Servo - November 2012

Very Large Servos

FIGURE 11. Hitec HS-645MG high torque servo.

FIGURE 12. Servo block from ServoCity.

test arm rod that I was using to measure the torque with the lab balance weights. The Hitec HS- M7990TH monster torque servo shown in Figure 15 weighs only 2.78 oz with an aluminum heatsink case, but has a massive 500 oz-in of torque at 6.0 volts. With titanium gears and dual ball bearings, a magnetic rotary encoder, and programming capability, this servo is perfect for large robots — either for leg or upper arm actuation. It exceeded my torque measurement capability and did not even slow down when grasped by my fingers.

ServoCity makes what is called a servo power gearbox shown in Figure 16. This particular setup used a Hitec HS-785HB sailboat winch servo with 3- 1/2 revolution capability to drive up to a 7:1 gear ratio to obtain 1,281 oz-in of torque and a full 180º of rotation. The model that I have tested has a 3:1 ratio and a 420º rotation for 549 oz-in of torque. For $120, you cannot find a servo anywhere with these capabilities, nor machine the brackets with the large ball bearings to take large loads. ServoCity (through its parent company, RobotZone) has a large variety of gear and mounting accessories for many servos.

Invenscience makes the Torxis i00800 shown in Figure 17, and the i01855 (industrial version) super high torque R/C servos with an ultimate torque of 3,200 oz-in. As with most very large servos, this one requires a separate 12 VDC supply at three amps capacity, but uses a standard R/C PWM input. It can travel 90º with an R/C input or up to 270º with a microcontroller's input.

Though a bit slower than a small servo, it has a 60º travel in . 5 seconds. It is all metal gear construction with dual roller bearings on the output shaft and an aluminum 6061-T6 horn. These are very massive and powerful servos that can handle any human-sized robot's requirements. In testing them, I had to be very careful of any pinching situation at the output horn.

FIGURE 13. Futaba 9352HV very high torque servo.

Rotary Actuator Servos from Robotis

The Dynamixel rotary actuator series that I mentioned earlier has become one of the most popular servos for advanced robotics experimentation. The basic AX- 12 actuator of the group is at the lower right in the two rows of actuators shown in Figure 18. It has a holding torque of 229 oz-in and operates from 7. 2 to 12 VDC. Operating on a TTL half duplex async serial protocol through the daisy-chain interconnection, this intelligent actuator is the servo for all 18 DOF in the Bioloid Premium kit.

Figure 19 shows the AX-12A and 10 of the different actuators in a speed vs. torque graph. The Robotis website, as well as the CrustCrawler and Trossen Robotics websites, have extensive information on these amazing servos.

The AX-12A and all of the AX and more advanced MX series can be mixed as all can operate on a single 12 VDC supply. The servo’s built-in microcontroller handles sensor management and position control.

Each actuator can track its speed, temperature, shaft position, voltage, and load. The most advanced of the series (shown on the bottom left) is the MX-106T with 1,416 oz-in of torque and an onboard 32-bit, 72 MHz Cortex M3 controller with a PID control algorithm (Proportional Integral Derivative or ‘three-term control’), and a contact-less magnetic encoder that can operate at up to 3 MB/s with the new TTL 2.0 bus structure.

I am very impressed with this whole series which I have been experimenting with using the Bioloid’s controller and various shop-made add-ons to test torque and speed. I have found the AX-12A actuators to be faultless in operation with the Bioloid Premium kit (which I will review very soon). The Dynamixel AX-12A and AX- 18 are plastic geared actuators with a cored motor.