Controlling Devices with Relays
I recently was given the curious task of making noise on a factory floor at set intervals. The company wanted to use this noise to provide important auditory cues for specific events. These events included the time employees should clock in, clock out, take a break and return from break. The solution I implemented has been working well, and everyone involved has been pleased with the results. The gratitude I receive from employees that clock in and clock out is related directly to the inaccuracy of their own personal timepiece—those with fast watches appreciate the apparent extra time they have before clocking in. Later, though, they curse me for being delayed in clocking out when the watches on their wrists clearly show it is time to go.
The company I work for uses barcode clock terminals placed throughout the plant to record the time that employees clock in and clock out. In addition, the barcode clocks also record what job each individual employee is performing. When an employee clocks in or out, the employee scans his or her identification badge and a barcode representing the job the person is doing. The clock terminal copies this data to a buffer, along with a timestamp of when the items were scanned. CMINet, a data collection system running on two Microsoft Windows NT systems, is used to acquire the data from the clocks. The clocks are polled every minute or so to retrieve the data held in the clock terminal's buffer. During polling, the clocks also are set to the local time of the machine polling the clocks. The data acquired from the clocks is later imported into the enterprise resource planning (ERP) system for job costing and payroll.
The company's policy on tardiness states, “employees will be considered tardy if they clock in one minute or more after their assigned start time.” Due to the company policy concerning tardiness, the event bells needed to be synchronized perfectly with the barcode clocks and the machines running CMINet. This synchronization would, in theory, alleviate the discrepancies between the recorded clock-in time, recorded clock-out times and the true local time. Correct time synchronization is key, but it is not discussed thoroughly in this article.
Considering the one-minute-or-more-late-equals-tardy rule and the fact that employees invariably become time-challenged when returning from break, it was decided that scheduled bell soundings would be useful. Therefore, a mission-critical noise-making system was needed, and I was given the task of implementing it.
Based on the issues at hand, this project had to meet the following requirements:
Provide auditory cues using a bell system. The bells need to be audible and annoying enough to be noticed even after prolonged exposure.
Provide a simple way of closing an electrical circuit over a computer.
Provide accurate time synchronization for all time-sensitive systems, including the machine that would trigger the bells, the machines running CMINet and, indirectly, the barcode clocks.
With the requirements defined I began to formulate a plan. Five new 12VAC bells and a 12VAC power supply were purchased. These are the same type of bells commonly used in alarm systems or any sounding system. The bells physically hammer themselves in the same manner as an old alarm clock, and they pump out the decibels. The bells needed to be wired so that connecting two wires would ring all the bells in the plant.
To control the bells, we needed to close the bells' circuit temporarily. I was pointed toward a specific relay device, the AR-2, that can be controlled by a computer over a serial port. This device would be used to complete the bell circuit, causing them to ring. The AR-2 is sold by Electronic Energy Control, Inc. (EECI) for about $44 US. It has two relays on a circuit board, and both can be connected as normally open (NO) or normally closed (NC) relays. Only one relay wired as NO would be used for this project. An enclosure (EN-B) to house the board also was purchased. An emergency on/off switch was mounted on the enclosure for use in case something went terribly wrong. The AR-2 is documented well enough that I knew it would be possible to write a program in C to control it. This program could be scheduled using cron jobs.
For time synchronization, the NTP protocol was chosen. NTP clients and servers are unlikely to have interoperability problems due to a well-defined protocol that is widely adopted. In addition, NTP clients can handle network latency, daylight saving time and local time zones. Free clients are available for almost every major operating system in use today. Computers using these clients can readily obtain time sources accurate to at least the millisecond. Good accuracy is exactly what we need when trying to synchronize multiple systems.
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