PLC finish line timer input response for race order

What you need

• A PLC with at least four fast or high-speed DC inputs, specified at 2 ms or better on the module data sheet
• Four photoelectric sensors with transistor outputs (PNP or NPN), plus the reflector each sensor's sheet specifies
• A 10 to 30 V DC supply sized for the sensors and matched to the PLC input voltage
• A start microswitch, if the race must report elapsed times
• A screen for live and final results
• The data sheets for the exact input modules installed, because the response figures move with them

Decide what the race must report

The requirement chooses the hardware. A place-only timer ranks the four finish events and needs no start switch. An elapsed-time timer starts a clock at the start pin and stops one clock per lane, which reports times and a ranking built from them. Set the tie rule now: when two timestamps match, or land closer together than the input response spread, the timer reports a tie. A printed tie is a usable result; a manufactured order invites a dispute the hardware has no data to settle.

Lane fairness is a race-method decision: running each car in each lane once and ranking on the total of four runs removes arguments about one lane being fast. Keep the per-run times so the totals can be checked.

The number that governs: input response

Input response is the time from the sensor output changing at the terminal to the input register showing it, and the module data sheet is the governing document. Catalogs for small PLCs list standard DC inputs at under 8 ms and dedicated fast input modules at 1 to 2 ms. Apply one rule to the finish signals: keep the worst-case response at or under half the margin the race must resolve. For a 0.01 s margin that means 5 ms, so the 1 to 2 ms class passes with room and an 8 ms standard input fails it.

The scan adds its own limit if order is ranked in ladder logic. Most small PLCs copy every input into the input image once at the top of each scan, so two beams that break inside one scan window arrive in the same snapshot and the program has no way to rank them. With a 10 ms scan, two finishes 6 ms apart look simultaneous in the image. Rank in hardware: interrupt inputs or a high-speed input capture function timestamps each transition outside the scan, so the data sheet response, not the scan period, sets the error.

Leave the analog inputs out of this. A photoelectric sensor gives a discrete transition, and an analog channel only delays it. Analog input step response figures in current PLC specification guides run in the tens of milliseconds once a notch filter is applied, commonly around 70 ms at 60 Hz and 80 ms at 50 Hz; at 5 ft/s that is 4.2 in of car travel before the value settles, seven hundredths of a second. Even the quickest analog figures printed in current specification sheets, around 5 ms maximum with modules processed in parallel, still take half the margin before the sensor is believed. Wire the beams to fast discrete inputs and spend the analog channels elsewhere.

Example: two cars one hundredth of a second apart

Take a heat where 1st and 2nd cross 0.01 s apart, with the cars moving 5 ft/s at the beams, a conservative figure for a late-race car; faster cars shrink the gaps and only ease the numbers.

• Nose gap: 5 ft/s × 0.01 s = 0.05 ft, which is 0.6 in between the two noses.
• Standard DC inputs, catalog worst case under 8 ms: the bounded order error between two lanes is 8 ms, because one channel can register near instantly while another takes the full figure. At 5 ft/s that maps to 5 × 0.008 = 0.04 ft = 0.48 in of car travel, or 0.8 hundredths. Two noses closer than 0.48 in can swap places.
• Fast inputs at 1 to 2 ms: worst case 2 ms maps to 5 × 0.002 = 0.01 ft = 0.12 in, about 0.2 hundredths. The margin is five times the worst-case error, the ratio that holds up on a disputed heat.
• Decision: same track, same cars, same sensors, opposite answers. The input choice is what decides this heat, so the finish beams go on the fast inputs.

Choose and aim the sensors

Retroreflective photoelectric sensors suit a finish line well: emitter and receiver share one housing, and the beam returns from a reflector mounted across the track. Use the reflector type the sensor's sheet calls for; polarizing retro models pair with corner-cube reflectors, and glass-bead tape is the low-cost option. Polarization is worth having here, because light bouncing off a glossy car body comes back scrambled and is rejected, which removes the false trip a shiny nose would otherwise cause.

Keep the effective beam narrow so a car breaks one lane's beam nose-first. A strip of tape with a slit across the sensor face trims the detection zone, and a T-square sets the beam at right angles to the track. Roll a dark matte car and a glossy car past each sensor and confirm each pass gives one clean transition; a miss is an aim or reflector problem to fix before the wiring gets blamed.

A through-beam arrangement works too: an emitter above each lane and a receiver below it. The receiver must present a transistor output that matches the PLC input; a bare photoresistor changes resistance and has nothing to switch, so it needs an output stage before it can drive an input. Most industrial photoelectric sensors run on 10 to 30 V DC, so match the supply to the sensor sheets and the input module.

Wire and configure the capture

1. Wire each lane's sensor output to its own fast input. Match PNP to a sourcing input and NPN to a sinking input, and share the 0 V common. Keep the start microswitch on a standard input; its delay shifts every lane equally and ranks no car.
2. Enable interrupt or high-speed capture on the four finish inputs, with a timestamp or sequence register per lane. The function's name changes by platform; look for interrupt inputs or input capture in the PLC manual, and take the response figures from your own modules' data sheets.
3. For elapsed time, start a 0.01 s time base from the start event and stop one clock per lane at its captured finish event. To show the clock live during a run, drive the screen from the PLC's current value as described in show an HMI timer counting from a PLC current value.
4. Apply the tie rule in the capture logic: equal timestamps, or two events inside the response spread, both report tie.
5. Latch the four results when the fourth beam breaks, then hand them to the display. While commissioning, put the raw lane bits on one diagnostics screen; display 16 PLC input and output bits on one screen shows the layout. Format the recorded clocks so leading zeros and decimals print the same every race; how to format a PLC timer duration display covers the formatting.
6. Store each run's time with car and lane, so four-run totals and lane swaps can be checked after the event.

Check it before race day

Roll test: push one car through each lane and watch the diagnostics screen. Exactly one bit must change per pass, and it must be that lane's bit. Any extra transition means the beam or the neighboring sensor's aim needs work.

One-switch test: jumper all four finish inputs to a single switch so every lane sees an identical event, and work the switch 100 times. Every pull must report the same order. Any pull that differs shows the capture path resolving one event inconsistently, so check wiring, output type and the capture configuration, then run the 100 again. Hold the track out of service until a run is 100 for 100; a capture that flips on a common switch will flip on close cars.

Recommendation

Recommendation: put the four finish beams on fast or high-speed inputs specified at 2 ms or better, rank the order in interrupt or high-speed capture hardware instead of the ladder scan, keep standard DC inputs for the start switch and every signal that never ranks a car, and release the track only after the one-switch test reports the same order on 100 straight pulls. When a heat lands inside the response spread anyway, report the tie the hardware measured; a printed tie ends an argument that a manufactured hundredth would start.

Questions