The “robo-referees” in baseball explained: How the automatic strike zone works with the challenge system developed in the minor leagues

An automated strike zone—known informally as a “robo-umpire”—has been in use for some time at the Triple-A level and occasionally in various other minor league leagues. Recently, MLB announced plans to use a variant of the automated ball-strike system (ABS), the Challenge System, in all remaining Triple-A games beginning June 25. This likely indicates that the ABS Challenge System will eventually be used in the majors.

This would be one of the most drastic structural changes in the major league game in a long time and is likely to leave fans with questions about what the ABS system is, how it works and what it will be like to see it in action. Below we’ll try to answer those questions that may be on your mind as you consider the future of the strike zone.

How is it possible to automate the strike zone?

The ABS system uses Hawk-Eye camera technology to determine whether each pitch is inside or outside a strike zone tailored to the individual hitter. Here is a general explanation from 2021 from three engineers who worked on the system used:

The ABS system design consists of four discrete subsystems:

• The MLB tracking system;
• An interface that the stadium operator uses to set the correct batsman;
• An MLB server that receives the tracking data and has the ball strike evaluation code;
• A low latency communications system to route calls to the referee.

As for the strike zone, as mentioned, it’s player-specific, meaning that Aaron Judge and Jose Altuve, for example, wouldn’t have the same zone parameters. It even means that Dansby Swanson and Tim Anderson – both listed at 6’1″ – wouldn’t have the same zones due to different stances. So no, this isn’t the rudimentary (and occasionally misleading) zone you’re used to seeing on televised games. Here’s what JJ Cooper of Baseball America reported last August about the ABS system’s latest round of zone refinements:

“Now the ABS system will use the Hawk-Eye system’s visual tracking to set the Triple-A strike zone. Because the Hawk-Eye system tracks each player’s limbs, the strike zone is now set individually for each player. The system sets the lower boundary of the strike zone at the level of the player’s knees. The upper boundary of the zone is set at two balls above the midpoint of the player’s left and right hip measurements. The goal is to place the upper boundary of the zone near the belt line.

“The new zone means that each player’s strike zone is tied to his body and stance individually, rather than to a universal formula. The new strike zone is intended to be more similar to the strike zone of human umpires, although the upper end is still intended to be lower than the upper end of the MLB strike zone.”

This level of customization is necessary to make an automated zone fair to all hitters, and the above has accomplished that quite well.

Why is an ABS system necessary?

This is seen as increasingly necessary as the umpires’ job at the plate becomes more difficult. The average major league fastball is 94.1 mph and getting faster every year. You’ll see plenty of hard, late sliders and even some changeups that reach the plate as quickly as the four-seamers of a generation ago. Add to that the emphasis on pitch framing, with catchers across the league more adept than ever at fooling umpires, and the task of calling balls and strikes has become nearly impossible. This is especially true for borderline throws.

For example, according to data from TruMedia, pitches to the corners that the batter hit were correctly called by the plate umpire only 58.7% of the time. In two-strike counts, umpires only correctly called pitches to the corners 46.3% of the time. All of these miscalls are unfair to players and fans, and it is unfair to ask umpires to keep trying the impossible. That is why an ABS system is needed in MLB.

What is the difference between a challenge system and a full ABS system?

In a full ABS system, every ball-strike call is automated, and the human umpire is there to relay these automated calls through the handset and perform other duties (safe-out calls at the plate, balks and catcher interference, checked swing calls, etc.). Even with the full system, the umpire must mentally make his own ball-strike calls in case the automated system has some sort of glitch and cannot make ball-strike decisions.

With the challenge system, the human umpire will determine whether a ball or a strike was called on all pitches. However, the pitcher, batter or catcher has the right to appeal calls that they believe were incorrect. At that point, the ABS system will decide whether the appealed call on the ball or strike is correct or incorrect. To prevent indiscriminate challenges, each team is given a limited number of incorrect challenges per game – say three. If your challenge results in an overturn, you keep it. There is an incentive to be judicious with challenges so you don’t lose them all in the first innings.

How will this work in practice?

Perhaps a video demonstration would be appropriate. Note:

As you can see, the batter challenges the strike call by physically signaling (tapping his helmet), and the umpire at the plate relays the challenge and gets the correct call. This all happens quickly and as smoothly as possible.

How soon will there be an ABS system in the professional league?

Here’s what MLB Commissioner Rob Manfred said in May about a possible timeline:

“We still have some technical issues; I don’t mean the technology, but technical issues in terms of how the system works. We haven’t made as much progress in the minor leagues this year as we had hoped at this point. I think it’s becoming more and more likely that it’s not going to work out in 2025.”

This means that there will be no ABS system in the major leagues until 2026 at the earliest. Until then, the strikes can be called strikes.