23. Analog outputs
FDTs with the analog interface board put each plate’s forces, moments and centre of pressure on sixteen ±10 V outputs, A1 to A16, for systems that record voltages: an EMG amplifier’s auxiliary inputs, a DAQ card, a motion capture system’s analog board. This chapter gives the scale and what the voltages mean. Source: Nur Engineering, Movetrics FDT analog outputs: scale per channel, 27 October 2026, for FDT firmware 1.2.8.22 (project files, hardware/Nur-Engineering_FDT_analog_outputs_2026-10-27.pdf). Machine-readable: contracts/analog-outputs.v1.json; in Python, movetrics_fdt.analog.
23.1 Scale
Section titled “23.1 Scale”0 V is zero load. Value = volts × units per V.
| Plate 1 | Plate 2 | Signal | Full scale at ±10 V | mV per unit | Units per V |
|---|---|---|---|---|---|
| A1 | A9 | Fz | ±8,000 N | 1.25 | 800 |
| A2 | A10 | Fx | ±2,000 N | 5.00 | 200 |
| A3 | A11 | Fy | ±2,000 N | 5.00 | 200 |
| A4 | A12 | Mz | ±720 N·m | 13.89 | 72 |
| A5 | A13 | Mx | ±720 N·m | 13.89 | 72 |
| A6 | A14 | My | ±720 N·m | 13.89 | 72 |
| A7 | A15 | CoP x | ±15 cm | 666.7 | 1.5 |
| A8 | A16 | CoP y | ±30 cm | 333.3 | 3 |
23.2 Signs
Section titled “23.2 Signs”Fz is positive for a load pressing on the plate. The other channels carry the sign of the same value in the FDT’s digital frame (the wire signs of chapter 05). Studio, and therefore the shared vocabulary (chapter 19), shows Fy and Mx with the opposite sign, so A3, A5, A11 and A13 are the reverse of fy and mx. movetrics_fdt.analog.to_vocabulary() flips them. The maker’s sheet does not state the sign or origin of the CoP outputs; check them with a known off-centre load before relying on them.
23.3 What the voltages can and cannot give you
Section titled “23.3 What the voltages can and cannot give you”- Resolution. The outputs are made from the stream’s whole numbers (N, N·m, cm), so they move in steps of one unit: 1.25 mV for Fz, 5 mV for Fx and Fy, 13.9 mV for a moment, and 1 cm for the centre of pressure. For CoP at millimetre level, compute it from the forces and moments (
plate_cop, chapter 19) or use the digital stream. - CoP threshold. The CoP outputs read 0 V while Fz is below 30 N. The digital vocabulary uses 50 N.
- Clipping. A value past full scale stays at the end of the range; it does not wrap. On the measured unit +10 V was reached at 99.4 % of full scale (about 7,950 N Fz) because of its zero offset. Treat anything within 1 % of ±10 V as clipped (
analog.clipped()). - Update rate. The outputs change only while the FDT streams (after START from Studio, the app or any host); they sit at 0 V after power-up and hold the last value after STOP. Each plate is updated at most 500 times per second: every frame at 500 frames/s, every second frame at 1,000 frames/s (chapter 05 section 4.10). Sampling them faster does not add information.
- Filtering and delay. The values are the FDT’s filtered values (50 Hz low-pass by default, set by the host), followed by a first-order low-pass at about 220 Hz in the output stage (33 kΩ, 22 nF, from the schematic, not measured). They lag the force by the frame latency of chapter 13 (about 12 ms at 500 frames/s), so an analog channel recorded next to EMG is not a time reference. For alignment use the sync jack or LSL (chapter 22).
- Zero. 0 V means the plate was zeroed: at power-up, at reset and on the Zero command. There is no separate analog zero.
23.4 Electrical
Section titled “23.4 Electrical”16-bit DAC, 0.305 mV per code. 50 Ω series resistor per output: a load of 10 kΩ or more keeps the divider error under 0.5 %. Gain set by 4.7 kΩ and 33 kΩ resistors of 1 %, so up to ±1.8 % worst case. On the measured unit a meter agreed with the digital values within 1 % at 164 N Fz and 24.6 N·m Mx, and zero read close to 0 mV on all outputs (27 October 2026, plates empty).
23.5 Firmware
Section titled “23.5 Firmware”The scale holds for firmware 1.2.8.21 and later with analog outputs (read in the 1.2.8.22 source on 28 Oct 2026: the same full scales, channel order and clipping as this chapter); 1.2.8.22 adds a small zero trim inside the FDT that needs nothing from the host. Before 1.2.8.21 the moment outputs of a double plate were 100 times too large and values past full scale wrapped around. ANALOG_ENABLE (0x34) switches the outputs on or off (chapter 04); they are on by default. The digital-only FDT variant has no analog board.
Status: maker’s document plus one metered unit (measured, 27 October 2026). Not checked by this SDK on hardware.