Royer has that control thanks to the Guitar Machine - a mechanical device allowing him to create a guitar and bass rhythm section with his feet, freeing his hands to play the banjo or lap steel guitar.
Royer, a Massachusetts native, invented the Guitar Machine while living in Tucson in the early 1990's. Now that he's back in the Boston area, he's taken the Guitar Machine out onto the streets and into the clubs of Cambridge, bringing, as the Boston Bluegrass Union's Gerry Katz says, "bluegrass to the masses."
For those skeptical about the viability of a one-man bluegrass band, a listen to Royer's new CD, "Royer's Guitar Machine Band," should be enough to dispel any doubts.
Not that Royer sticks to the bluegrass straight and narrow - the CD's material ranges through old-time numbers like Uncle Dave Macon's "Tell Her To Come Back Home" to Royer's own, waltz-time take on Hazel Dickens' "Won't You Come And Sing For Me" - but when he wants to, he can do it pretty well.
His version of the "Tennessee Breakdown" captures the spirit of Don Reno's original quite nicely, and his arrangements of tunes like "Bill Cheatum" and "Sally Goodin" sound...well, they sound like a bluegrass band.
"I'm trying to stick to making it a bluegrass sound," Royer says. "It's really challenging to get the bass alternating, but I'm striving to get the rhythm solid."
That's the purpose of the Guitar Machine. "When it was new - I built the first one in 1994 - it was interesting and different in and of itself. The place I was living in Tucson had lots of junk - old washing machines, things like that - and I was always fooling around with it. I made a drum machine and a couple of other things, but the guitar was the one that really worked. As I kept doing it, though, the music started to become much more important."
As a youngster, Royer was mostly preoccupied with punk rock. That interest led him to Boston in the mid-1980's, but things didn't pan out for him, and his move to Tucson followed.
There, he became acquainted with bluegrass and other kinds of roots music. "I'd always been interested in fingerpicking," he says. "And I got a banjo and did some fingerpicking on that, but I didn't learn the bluegrass style right away. As I started listening to it more, though, I wanted to learn how to play the banjo that way, and that really drove things along."
"Things have changed in the way I look at the world," Royer continues. "When I played guitar, when I played rock, I didn't want to imitate anyone. But somehow I changed the way I thought about learning, and I decided I would try to learn this very traditional thing for a couple of years."
Though he took a few lessons from Lynn Morris, a well-known banjo player and bluegrass band leader, he worked mostly from recordings.
"When you learn how Earl (Scruggs, the creator of the bluegrass banjo style) did it, it opens up all of these subtle things. I heard stuff I'd never heard before."
The Guitar Machine has gone through several editions as Royer's learned more about bluegrass - and engineering.
"The first machine had just a guitar with four chords and a hi-hat, that's all. We strapped it to the top of the car when I moved back to Boston in 1994."
Later machines have incorporated a copper framework (for lighter weight and easier carrying), an electric bass and more chords. "The left foot pedals control what chord is played on the guitar and what notes are used on the bass," he says. "The right foot is the rhythm, with a bass note on the downbeat and a guitar strum on the up-beat."
With a banjo held conventionally on his lap, Royer has the makings of a bluegrass band.
Working on the streets and in the subways has become Royer's main way to reach audiences - enough that, with some club and festival dates plus the occasional odd carpentry or painting job, he's able to make a living with his Guitar Machine.
"I've had some club gigs, mostly for alternative country and rockabilly audiences. What I do goes over pretty well; just about any kind of audience enjoys the music, or at least the Machine. I like playing in clubs, and I'm trying to do more of it, but it's tough to make as much money there as I can out on the street.
"I'd like to work at more bluegrass festivals, too," he adds, "but it can be hard to explain what I do, and being up on a stage isn't always the best place for me to be; when I'm right out there on the ground is when I get my best response. A lot of times it comes from the kids."
Royer doesn't rule out working in a more conventional bluegrass setting than his Machine provides - in fact, he says, he's definitely interested.
"I'd like to be in a bluegrass band, but it's hard for me to find folks playing the kind of stuff I like best. It seems like a lot of Boston bluegrass pickers, including a lot of the best, are more into a progressive sound, or newgrass," something he's not as interested in playing.
In the meantime, though, he's hard at work on a new - and, he says, better - Guitar Machine.
"It's going to be all acoustic," he enthuses. "It's about half way done right now. The main thing is to get a bass sound that's more like a standup. I'm building a special bass from scratch, with a big, hollow neck to get more of a sound chamber; since I'm not holding it in my hand, it doesn't matter whether it's too big to fit my hand. I really want to get a more traditional sound. Besides that, the mechanisms will be smoother, and maybe a little more sophisticated. The whole thing will just sound better."
Once his new Machine is built, Royer says, "then I'll record again. This first album has been doing pretty well; I've sold a lot of copies, mostly while playing on the street or at gigs, not in the big stores."
That's not surprising. After all, on first glance, the idea of a musician creating a genuine bluegrass band sound all by himself seems implausible; still, if anyone is able to do it, it's Eric Royer - not only because he turned out to have the mechanical aptitude necessary to build his Machine, but more importantly, because his attitude about bluegrass and how to play it is right in the classic vein.
"Something changed in my brain," he says."The rock music I grew up with had so much of an emphasis on originality, but in bluegrass, playing traditional tunes is a big part of the fun. All I want to do is to be good at it."