Cylindrical Objects Moving on a Conveyor

A simple flexible rod with two microswitches and a reversible counter might be sufficient, but it depends on other factors.....

Use a quadrature encoder,or a grey code encoder.

https://www.ni.com/tutorial/7109/en/

I like your way, using a quadrature encoder. Assuming the cylinders are in "single file", two light beams across the conveyor belt interrupted by the cylinders should operate the same way as an optical encoder. GA.

The simplest way is to have a direction sensor and a sensor to indicate a cylinder is present. The direction sensor could be a switch with a pad attached which rides on the belt. Motion of the belt would turn the switch on or off. The cylinder sensor could be a light beam and sensor. Assume the light sensor (L) outputs 0 when cylinder is present and 1 when it is absent. Assume the direction sensor (D) outputs 1 for forward and 0 for backward.

Direction Sensor D=1 (forward): Count up on falling light sensor signal L = 1->0

Direction Sensor D=0 (reverse): Count down on rising light sensor signal L = 0->1

This can be implemented with logic and up/down counters, purchased modules, or a microcontroller can easily do the job with a couple of digital inputs.

Really need to know the size, weight, material composition of objects, density of objects, and speed of objects, type of conveyor, to work out a proper method...

https://www.monk-conveyors.com/videos

https://www.conveyco.com/warehouse-automation-guide/

https://instrumentationtools.com/plc-program-for-counting-moving-objects-on-conveyor/

ref link

...also need to know the number of objects you need to count at a time so your counter goes high enough....

That might not work. For instance, what happens if the object changes direction while right over the sensor?

That may be solvable, but it might be trickier than first imagined.

That might not work. For instance, what happens if the object changes direction while right over the sensor?

In #3, I think I addressed that situation, changing from falling level signal transition of the cylinder detector to rising level when the direction reversed. Reversing belt direction reverses time, reversing transition sensing from falling to rising.

There is a lot missing from this question. It would be a lot more complex if the cylinders are not "single-file". That would require a view from the top and a lot more sophisticated algorithm. I'm going to assume "single-file" to make the problem tractable.

You could use two light beams across the conveyor belt instead of a sensor to detect belt direction. This method proposed by HiTekRedNek in #2 using a quadrature detector should work in either direction no matter where the belt stops.

You could have 3 beams with 2 consecutive signals for a positive or negative count, that way the belt could stop in either direction and an accurate count would be maintained...

That is indeed a little trickier than first imagined.

Even more challenging (and not ruled out by the given conditions), what if the cylinders are lying down, crosswise to the belt, so even if the belt changes direction they keep rolling? What if some cylinders touch or overlap one another?

(Yeah, I'm already envisioning Lucy and Ethel in the candy factory.)

That's why all the parameters must be known to come up with a foolproof methodology for detection....then finding the cheapest way to implement that strategy...a mixed bag would be more challenging, but then may be corrected with an additional mechanism that presented the objects so they were uniform in presentation...

How about a simple latch above the conveyor to prevent the items coming back.

That would be a perfect example of domino theory.

If 100 cylinders are made and the equipment rejects 3 I have 97 cylinders on the conveyer belt.

If the conveyer is slightly inclined or stepped a bit, wouldn't this keep cylinders from rolling backwards after counting?

OP needs to input more fot better output.. err. Input.

There is a woeful lack of information to be able to tackle this problem. Knowing the conveyor direction can be determined in a variety of ways, as mentioned by others but nothing is said of the orientation of the cylinders, can the cylinders overlap each other on the conveyor? How big are the objects? How fast are they moving on the conveyor? Perhaps a better place to count the cylinders would be as they are transferred off the conveyor but we don't know how they are collected. Applications like this are fun but "more input" as said No. 5...

If the objects are somewhat repeatably positioned the minimum you need is 2 sensors, A and B, optic through beam is probably the best. In one direction the sensor inputs would be A-B, the reverse would be B-A.

If they are not repeatably positioned use an industrial vision sensor and conveyor encoder. A good programmer should be able to write some machine code to capture images at a rate matched to the conveyor and the vision sensor image size and the encoder will give you speed and direction.