a rotary switch. The rest of you have been sucked in by some marketing hype that makes a crude device seem 'better' ( worth more, you pay more ) than what it really is. By the 'expanded' definition, a PLC drum would be electronic camming.
At least I know the difference.
I would also like to add powdered metal presses to the list of applications that can use cams. Now PLCs with motion control cards can change the cam profiles instantly reducing down time between different part types.
I know this is just a argument of terms. I get heated when I have to explain difference to someone who is comparing features and they are comparing rotational switchs to true camming. They think they are the same thing because some sales/marketing guy told him so.
Terry Woods said:
If it is a cam then the resulting action should be a function of the cam position. An engine's valve extends as a function of its cam's rotational position now matter what the speed within reason. A cam works in reverse too.
Once the rotary switch makes a contact that action takes place independent of the rotary switches action. This is not camming.
That makes it a rotary switch. The rest of you have been sucked in by some marketing hype that makes a crude device seem 'better' ( worth more, you pay more ) than what it really is. By the 'expanded' definition, a PLC drum would be electronic camming.
At least I know the difference.
I would also like to add powdered metal presses to the list of applications that can use cams. Now PLCs with motion control cards can change the cam profiles instantly reducing down time between different part types.
I know this is just a argument of terms. I get heated when I have to explain difference to someone who is comparing features and they are comparing rotational switchs to true camming. They think they are the same thing because some sales/marketing guy told him so.
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